X-Git-Url: https://perl5.git.perl.org/perl5.git/blobdiff_plain/d35dd6c678badc24d545f8b7b7a3ebdf0fb0b355..c149d39e9196847264957ce3d52e0590fcf7a5ca:/pod/perlre.pod diff --git a/pod/perlre.pod b/pod/perlre.pod index acc1ad5..b6d6677 100644 --- a/pod/perlre.pod +++ b/pod/perlre.pod @@ -31,8 +31,8 @@ L. X X X X Treat string as multiple lines. That is, change "^" and "$" from matching -the start or end of the string to matching the start or end of any -line anywhere within the string. +the start or end of line only at the left and right ends of the string to +matching them anywhere within the string. =item s X X X @@ -51,13 +51,53 @@ X Do case-insensitive pattern matching. -If C is in effect, the case map is taken from the current -locale. See L. +If locale matching rules are in effect, the case map is taken from the +current +locale for code points less than 255, and from Unicode rules for larger +code points. However, matches that would cross the Unicode +rules/non-Unicode rules boundary (ords 255/256) will not succeed. See +L. + +There are a number of Unicode characters that match multiple characters +under C. For example, C +should match the sequence C. Perl is not +currently able to do this when the multiple characters are in the pattern and +are split between groupings, or when one or more are quantified. Thus + + "\N{LATIN SMALL LIGATURE FI}" =~ /fi/i; # Matches + "\N{LATIN SMALL LIGATURE FI}" =~ /[fi][fi]/i; # Doesn't match! + "\N{LATIN SMALL LIGATURE FI}" =~ /fi*/i; # Doesn't match! + + # The below doesn't match, and it isn't clear what $1 and $2 would + # be even if it did!! + "\N{LATIN SMALL LIGATURE FI}" =~ /(f)(i)/i; # Doesn't match! + +Perl doesn't match multiple characters in an inverted bracketed +character class, which otherwise could be highly confusing. See +L. + +Another bug involves character classes that match both a sequence of +multiple characters, and an initial sub-string of that sequence. For +example, + + /[s\xDF]/i + +should match both a single and a double "s", since C<\xDF> (on ASCII +platforms) matches "ss". However, this bug +(L<[perl #89774]|https://rt.perl.org/rt3/Ticket/Display.html?id=89774>) +causes it to only match a single "s", even if the final larger match +fails, and matching the double "ss" would have succeeded. + +Also, Perl matching doesn't fully conform to the current Unicode C +recommendations, which ask that the matching be made upon the NFD +(Normalization Form Decomposed) of the text. However, Unicode is +in the process of reconsidering and revising their recommendations. =item x X Extend your pattern's legibility by permitting whitespace and comments. +Details in L"/x"> =item p X
, and C modifiers are special in that -they can only be enabled, not disabled, and the C, C, and C -modifiers are mutually exclusive: specifying one de-specifies the -others, and a maximum of one may appear in the construct. Thus, for -example, C<(?-p)>, C<(?-d:...)>, and C<(?-dl:...)> will warn when -compiled under C. +Note that the C, C, C, C, and C modifiers are special in +that they can only be enabled, not disabled, and the C, C, C, and +C modifiers are mutually exclusive: specifying one de-specifies the +others, and a maximum of one (or two C's) may appear in the +construct. Thus, for +example, C<(?-p)> will warn when compiled under C; +C<(?-d:...)> and C<(?dl:...)> are fatal errors. Note also that the C modifier is special in that its presence anywhere in a pattern has a global effect. @@ -689,9 +1027,9 @@ anywhere in a pattern has a global effect. =item C<(?:pattern)> X<(?:)> -=item C<(?dluimsx-imsx:pattern)> +=item C<(?adluimsx-imsx:pattern)> -=item C<(?^luimsx:pattern)> +=item C<(?^aluimsx:pattern)> X<(?^:)> This is for clustering, not capturing; it groups subexpressions like @@ -707,7 +1045,7 @@ but doesn't spit out extra fields. It's also cheaper not to capture characters if you don't need to. Any letters between C> and C<:> act as flags modifiers as with -C<(?dluimsx-imsx)>. For example, +C<(?adluimsx-imsx)>. For example, /(?s-i:more.*than).*million/i @@ -726,7 +1064,7 @@ is equivalent to (?x-ims:foo) The caret tells Perl that this cluster doesn't inherit the flags of any -surrounding pattern, but to go back to the system defaults (C), +surrounding pattern, but uses the system defaults (C), modified by any flags specified. The caret allows for simpler stringification of compiled regular @@ -761,7 +1099,7 @@ following this construct will be numbered as though the construct contained only one branch, that being the one with the most capture groups in it. -This construct will be useful when you want to capture one of a +This construct is useful when you want to capture one of a number of alternative matches. Consider the following pattern. The numbers underneath show in @@ -794,7 +1132,7 @@ named C<< b >> are aliases for the group belonging to C<< $1 >>. =item Look-Around Assertions X X X X -Look-around assertions are zero width patterns which match a specific +Look-around assertions are zero-width patterns which match a specific pattern without including it in C<$&>. Positive assertions match when their subpattern matches, negative assertions match when their subpattern fails. Look-behind matches text up to the current match position, @@ -819,14 +1157,7 @@ use this for look-behind. If you are looking for a "bar" that isn't preceded by a "foo", C(?!foo)bar/> will not do what you want. That's because the C<(?!foo)> is just saying that the next thing cannot be "foo"--and it's not, it's a "bar", so "foobar" will -match. You would have to do something like C(?!foo)...bar/> for that. We -say "like" because there's the case of your "bar" not having three characters -before it. You could cover that this way: C(?:(?!foo)...|^.{0,2})bar/>. -Sometimes it's still easier just to say: - - if (/bar/ && $` !~ /foo$/) - -For look-behind see below. +match. Use look-behind instead (see below). =item C<(?<=pattern)> C<\K> X<(?<=)> X X X<\K> @@ -837,7 +1168,7 @@ Works only for fixed-width look-behind. There is a special form of this construct, called C<\K>, which causes the regex engine to "keep" everything it had matched prior to the C<\K> and -not include it in C<$&>. This effectively provides variable length +not include it in C<$&>. This effectively provides variable-length look-behind. The use of C<\K> inside of another look-around assertion is allowed, but the behaviour is currently not well defined. @@ -867,8 +1198,10 @@ only for fixed-width look-behind. X<< (?) >> X<(?'NAME')> X X A named capture group. Identical in every respect to normal capturing -parentheses C<()> but for the additional fact that C<%+> or C<%-> may be -used after a successful match to refer to a named group. See C +parentheses C<()> but for the additional fact that the group +can be referred to by name in various regular expression +constructs (like C<\g{NAME}>) and can be accessed by name +after a successful match via C<%+> or C<%->. See L for more details on the C<%+> and C<%-> hashes. If multiple distinct capture groups have the same name then the @@ -920,87 +1253,124 @@ X<(?{})> X X X B: This extended regular expression feature is considered experimental, and may be changed without notice. Code executed that has side effects may not perform identically from version to version -due to the effect of future optimisations in the regex engine. +due to the effect of future optimisations in the regex engine. The +implementation of this feature was radically overhauled for the 5.18.0 +release, and its behaviour in earlier versions of perl was much buggier, +especially in relation to parsing, lexical vars, scoping, recursion and +reentrancy. -This zero-width assertion evaluates any embedded Perl code. It -always succeeds, and its C is not interpolated. Currently, -the rules to determine where the C ends are somewhat convoluted. +This zero-width assertion executes any embedded Perl code. It always +succeeds, and its return value is set as C<$^R>. -This feature can be used together with the special variable C<$^N> to -capture the results of submatches in variables without having to keep -track of the number of nested parentheses. For example: +In literal patterns, the code is parsed at the same time as the +surrounding code. While within the pattern, control is passed temporarily +back to the perl parser, until the logically-balancing closing brace is +encountered. This is similar to the way that an array index expression in +a literal string is handled, for example - $_ = "The brown fox jumps over the lazy dog"; - /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i; - print "color = $color, animal = $animal\n"; + "abc$array[ 1 + f('[') + g()]def" + +In particular, braces do not need to be balanced: + + /abc(?{ f('{'); })/def/ + +Even in a pattern that is interpolated and compiled at run-time, literal +code blocks will be compiled once, at perl compile time; the following +prints "ABCD": -Inside the C<(?{...})> block, C<$_> refers to the string the regular + print "D"; + my $qr = qr/(?{ BEGIN { print "A" } })/; + my $foo = "foo"; + /$foo$qr(?{ BEGIN { print "B" } })/; + BEGIN { print "C" } + +In patterns where the text of the code is derived from run-time +information rather than appearing literally in a source code /pattern/, +the code is compiled at the same time that the pattern is compiled, and +fro reasons of security, C must be in scope. This is to +stop user-supplied patterns containing code snippets from being +executable. + +In situations where you need enable this with C, you should +also have taint checking enabled. Better yet, use the carefully +constrained evaluation within a Safe compartment. See L for +details about both these mechanisms. + +From the viewpoint of parsing, lexical variable scope and closures, + + /AAA(?{ BBB })CCC/ + +behaves approximately like + + /AAA/ && do { BBB } && /CCC/ + +Similarly, + + qr/AAA(?{ BBB })CCC/ + +behaves approximately like + + sub { /AAA/ && do { BBB } && /CCC/ } + +In particular: + + { my $i = 1; $r = qr/(?{ print $i })/ } + my $i = 2; + /$r/; # prints "1" + +Inside a C<(?{...})> block, C<$_> refers to the string the regular expression is matching against. You can also use C to know what is the current position of matching within this string. -The C is properly scoped in the following sense: If the assertion -is backtracked (compare L<"Backtracking">), all changes introduced after -Cization are undone, so that +The code block introduces a new scope from the perspective of lexical +variable declarations, but B from the perspective of C and +similar localizing behaviours. So later code blocks within the same +pattern will still see the values which were localized in earlier blocks. +These accumulated localizations are undone either at the end of a +successful match, or if the assertion is backtracked (compare +L<"Backtracking">). For example, $_ = 'a' x 8; m< - (?{ $cnt = 0 }) # Initialize $cnt. + (?{ $cnt = 0 }) # Initialize $cnt. ( a (?{ - local $cnt = $cnt + 1; # Update $cnt, backtracking-safe. + local $cnt = $cnt + 1; # Update $cnt, + # backtracking-safe. }) )* aaaa - (?{ $res = $cnt }) # On success copy to - # non-localized location. + (?{ $res = $cnt }) # On success copy to + # non-localized location. >x; -will set C<$res = 4>. Note that after the match, C<$cnt> returns to the globally -introduced value, because the scopes that restrict C operators -are unwound. +will initially increment C<$cnt> up to 8; then during backtracking, its +value will be unwound back to 4, which is the value assigned to C<$res>. +At the end of the regex execution, $cnt will be wound back to its initial +value of 0. + +This assertion may be used as the condition in a + + (?(condition)yes-pattern|no-pattern) -This assertion may be used as a C<(?(condition)yes-pattern|no-pattern)> -switch. If I used in this way, the result of evaluation of -C is put into the special variable C<$^R>. This happens -immediately, so C<$^R> can be used from other C<(?{ code })> assertions -inside the same regular expression. +switch. If I used in this way, the result of evaluation of C +is put into the special variable C<$^R>. This happens immediately, so +C<$^R> can be used from other C<(?{ code })> assertions inside the same +regular expression. The assignment to C<$^R> above is properly localized, so the old value of C<$^R> is restored if the assertion is backtracked; compare L<"Backtracking">. -For reasons of security, this construct is forbidden if the regular -expression involves run-time interpolation of variables, unless the -perilous C pragma has been used (see L), or the -variables contain results of C operator (see -Lmsixpo">). +Note that the special variable C<$^N> is particularly useful with code +blocks to capture the results of submatches in variables without having to +keep track of the number of nested parentheses. For example: -This restriction is due to the wide-spread and remarkably convenient -custom of using run-time determined strings as patterns. For example: + $_ = "The brown fox jumps over the lazy dog"; + /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i; + print "color = $color, animal = $animal\n"; - $re = <>; - chomp $re; - $string =~ /$re/; - -Before Perl knew how to execute interpolated code within a pattern, -this operation was completely safe from a security point of view, -although it could raise an exception from an illegal pattern. If -you turn on the C, though, it is no longer secure, -so you should only do so if you are also using taint checking. -Better yet, use the carefully constrained evaluation within a Safe -compartment. See L for details about both these mechanisms. - -B: Use of lexical (C) variables in these blocks is -broken. The result is unpredictable and will make perl unstable. The -workaround is to use global (C) variables. - -B: In perl 5.12.x and earlier, the regex engine -was not re-entrant, so interpolated code could not -safely invoke the regex engine either directly with -C or C), or indirectly with functions such as -C. Invoking the regex engine in these blocks would make perl -unstable. =item C<(??{ code })> X<(??{})> @@ -1011,61 +1381,60 @@ experimental, and may be changed without notice. Code executed that has side effects may not perform identically from version to version due to the effect of future optimisations in the regex engine. -This is a "postponed" regular subexpression. The C is evaluated -at run time, at the moment this subexpression may match. The result -of evaluation is considered as a regular expression and matched as -if it were inserted instead of this construct. Note that this means -that the contents of capture groups defined inside an eval'ed pattern -are not available outside of the pattern, and vice versa, there is no -way for the inner pattern to refer to a capture group defined outside. -Thus, +This is a "postponed" regular subexpression. It behaves in I the +same way as a C<(?{ code })> code block as described above, except that +its return value, rather than being assigned to C<$^R>, is treated as a +pattern, compiled if it's a string (or used as-is if its a qr// object), +then matched as if it were inserted instead of this construct. - ('a' x 100)=~/(??{'(.)' x 100})/ +During the matching of this sub-pattern, it has its own set of +captures which are valid during the sub-match, but are discarded once +control returns to the main pattern. For example, the following matches, +with the inner pattern capturing "B" and matching "BB", while the outer +pattern captures "A"; -B match, it will B set $1. + my $inner = '(.)\1'; + "ABBA" =~ /^(.)(??{ $inner })\1/; + print $1; # prints "A"; -The C is not interpolated. As before, the rules to determine -where the C ends are currently somewhat convoluted. +Note that this means that there is no way for the inner pattern to refer +to a capture group defined outside. (The code block itself can use C<$1>, +etc., to refer to the enclosing pattern's capture groups.) Thus, although + + ('a' x 100)=~/(??{'(.)' x 100})/ + +I match, it will I set $1 on exit. The following pattern matches a parenthesized group: - $re = qr{ - \( - (?: - (?> [^()]+ ) # Non-parens without backtracking - | - (??{ $re }) # Group with matching parens - )* - \) - }x; + $re = qr{ + \( + (?: + (?> [^()]+ ) # Non-parens without backtracking + | + (??{ $re }) # Group with matching parens + )* + \) + }x; See also C<(?PARNO)> for a different, more efficient way to accomplish the same task. -For reasons of security, this construct is forbidden if the regular -expression involves run-time interpolation of variables, unless the -perilous C pragma has been used (see L), or the -variables contain results of C operator (see -LSTRINGEmsixpo">). - -In perl 5.12.x and earlier, because the regex engine was not re-entrant, -delayed code could not safely invoke the regex engine either directly with -C or C), or indirectly with functions such as C. - -Recursing deeper than 50 times without consuming any input string will -result in a fatal error. The maximum depth is compiled into perl, so -changing it requires a custom build. +Executing a postponed regular expression 50 times without consuming any +input string will result in a fatal error. The maximum depth is compiled +into perl, so changing it requires a custom build. =item C<(?PARNO)> C<(?-PARNO)> C<(?+PARNO)> C<(?R)> C<(?0)> X<(?PARNO)> X<(?1)> X<(?R)> X<(?0)> X<(?-1)> X<(?+1)> X<(?-PARNO)> X<(?+PARNO)> X X X X -Similar to C<(??{ code })> except it does not involve compiling any code, -instead it treats the contents of a capture group as an independent -pattern that must match at the current position. Capture groups -contained by the pattern will have the value as determined by the -outermost recursion. +Similar to C<(??{ code })> except that it does not involve executing any +code or potentially compiling a returned pattern string; instead it treats +the part of the current pattern contained within a specified capture group +as an independent pattern that must match at the current position. +Capture groups contained by the pattern will have the value as determined +by the outermost recursion. PARNO is a sequence of digits (not starting with 0) whose value reflects the paren-number of the capture group to recurse to. C<(?R)> recurses to @@ -1081,15 +1450,15 @@ included. The following pattern matches a function foo() which may contain balanced parentheses as the argument. - $re = qr{ ( # paren group 1 (full function) + $re = qr{ ( # paren group 1 (full function) foo - ( # paren group 2 (parens) + ( # paren group 2 (parens) \( - ( # paren group 3 (contents of parens) + ( # paren group 3 (contents of parens) (?: - (?> [^()]+ ) # Non-parens without backtracking + (?> [^()]+ ) # Non-parens without backtracking | - (?2) # Recurse to start of paren group 2 + (?2) # Recurse to start of paren group 2 )* ) \) @@ -1120,7 +1489,7 @@ easier to embed recursive patterns inside of a C construct for later use: my $parens = qr/(\((?:[^()]++|(?-1))*+\))/; - if (/foo $parens \s+ + \s+ bar $parens/x) { + if (/foo $parens \s+ \+ \s+ bar $parens/x) { # do something here... } @@ -1150,11 +1519,15 @@ X<(?()> =item C<(?(condition)yes-pattern)> -Conditional expression. C<(condition)> should be either an integer in +Conditional expression. Matches C if C yields +a true value, matches C otherwise. A missing pattern always +matches. + +C<(condition)> should be one of: 1) an integer in parentheses (which is valid if the corresponding pair of parentheses -matched), a look-ahead/look-behind/evaluate zero-width assertion, a +matched); 2) a look-ahead/look-behind/evaluate zero-width assertion; 3) a name in angle brackets or single quotes (which is valid if a group -with the given name matched), or the special symbol (R) (true when +with the given name matched); or 4) the special symbol (R) (true when evaluated inside of recursion or eval). Additionally the R may be followed by a number, (which will be true when evaluated when recursing inside of the appropriate group), or by C<&NAME>, in which case it will @@ -1172,9 +1545,14 @@ Checks if the numbered capturing group has matched something. Checks if a group with the given name has matched something. +=item (?=...) (?!...) (?<=...) (? predicate, which never executes directly -its yes-pattern, and does not allow a no-pattern. This allows to define -subpatterns which will be executed only by using the recursion mechanism. +A special form is the C<(DEFINE)> predicate, which never executes its +yes-pattern directly, and does not allow a no-pattern. This allows one to +define subpatterns which will be executed only by the recursion mechanism. This way, you can define a set of regular expression rules that can be bundled into any pattern you choose. @@ -1240,6 +1618,19 @@ after the recursion returns, so the extra layer of capturing groups is necessary. Thus C<$+{NAME_PAT}> would not be defined even though C<$+{NAME}> would be. +Finally, keep in mind that subpatterns created inside a DEFINE block +count towards the absolute and relative number of captures, so this: + + my @captures = "a" =~ /(.) # First capture + (?(DEFINE) + (? 1 ) # Second capture + )/x; + say scalar @captures; + +Will output 2, not 1. This is particularly important if you intend to +compile the definitions with the C operator, and later +interpolate them in another pattern. + =item C<< (?>pattern) >> X X X X @@ -1260,9 +1651,13 @@ group C (see L<"Backtracking">). In particular, C inside C will match fewer characters than a standalone C, since this makes the tail match. +C<< (?>pattern) >> does not disable backtracking altogether once it has +matched. It is still possible to backtrack past the construct, but not +into it. So C<< ((?>a*)|(?>b*))ar >> will still match "bar". + An effect similar to C<< (?>pattern) >> may be achieved by writing -C<(?=(pattern))\g1>. This matches the same substring as a standalone -C, and the following C<\g1> eats the matched string; it therefore +C<(?=(pattern))\g{-1}>. This matches the same substring as a standalone +C, and the following C<\g{-1}> eats the matched string; it therefore makes a zero-length assertion into an analogue of C<< (?>...) >>. (The difference between these two constructs is that the second one uses a capturing group, thus shifting ordinals of backreferences @@ -1302,7 +1697,8 @@ hung. However, a tiny change to this pattern which uses C<< (?>...) >> matches exactly when the one above does (verifying this yourself would be a productive exercise), but finishes in a fourth the time when used on a similar string with 1000000 Cs. Be aware, -however, that this pattern currently triggers a warning message under +however, that, when this construct is followed by a +quantifier, it currently triggers a warning message under the C pragma or B<-w> switch saying it C<"matches null string many times in regex">. @@ -1373,14 +1769,14 @@ C<(*MARK:NAME)> pattern executed. See the explanation for the C<(*MARK:NAME)> verb below for more details. B C<$REGERROR> and C<$REGMARK> are not magic variables like C<$1> -and most other regex related variables. They are not local to a scope, nor +and most other regex-related variables. They are not local to a scope, nor readonly, but instead are volatile package variables similar to C<$AUTOLOAD>. Use C to localize changes to them to a specific scope if necessary. If a pattern does not contain a special backtracking verb that allows an argument, then C<$REGERROR> and C<$REGMARK> are not touched at all. -=over 4 +=over 3 =item Verbs that take an argument @@ -1421,7 +1817,7 @@ If we add a C<(*PRUNE)> before the count like the following 'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/; print "Count=$count\n"; -we prevent backtracking and find the count of the longest matching +we prevent backtracking and find the count of the longest matching string at each matching starting point like so: aaab @@ -1437,7 +1833,6 @@ replaced with a C<< (?>pattern) >> with no functional difference; however, C<(*PRUNE)> can be used to handle cases that cannot be expressed using a C<< (?>pattern) >> alone. - =item C<(*SKIP)> C<(*SKIP:NAME)> X<(*SKIP)> @@ -1455,11 +1850,11 @@ encountered, then the C<(*SKIP)> operator has no effect. When used without a name the "skip point" is where the match point was when executing the (*SKIP) pattern. -Compare the following to the examples in C<(*PRUNE)>, note the string +Compare the following to the examples in C<(*PRUNE)>; note the string is twice as long: - 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/; - print "Count=$count\n"; + 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/; + print "Count=$count\n"; outputs @@ -1472,7 +1867,7 @@ executed, the next starting point will be where the cursor was when the C<(*SKIP)> was executed. =item C<(*MARK:NAME)> C<(*:NAME)> -X<(*MARK)> C<(*MARK:NAME)> C<(*:NAME)> +X<(*MARK)> X<(*MARK:NAME)> X<(*:NAME)> This zero-width pattern can be used to mark the point reached in a string when a certain part of the pattern has been successfully matched. This @@ -1500,16 +1895,18 @@ failing the match and has provided its own name to use, the C<$REGERROR> variable will be set to the name of the most recently executed C<(*MARK:NAME)>. -See C<(*SKIP)> for more details. +See L(*SKIP)> for more details. As a shortcut C<(*MARK:NAME)> can be written C<(*:NAME)>. =item C<(*THEN)> C<(*THEN:NAME)> -This is similar to the "cut group" operator C<::> from Perl 6. Like +This is similar to the "cut group" operator C<::> from Perl 6. Like C<(*PRUNE)>, this verb always matches, and when backtracked into on failure, it causes the regex engine to try the next alternation in the -innermost enclosing group (capturing or otherwise). +innermost enclosing group (capturing or otherwise) that has alternations. +The two branches of a C<(?(condition)yes-pattern|no-pattern)> do not +count as an alternation, as far as C<(*THEN)> is concerned. Its name comes from the observation that this operation combined with the alternation operator (C<|>) can be used to create what is essentially a @@ -1528,15 +1925,21 @@ is the same as but - / ( A (*THEN) B | C (*THEN) D ) / + / ( A (*THEN) B | C ) / is not the same as - / ( A (*PRUNE) B | C (*PRUNE) D ) / + / ( A (*PRUNE) B | C ) / as after matching the A but failing on the B the C<(*THEN)> verb will backtrack and try C; but the C<(*PRUNE)> verb will simply fail. +=back + +=item Verbs without an argument + +=over 4 + =item C<(*COMMIT)> X<(*COMMIT)> @@ -1546,8 +1949,8 @@ into on failure it causes the match to fail outright. No further attempts to find a valid match by advancing the start pointer will occur again. For example, - 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/; - print "Count=$count\n"; + 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/; + print "Count=$count\n"; outputs @@ -1558,12 +1961,6 @@ In other words, once the C<(*COMMIT)> has been entered, and if the pattern does not match, the regex engine will not try any further matching on the rest of the string. -=back - -=item Verbs without an argument - -=over 4 - =item C<(*FAIL)> C<(*F)> X<(*FAIL)> X<(*F)> @@ -1592,7 +1989,7 @@ For instance: 'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x; will match, and C<$1> will be C and C<$2> will be C, C<$3> will not -be set. If another branch in the inner parentheses were matched, such as in the +be set. If another branch in the inner parentheses was matched, such as in the string 'ACDE', then the C and C would have to be matched as well. =back @@ -1751,7 +2148,7 @@ let C<\D*> expand to "ABC", this would have caused the whole pattern to fail. The search engine will initially match C<\D*> with "ABC". Then it will -try to match C<(?!123> with "123", which fails. But because +try to match C<(?!123)> with "123", which fails. But because a quantifier (C<\D*>) has been used in the regular expression, the search engine can backtrack and retry the match differently in the hope of matching the complete regular expression. @@ -1799,12 +2196,12 @@ match takes a long time to finish. A powerful tool for optimizing such beasts is what is known as an "independent group", -which does not backtrack (see Lpattern) >>>). Note also that +which does not backtrack (see Lpattern) >>>). Note also that zero-length look-ahead/look-behind assertions will not backtrack to make the tail match, since they are in "logical" context: only whether they match is considered relevant. For an example where side-effects of look-ahead I have influenced the -following match, see Lpattern) >>>. +following match, see Lpattern) >>>. =head2 Version 8 Regular Expressions X X X @@ -1820,7 +2217,7 @@ character; "\\" matches a "\"). This escape mechanism is also required for the character used as the pattern delimiter. A series of characters matches that series of characters in the target -string, so the pattern C would match "blurfl" in the target +string, so the pattern C would match "blurfl" in the target string. You can specify a character class, by enclosing a list of characters @@ -1859,9 +2256,9 @@ You can specify a series of alternatives for a pattern using "|" to separate them, so that C will match any of "fee", "fie", or "foe" in the target string (as would C). The first alternative includes everything from the last pattern delimiter -("(", "[", or the beginning of the pattern) up to the first "|", and +("(", "(?:", etc. or the beginning of the pattern) up to the first "|", and the last alternative contains everything from the last "|" to the next -pattern delimiter. That's why it's common practice to include +closing pattern delimiter. That's why it's common practice to include alternatives in parentheses: to minimize confusion about where they start and end. @@ -1879,7 +2276,7 @@ so if you write C<[fee|fie|foe]> you're really only matching C<[feio|]>. Within a pattern, you may designate subpatterns for later reference by enclosing them in parentheses, and you may refer back to the Ith subpattern later in the pattern using the metacharacter -\I. Subpatterns are numbered based on the left to right order +\I or \gI. Subpatterns are numbered based on the left to right order of their opening parenthesis. A backreference matches whatever actually matched the subpattern in the string being examined, not the rules for that subpattern. Therefore, C<(0|0x)\d*\s\g1\d*> will @@ -1943,7 +2340,7 @@ However, long experience has shown that many programming tasks may be significantly simplified by using repeated subexpressions that may match zero-length substrings. Here's a simple example being: - @chars = split //, $string; # // is not magic in split + @chars = split //, $string; # // is not magic in split ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// / Thus Perl allows such constructs, by I matches a zero-length string, it stops +the C<*>. + +The higher-level loops preserve an additional state between iterations: whether the last match was zero-length. To break the loop, the following match after a zero-length match is prohibited to have a length of zero. This prohibition interacts with backtracking (see L<"Backtracking">), @@ -1995,7 +2414,7 @@ Each of the elementary pieces of regular expressions which were described before (such as C or C<\Z>) could match at most one substring at the given position of the input string. However, in a typical regular expression these elementary pieces are combined into more complicated -patterns using combining operators C, C, C etc +patterns using combining operators C, C, C etc. (in these examples C and C are regular subexpressions). Such combinations can include alternatives, leading to a problem of choice: @@ -2024,11 +2443,11 @@ Consider two possible matches, C and C, C and C are substrings which can be matched by C, C and C are substrings which can be matched by C. -If C is better match for C than C, C is a better +If C is a better match for C than C, C is a better match than C. If C and C coincide: C is a better match than C if -C is better match for C than C. +C is a better match for C than C. =item C @@ -2092,8 +2511,13 @@ than a match at a later position. =head2 Creating Custom RE Engines -Overloaded constants (see L) provide a simple way to extend -the functionality of the RE engine. +As of Perl 5.10.0, one can create custom regular expression engines. This +is not for the faint of heart, as they have to plug in at the C level. See +L for more details. + +As an alternative, overloaded constants (see L) provide a simple +way to extend the functionality of the RE engine, by substituting one +pattern for another. Suppose that we want to enable a new RE escape-sequence C<\Y|> which matches at a boundary between whitespace characters and non-whitespace @@ -2139,11 +2563,11 @@ part of this regular expression needs to be converted explicitly $re = customre::convert $re; /\Y|$re\Y|/; -=head1 PCRE/Python Support +=head2 PCRE/Python Support -As of Perl 5.10.0, Perl supports several Python/PCRE specific extensions +As of Perl 5.10.0, Perl supports several Python/PCRE-specific extensions to the regex syntax. While Perl programmers are encouraged to use the -Perl specific syntax, the following are also accepted: +Perl-specific syntax, the following are also accepted: =over 4 @@ -2163,17 +2587,11 @@ Subroutine call to a named capture group. Equivalent to C<< (?&NAME) >>. =head1 BUGS -There are numerous problems with case insensitive matching of characters -outside the ASCII range, especially with those whose folds are multiple -characters, such as ligatures like C. - -In a bracketed character class with case insensitive matching, ranges only work -for ASCII characters. For example, -C -doesn't match all the Russian upper and lower case letters. - Many regular expression constructs don't work on EBCDIC platforms. +There are a number of issues with regard to case-insensitive matching +in Unicode rules. See C under L above. + This document varies from difficult to understand to completely and utterly opaque. The wandering prose riddled with jargon is hard to fathom in several places.
, and C modifiers are special in +that they can only be enabled, not disabled, and the C, C, C, and +C modifiers are mutually exclusive: specifying one de-specifies the +others, and a maximum of one (or two C's) may appear in the +construct. Thus, for +example, C<(?-p)> will warn when compiled under C; +C<(?-d:...)> and C<(?dl:...)> are fatal errors. Note also that the C modifier is special in that its presence anywhere in a pattern has a global effect. @@ -689,9 +1027,9 @@ anywhere in a pattern has a global effect. =item C<(?:pattern)> X<(?:)> -=item C<(?dluimsx-imsx:pattern)> +=item C<(?adluimsx-imsx:pattern)> -=item C<(?^luimsx:pattern)> +=item C<(?^aluimsx:pattern)> X<(?^:)> This is for clustering, not capturing; it groups subexpressions like @@ -707,7 +1045,7 @@ but doesn't spit out extra fields. It's also cheaper not to capture characters if you don't need to. Any letters between C> and C<:> act as flags modifiers as with -C<(?dluimsx-imsx)>. For example, +C<(?adluimsx-imsx)>. For example, /(?s-i:more.*than).*million/i @@ -726,7 +1064,7 @@ is equivalent to (?x-ims:foo) The caret tells Perl that this cluster doesn't inherit the flags of any -surrounding pattern, but to go back to the system defaults (C), +surrounding pattern, but uses the system defaults (C), modified by any flags specified. The caret allows for simpler stringification of compiled regular @@ -761,7 +1099,7 @@ following this construct will be numbered as though the construct contained only one branch, that being the one with the most capture groups in it. -This construct will be useful when you want to capture one of a +This construct is useful when you want to capture one of a number of alternative matches. Consider the following pattern. The numbers underneath show in @@ -794,7 +1132,7 @@ named C<< b >> are aliases for the group belonging to C<< $1 >>. =item Look-Around Assertions X X X X -Look-around assertions are zero width patterns which match a specific +Look-around assertions are zero-width patterns which match a specific pattern without including it in C<$&>. Positive assertions match when their subpattern matches, negative assertions match when their subpattern fails. Look-behind matches text up to the current match position, @@ -819,14 +1157,7 @@ use this for look-behind. If you are looking for a "bar" that isn't preceded by a "foo", C(?!foo)bar/> will not do what you want. That's because the C<(?!foo)> is just saying that the next thing cannot be "foo"--and it's not, it's a "bar", so "foobar" will -match. You would have to do something like C(?!foo)...bar/> for that. We -say "like" because there's the case of your "bar" not having three characters -before it. You could cover that this way: C(?:(?!foo)...|^.{0,2})bar/>. -Sometimes it's still easier just to say: - - if (/bar/ && $` !~ /foo$/) - -For look-behind see below. +match. Use look-behind instead (see below). =item C<(?<=pattern)> C<\K> X<(?<=)> X X X<\K> @@ -837,7 +1168,7 @@ Works only for fixed-width look-behind. There is a special form of this construct, called C<\K>, which causes the regex engine to "keep" everything it had matched prior to the C<\K> and -not include it in C<$&>. This effectively provides variable length +not include it in C<$&>. This effectively provides variable-length look-behind. The use of C<\K> inside of another look-around assertion is allowed, but the behaviour is currently not well defined. @@ -867,8 +1198,10 @@ only for fixed-width look-behind. X<< (?) >> X<(?'NAME')> X X A named capture group. Identical in every respect to normal capturing -parentheses C<()> but for the additional fact that C<%+> or C<%-> may be -used after a successful match to refer to a named group. See C +parentheses C<()> but for the additional fact that the group +can be referred to by name in various regular expression +constructs (like C<\g{NAME}>) and can be accessed by name +after a successful match via C<%+> or C<%->. See L for more details on the C<%+> and C<%-> hashes. If multiple distinct capture groups have the same name then the @@ -920,87 +1253,124 @@ X<(?{})> X X X B: This extended regular expression feature is considered experimental, and may be changed without notice. Code executed that has side effects may not perform identically from version to version -due to the effect of future optimisations in the regex engine. +due to the effect of future optimisations in the regex engine. The +implementation of this feature was radically overhauled for the 5.18.0 +release, and its behaviour in earlier versions of perl was much buggier, +especially in relation to parsing, lexical vars, scoping, recursion and +reentrancy. -This zero-width assertion evaluates any embedded Perl code. It -always succeeds, and its C is not interpolated. Currently, -the rules to determine where the C ends are somewhat convoluted. +This zero-width assertion executes any embedded Perl code. It always +succeeds, and its return value is set as C<$^R>. -This feature can be used together with the special variable C<$^N> to -capture the results of submatches in variables without having to keep -track of the number of nested parentheses. For example: +In literal patterns, the code is parsed at the same time as the +surrounding code. While within the pattern, control is passed temporarily +back to the perl parser, until the logically-balancing closing brace is +encountered. This is similar to the way that an array index expression in +a literal string is handled, for example - $_ = "The brown fox jumps over the lazy dog"; - /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i; - print "color = $color, animal = $animal\n"; + "abc$array[ 1 + f('[') + g()]def" + +In particular, braces do not need to be balanced: + + /abc(?{ f('{'); })/def/ + +Even in a pattern that is interpolated and compiled at run-time, literal +code blocks will be compiled once, at perl compile time; the following +prints "ABCD": -Inside the C<(?{...})> block, C<$_> refers to the string the regular + print "D"; + my $qr = qr/(?{ BEGIN { print "A" } })/; + my $foo = "foo"; + /$foo$qr(?{ BEGIN { print "B" } })/; + BEGIN { print "C" } + +In patterns where the text of the code is derived from run-time +information rather than appearing literally in a source code /pattern/, +the code is compiled at the same time that the pattern is compiled, and +fro reasons of security, C must be in scope. This is to +stop user-supplied patterns containing code snippets from being +executable. + +In situations where you need enable this with C, you should +also have taint checking enabled. Better yet, use the carefully +constrained evaluation within a Safe compartment. See L for +details about both these mechanisms. + +From the viewpoint of parsing, lexical variable scope and closures, + + /AAA(?{ BBB })CCC/ + +behaves approximately like + + /AAA/ && do { BBB } && /CCC/ + +Similarly, + + qr/AAA(?{ BBB })CCC/ + +behaves approximately like + + sub { /AAA/ && do { BBB } && /CCC/ } + +In particular: + + { my $i = 1; $r = qr/(?{ print $i })/ } + my $i = 2; + /$r/; # prints "1" + +Inside a C<(?{...})> block, C<$_> refers to the string the regular expression is matching against. You can also use C to know what is the current position of matching within this string. -The C is properly scoped in the following sense: If the assertion -is backtracked (compare L<"Backtracking">), all changes introduced after -Cization are undone, so that +The code block introduces a new scope from the perspective of lexical +variable declarations, but B from the perspective of C and +similar localizing behaviours. So later code blocks within the same +pattern will still see the values which were localized in earlier blocks. +These accumulated localizations are undone either at the end of a +successful match, or if the assertion is backtracked (compare +L<"Backtracking">). For example, $_ = 'a' x 8; m< - (?{ $cnt = 0 }) # Initialize $cnt. + (?{ $cnt = 0 }) # Initialize $cnt. ( a (?{ - local $cnt = $cnt + 1; # Update $cnt, backtracking-safe. + local $cnt = $cnt + 1; # Update $cnt, + # backtracking-safe. }) )* aaaa - (?{ $res = $cnt }) # On success copy to - # non-localized location. + (?{ $res = $cnt }) # On success copy to + # non-localized location. >x; -will set C<$res = 4>. Note that after the match, C<$cnt> returns to the globally -introduced value, because the scopes that restrict C operators -are unwound. +will initially increment C<$cnt> up to 8; then during backtracking, its +value will be unwound back to 4, which is the value assigned to C<$res>. +At the end of the regex execution, $cnt will be wound back to its initial +value of 0. + +This assertion may be used as the condition in a + + (?(condition)yes-pattern|no-pattern) -This assertion may be used as a C<(?(condition)yes-pattern|no-pattern)> -switch. If I used in this way, the result of evaluation of -C is put into the special variable C<$^R>. This happens -immediately, so C<$^R> can be used from other C<(?{ code })> assertions -inside the same regular expression. +switch. If I used in this way, the result of evaluation of C +is put into the special variable C<$^R>. This happens immediately, so +C<$^R> can be used from other C<(?{ code })> assertions inside the same +regular expression. The assignment to C<$^R> above is properly localized, so the old value of C<$^R> is restored if the assertion is backtracked; compare L<"Backtracking">. -For reasons of security, this construct is forbidden if the regular -expression involves run-time interpolation of variables, unless the -perilous C pragma has been used (see L), or the -variables contain results of C operator (see -Lmsixpo">). +Note that the special variable C<$^N> is particularly useful with code +blocks to capture the results of submatches in variables without having to +keep track of the number of nested parentheses. For example: -This restriction is due to the wide-spread and remarkably convenient -custom of using run-time determined strings as patterns. For example: + $_ = "The brown fox jumps over the lazy dog"; + /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i; + print "color = $color, animal = $animal\n"; - $re = <>; - chomp $re; - $string =~ /$re/; - -Before Perl knew how to execute interpolated code within a pattern, -this operation was completely safe from a security point of view, -although it could raise an exception from an illegal pattern. If -you turn on the C, though, it is no longer secure, -so you should only do so if you are also using taint checking. -Better yet, use the carefully constrained evaluation within a Safe -compartment. See L for details about both these mechanisms. - -B: Use of lexical (C) variables in these blocks is -broken. The result is unpredictable and will make perl unstable. The -workaround is to use global (C) variables. - -B: In perl 5.12.x and earlier, the regex engine -was not re-entrant, so interpolated code could not -safely invoke the regex engine either directly with -C or C), or indirectly with functions such as -C. Invoking the regex engine in these blocks would make perl -unstable. =item C<(??{ code })> X<(??{})> @@ -1011,61 +1381,60 @@ experimental, and may be changed without notice. Code executed that has side effects may not perform identically from version to version due to the effect of future optimisations in the regex engine. -This is a "postponed" regular subexpression. The C is evaluated -at run time, at the moment this subexpression may match. The result -of evaluation is considered as a regular expression and matched as -if it were inserted instead of this construct. Note that this means -that the contents of capture groups defined inside an eval'ed pattern -are not available outside of the pattern, and vice versa, there is no -way for the inner pattern to refer to a capture group defined outside. -Thus, +This is a "postponed" regular subexpression. It behaves in I the +same way as a C<(?{ code })> code block as described above, except that +its return value, rather than being assigned to C<$^R>, is treated as a +pattern, compiled if it's a string (or used as-is if its a qr// object), +then matched as if it were inserted instead of this construct. - ('a' x 100)=~/(??{'(.)' x 100})/ +During the matching of this sub-pattern, it has its own set of +captures which are valid during the sub-match, but are discarded once +control returns to the main pattern. For example, the following matches, +with the inner pattern capturing "B" and matching "BB", while the outer +pattern captures "A"; -B match, it will B set $1. + my $inner = '(.)\1'; + "ABBA" =~ /^(.)(??{ $inner })\1/; + print $1; # prints "A"; -The C is not interpolated. As before, the rules to determine -where the C ends are currently somewhat convoluted. +Note that this means that there is no way for the inner pattern to refer +to a capture group defined outside. (The code block itself can use C<$1>, +etc., to refer to the enclosing pattern's capture groups.) Thus, although + + ('a' x 100)=~/(??{'(.)' x 100})/ + +I match, it will I set $1 on exit. The following pattern matches a parenthesized group: - $re = qr{ - \( - (?: - (?> [^()]+ ) # Non-parens without backtracking - | - (??{ $re }) # Group with matching parens - )* - \) - }x; + $re = qr{ + \( + (?: + (?> [^()]+ ) # Non-parens without backtracking + | + (??{ $re }) # Group with matching parens + )* + \) + }x; See also C<(?PARNO)> for a different, more efficient way to accomplish the same task. -For reasons of security, this construct is forbidden if the regular -expression involves run-time interpolation of variables, unless the -perilous C pragma has been used (see L), or the -variables contain results of C operator (see -LSTRINGEmsixpo">). - -In perl 5.12.x and earlier, because the regex engine was not re-entrant, -delayed code could not safely invoke the regex engine either directly with -C or C), or indirectly with functions such as C. - -Recursing deeper than 50 times without consuming any input string will -result in a fatal error. The maximum depth is compiled into perl, so -changing it requires a custom build. +Executing a postponed regular expression 50 times without consuming any +input string will result in a fatal error. The maximum depth is compiled +into perl, so changing it requires a custom build. =item C<(?PARNO)> C<(?-PARNO)> C<(?+PARNO)> C<(?R)> C<(?0)> X<(?PARNO)> X<(?1)> X<(?R)> X<(?0)> X<(?-1)> X<(?+1)> X<(?-PARNO)> X<(?+PARNO)> X X X X -Similar to C<(??{ code })> except it does not involve compiling any code, -instead it treats the contents of a capture group as an independent -pattern that must match at the current position. Capture groups -contained by the pattern will have the value as determined by the -outermost recursion. +Similar to C<(??{ code })> except that it does not involve executing any +code or potentially compiling a returned pattern string; instead it treats +the part of the current pattern contained within a specified capture group +as an independent pattern that must match at the current position. +Capture groups contained by the pattern will have the value as determined +by the outermost recursion. PARNO is a sequence of digits (not starting with 0) whose value reflects the paren-number of the capture group to recurse to. C<(?R)> recurses to @@ -1081,15 +1450,15 @@ included. The following pattern matches a function foo() which may contain balanced parentheses as the argument. - $re = qr{ ( # paren group 1 (full function) + $re = qr{ ( # paren group 1 (full function) foo - ( # paren group 2 (parens) + ( # paren group 2 (parens) \( - ( # paren group 3 (contents of parens) + ( # paren group 3 (contents of parens) (?: - (?> [^()]+ ) # Non-parens without backtracking + (?> [^()]+ ) # Non-parens without backtracking | - (?2) # Recurse to start of paren group 2 + (?2) # Recurse to start of paren group 2 )* ) \) @@ -1120,7 +1489,7 @@ easier to embed recursive patterns inside of a C construct for later use: my $parens = qr/(\((?:[^()]++|(?-1))*+\))/; - if (/foo $parens \s+ + \s+ bar $parens/x) { + if (/foo $parens \s+ \+ \s+ bar $parens/x) { # do something here... } @@ -1150,11 +1519,15 @@ X<(?()> =item C<(?(condition)yes-pattern)> -Conditional expression. C<(condition)> should be either an integer in +Conditional expression. Matches C if C yields +a true value, matches C otherwise. A missing pattern always +matches. + +C<(condition)> should be one of: 1) an integer in parentheses (which is valid if the corresponding pair of parentheses -matched), a look-ahead/look-behind/evaluate zero-width assertion, a +matched); 2) a look-ahead/look-behind/evaluate zero-width assertion; 3) a name in angle brackets or single quotes (which is valid if a group -with the given name matched), or the special symbol (R) (true when +with the given name matched); or 4) the special symbol (R) (true when evaluated inside of recursion or eval). Additionally the R may be followed by a number, (which will be true when evaluated when recursing inside of the appropriate group), or by C<&NAME>, in which case it will @@ -1172,9 +1545,14 @@ Checks if the numbered capturing group has matched something. Checks if a group with the given name has matched something. +=item (?=...) (?!...) (?<=...) (? predicate, which never executes directly -its yes-pattern, and does not allow a no-pattern. This allows to define -subpatterns which will be executed only by using the recursion mechanism. +A special form is the C<(DEFINE)> predicate, which never executes its +yes-pattern directly, and does not allow a no-pattern. This allows one to +define subpatterns which will be executed only by the recursion mechanism. This way, you can define a set of regular expression rules that can be bundled into any pattern you choose. @@ -1240,6 +1618,19 @@ after the recursion returns, so the extra layer of capturing groups is necessary. Thus C<$+{NAME_PAT}> would not be defined even though C<$+{NAME}> would be. +Finally, keep in mind that subpatterns created inside a DEFINE block +count towards the absolute and relative number of captures, so this: + + my @captures = "a" =~ /(.) # First capture + (?(DEFINE) + (? 1 ) # Second capture + )/x; + say scalar @captures; + +Will output 2, not 1. This is particularly important if you intend to +compile the definitions with the C operator, and later +interpolate them in another pattern. + =item C<< (?>pattern) >> X X X X @@ -1260,9 +1651,13 @@ group C (see L<"Backtracking">). In particular, C inside C will match fewer characters than a standalone C, since this makes the tail match. +C<< (?>pattern) >> does not disable backtracking altogether once it has +matched. It is still possible to backtrack past the construct, but not +into it. So C<< ((?>a*)|(?>b*))ar >> will still match "bar". + An effect similar to C<< (?>pattern) >> may be achieved by writing -C<(?=(pattern))\g1>. This matches the same substring as a standalone -C, and the following C<\g1> eats the matched string; it therefore +C<(?=(pattern))\g{-1}>. This matches the same substring as a standalone +C, and the following C<\g{-1}> eats the matched string; it therefore makes a zero-length assertion into an analogue of C<< (?>...) >>. (The difference between these two constructs is that the second one uses a capturing group, thus shifting ordinals of backreferences @@ -1302,7 +1697,8 @@ hung. However, a tiny change to this pattern which uses C<< (?>...) >> matches exactly when the one above does (verifying this yourself would be a productive exercise), but finishes in a fourth the time when used on a similar string with 1000000 Cs. Be aware, -however, that this pattern currently triggers a warning message under +however, that, when this construct is followed by a +quantifier, it currently triggers a warning message under the C pragma or B<-w> switch saying it C<"matches null string many times in regex">. @@ -1373,14 +1769,14 @@ C<(*MARK:NAME)> pattern executed. See the explanation for the C<(*MARK:NAME)> verb below for more details. B C<$REGERROR> and C<$REGMARK> are not magic variables like C<$1> -and most other regex related variables. They are not local to a scope, nor +and most other regex-related variables. They are not local to a scope, nor readonly, but instead are volatile package variables similar to C<$AUTOLOAD>. Use C to localize changes to them to a specific scope if necessary. If a pattern does not contain a special backtracking verb that allows an argument, then C<$REGERROR> and C<$REGMARK> are not touched at all. -=over 4 +=over 3 =item Verbs that take an argument @@ -1421,7 +1817,7 @@ If we add a C<(*PRUNE)> before the count like the following 'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/; print "Count=$count\n"; -we prevent backtracking and find the count of the longest matching +we prevent backtracking and find the count of the longest matching string at each matching starting point like so: aaab @@ -1437,7 +1833,6 @@ replaced with a C<< (?>pattern) >> with no functional difference; however, C<(*PRUNE)> can be used to handle cases that cannot be expressed using a C<< (?>pattern) >> alone. - =item C<(*SKIP)> C<(*SKIP:NAME)> X<(*SKIP)> @@ -1455,11 +1850,11 @@ encountered, then the C<(*SKIP)> operator has no effect. When used without a name the "skip point" is where the match point was when executing the (*SKIP) pattern. -Compare the following to the examples in C<(*PRUNE)>, note the string +Compare the following to the examples in C<(*PRUNE)>; note the string is twice as long: - 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/; - print "Count=$count\n"; + 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/; + print "Count=$count\n"; outputs @@ -1472,7 +1867,7 @@ executed, the next starting point will be where the cursor was when the C<(*SKIP)> was executed. =item C<(*MARK:NAME)> C<(*:NAME)> -X<(*MARK)> C<(*MARK:NAME)> C<(*:NAME)> +X<(*MARK)> X<(*MARK:NAME)> X<(*:NAME)> This zero-width pattern can be used to mark the point reached in a string when a certain part of the pattern has been successfully matched. This @@ -1500,16 +1895,18 @@ failing the match and has provided its own name to use, the C<$REGERROR> variable will be set to the name of the most recently executed C<(*MARK:NAME)>. -See C<(*SKIP)> for more details. +See L(*SKIP)> for more details. As a shortcut C<(*MARK:NAME)> can be written C<(*:NAME)>. =item C<(*THEN)> C<(*THEN:NAME)> -This is similar to the "cut group" operator C<::> from Perl 6. Like +This is similar to the "cut group" operator C<::> from Perl 6. Like C<(*PRUNE)>, this verb always matches, and when backtracked into on failure, it causes the regex engine to try the next alternation in the -innermost enclosing group (capturing or otherwise). +innermost enclosing group (capturing or otherwise) that has alternations. +The two branches of a C<(?(condition)yes-pattern|no-pattern)> do not +count as an alternation, as far as C<(*THEN)> is concerned. Its name comes from the observation that this operation combined with the alternation operator (C<|>) can be used to create what is essentially a @@ -1528,15 +1925,21 @@ is the same as but - / ( A (*THEN) B | C (*THEN) D ) / + / ( A (*THEN) B | C ) / is not the same as - / ( A (*PRUNE) B | C (*PRUNE) D ) / + / ( A (*PRUNE) B | C ) / as after matching the A but failing on the B the C<(*THEN)> verb will backtrack and try C; but the C<(*PRUNE)> verb will simply fail. +=back + +=item Verbs without an argument + +=over 4 + =item C<(*COMMIT)> X<(*COMMIT)> @@ -1546,8 +1949,8 @@ into on failure it causes the match to fail outright. No further attempts to find a valid match by advancing the start pointer will occur again. For example, - 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/; - print "Count=$count\n"; + 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/; + print "Count=$count\n"; outputs @@ -1558,12 +1961,6 @@ In other words, once the C<(*COMMIT)> has been entered, and if the pattern does not match, the regex engine will not try any further matching on the rest of the string. -=back - -=item Verbs without an argument - -=over 4 - =item C<(*FAIL)> C<(*F)> X<(*FAIL)> X<(*F)> @@ -1592,7 +1989,7 @@ For instance: 'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x; will match, and C<$1> will be C and C<$2> will be C, C<$3> will not -be set. If another branch in the inner parentheses were matched, such as in the +be set. If another branch in the inner parentheses was matched, such as in the string 'ACDE', then the C and C would have to be matched as well. =back @@ -1751,7 +2148,7 @@ let C<\D*> expand to "ABC", this would have caused the whole pattern to fail. The search engine will initially match C<\D*> with "ABC". Then it will -try to match C<(?!123> with "123", which fails. But because +try to match C<(?!123)> with "123", which fails. But because a quantifier (C<\D*>) has been used in the regular expression, the search engine can backtrack and retry the match differently in the hope of matching the complete regular expression. @@ -1799,12 +2196,12 @@ match takes a long time to finish. A powerful tool for optimizing such beasts is what is known as an "independent group", -which does not backtrack (see Lpattern) >>>). Note also that +which does not backtrack (see Lpattern) >>>). Note also that zero-length look-ahead/look-behind assertions will not backtrack to make the tail match, since they are in "logical" context: only whether they match is considered relevant. For an example where side-effects of look-ahead I have influenced the -following match, see Lpattern) >>>. +following match, see Lpattern) >>>. =head2 Version 8 Regular Expressions X X X @@ -1820,7 +2217,7 @@ character; "\\" matches a "\"). This escape mechanism is also required for the character used as the pattern delimiter. A series of characters matches that series of characters in the target -string, so the pattern C would match "blurfl" in the target +string, so the pattern C would match "blurfl" in the target string. You can specify a character class, by enclosing a list of characters @@ -1859,9 +2256,9 @@ You can specify a series of alternatives for a pattern using "|" to separate them, so that C will match any of "fee", "fie", or "foe" in the target string (as would C). The first alternative includes everything from the last pattern delimiter -("(", "[", or the beginning of the pattern) up to the first "|", and +("(", "(?:", etc. or the beginning of the pattern) up to the first "|", and the last alternative contains everything from the last "|" to the next -pattern delimiter. That's why it's common practice to include +closing pattern delimiter. That's why it's common practice to include alternatives in parentheses: to minimize confusion about where they start and end. @@ -1879,7 +2276,7 @@ so if you write C<[fee|fie|foe]> you're really only matching C<[feio|]>. Within a pattern, you may designate subpatterns for later reference by enclosing them in parentheses, and you may refer back to the Ith subpattern later in the pattern using the metacharacter -\I. Subpatterns are numbered based on the left to right order +\I or \gI. Subpatterns are numbered based on the left to right order of their opening parenthesis. A backreference matches whatever actually matched the subpattern in the string being examined, not the rules for that subpattern. Therefore, C<(0|0x)\d*\s\g1\d*> will @@ -1943,7 +2340,7 @@ However, long experience has shown that many programming tasks may be significantly simplified by using repeated subexpressions that may match zero-length substrings. Here's a simple example being: - @chars = split //, $string; # // is not magic in split + @chars = split //, $string; # // is not magic in split ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// / Thus Perl allows such constructs, by I matches a zero-length string, it stops +the C<*>. + +The higher-level loops preserve an additional state between iterations: whether the last match was zero-length. To break the loop, the following match after a zero-length match is prohibited to have a length of zero. This prohibition interacts with backtracking (see L<"Backtracking">), @@ -1995,7 +2414,7 @@ Each of the elementary pieces of regular expressions which were described before (such as C or C<\Z>) could match at most one substring at the given position of the input string. However, in a typical regular expression these elementary pieces are combined into more complicated -patterns using combining operators C, C, C etc +patterns using combining operators C, C, C etc. (in these examples C and C are regular subexpressions). Such combinations can include alternatives, leading to a problem of choice: @@ -2024,11 +2443,11 @@ Consider two possible matches, C and C, C and C are substrings which can be matched by C, C and C are substrings which can be matched by C. -If C is better match for C than C, C is a better +If C is a better match for C than C, C is a better match than C. If C and C coincide: C is a better match than C if -C is better match for C than C. +C is a better match for C than C. =item C @@ -2092,8 +2511,13 @@ than a match at a later position. =head2 Creating Custom RE Engines -Overloaded constants (see L) provide a simple way to extend -the functionality of the RE engine. +As of Perl 5.10.0, one can create custom regular expression engines. This +is not for the faint of heart, as they have to plug in at the C level. See +L for more details. + +As an alternative, overloaded constants (see L) provide a simple +way to extend the functionality of the RE engine, by substituting one +pattern for another. Suppose that we want to enable a new RE escape-sequence C<\Y|> which matches at a boundary between whitespace characters and non-whitespace @@ -2139,11 +2563,11 @@ part of this regular expression needs to be converted explicitly $re = customre::convert $re; /\Y|$re\Y|/; -=head1 PCRE/Python Support +=head2 PCRE/Python Support -As of Perl 5.10.0, Perl supports several Python/PCRE specific extensions +As of Perl 5.10.0, Perl supports several Python/PCRE-specific extensions to the regex syntax. While Perl programmers are encouraged to use the -Perl specific syntax, the following are also accepted: +Perl-specific syntax, the following are also accepted: =over 4 @@ -2163,17 +2587,11 @@ Subroutine call to a named capture group. Equivalent to C<< (?&NAME) >>. =head1 BUGS -There are numerous problems with case insensitive matching of characters -outside the ASCII range, especially with those whose folds are multiple -characters, such as ligatures like C. - -In a bracketed character class with case insensitive matching, ranges only work -for ASCII characters. For example, -C -doesn't match all the Russian upper and lower case letters. - Many regular expression constructs don't work on EBCDIC platforms. +There are a number of issues with regard to case-insensitive matching +in Unicode rules. See C under L above. + This document varies from difficult to understand to completely and utterly opaque. The wandering prose riddled with jargon is hard to fathom in several places.
modifier is special in that its presence anywhere in a pattern has a global effect. @@ -689,9 +1027,9 @@ anywhere in a pattern has a global effect. =item C<(?:pattern)> X<(?:)> -=item C<(?dluimsx-imsx:pattern)> +=item C<(?adluimsx-imsx:pattern)> -=item C<(?^luimsx:pattern)> +=item C<(?^aluimsx:pattern)> X<(?^:)> This is for clustering, not capturing; it groups subexpressions like @@ -707,7 +1045,7 @@ but doesn't spit out extra fields. It's also cheaper not to capture characters if you don't need to. Any letters between C> and C<:> act as flags modifiers as with -C<(?dluimsx-imsx)>. For example, +C<(?adluimsx-imsx)>. For example, /(?s-i:more.*than).*million/i @@ -726,7 +1064,7 @@ is equivalent to (?x-ims:foo) The caret tells Perl that this cluster doesn't inherit the flags of any -surrounding pattern, but to go back to the system defaults (C), +surrounding pattern, but uses the system defaults (C), modified by any flags specified. The caret allows for simpler stringification of compiled regular @@ -761,7 +1099,7 @@ following this construct will be numbered as though the construct contained only one branch, that being the one with the most capture groups in it. -This construct will be useful when you want to capture one of a +This construct is useful when you want to capture one of a number of alternative matches. Consider the following pattern. The numbers underneath show in @@ -794,7 +1132,7 @@ named C<< b >> are aliases for the group belonging to C<< $1 >>. =item Look-Around Assertions X X X X -Look-around assertions are zero width patterns which match a specific +Look-around assertions are zero-width patterns which match a specific pattern without including it in C<$&>. Positive assertions match when their subpattern matches, negative assertions match when their subpattern fails. Look-behind matches text up to the current match position, @@ -819,14 +1157,7 @@ use this for look-behind. If you are looking for a "bar" that isn't preceded by a "foo", C(?!foo)bar/> will not do what you want. That's because the C<(?!foo)> is just saying that the next thing cannot be "foo"--and it's not, it's a "bar", so "foobar" will -match. You would have to do something like C(?!foo)...bar/> for that. We -say "like" because there's the case of your "bar" not having three characters -before it. You could cover that this way: C(?:(?!foo)...|^.{0,2})bar/>. -Sometimes it's still easier just to say: - - if (/bar/ && $` !~ /foo$/) - -For look-behind see below. +match. Use look-behind instead (see below). =item C<(?<=pattern)> C<\K> X<(?<=)> X X X<\K> @@ -837,7 +1168,7 @@ Works only for fixed-width look-behind. There is a special form of this construct, called C<\K>, which causes the regex engine to "keep" everything it had matched prior to the C<\K> and -not include it in C<$&>. This effectively provides variable length +not include it in C<$&>. This effectively provides variable-length look-behind. The use of C<\K> inside of another look-around assertion is allowed, but the behaviour is currently not well defined. @@ -867,8 +1198,10 @@ only for fixed-width look-behind. X<< (?) >> X<(?'NAME')> X X A named capture group. Identical in every respect to normal capturing -parentheses C<()> but for the additional fact that C<%+> or C<%-> may be -used after a successful match to refer to a named group. See C +parentheses C<()> but for the additional fact that the group +can be referred to by name in various regular expression +constructs (like C<\g{NAME}>) and can be accessed by name +after a successful match via C<%+> or C<%->. See L for more details on the C<%+> and C<%-> hashes. If multiple distinct capture groups have the same name then the @@ -920,87 +1253,124 @@ X<(?{})> X X X B: This extended regular expression feature is considered experimental, and may be changed without notice. Code executed that has side effects may not perform identically from version to version -due to the effect of future optimisations in the regex engine. +due to the effect of future optimisations in the regex engine. The +implementation of this feature was radically overhauled for the 5.18.0 +release, and its behaviour in earlier versions of perl was much buggier, +especially in relation to parsing, lexical vars, scoping, recursion and +reentrancy. -This zero-width assertion evaluates any embedded Perl code. It -always succeeds, and its C is not interpolated. Currently, -the rules to determine where the C ends are somewhat convoluted. +This zero-width assertion executes any embedded Perl code. It always +succeeds, and its return value is set as C<$^R>. -This feature can be used together with the special variable C<$^N> to -capture the results of submatches in variables without having to keep -track of the number of nested parentheses. For example: +In literal patterns, the code is parsed at the same time as the +surrounding code. While within the pattern, control is passed temporarily +back to the perl parser, until the logically-balancing closing brace is +encountered. This is similar to the way that an array index expression in +a literal string is handled, for example - $_ = "The brown fox jumps over the lazy dog"; - /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i; - print "color = $color, animal = $animal\n"; + "abc$array[ 1 + f('[') + g()]def" + +In particular, braces do not need to be balanced: + + /abc(?{ f('{'); })/def/ + +Even in a pattern that is interpolated and compiled at run-time, literal +code blocks will be compiled once, at perl compile time; the following +prints "ABCD": -Inside the C<(?{...})> block, C<$_> refers to the string the regular + print "D"; + my $qr = qr/(?{ BEGIN { print "A" } })/; + my $foo = "foo"; + /$foo$qr(?{ BEGIN { print "B" } })/; + BEGIN { print "C" } + +In patterns where the text of the code is derived from run-time +information rather than appearing literally in a source code /pattern/, +the code is compiled at the same time that the pattern is compiled, and +fro reasons of security, C must be in scope. This is to +stop user-supplied patterns containing code snippets from being +executable. + +In situations where you need enable this with C, you should +also have taint checking enabled. Better yet, use the carefully +constrained evaluation within a Safe compartment. See L for +details about both these mechanisms. + +From the viewpoint of parsing, lexical variable scope and closures, + + /AAA(?{ BBB })CCC/ + +behaves approximately like + + /AAA/ && do { BBB } && /CCC/ + +Similarly, + + qr/AAA(?{ BBB })CCC/ + +behaves approximately like + + sub { /AAA/ && do { BBB } && /CCC/ } + +In particular: + + { my $i = 1; $r = qr/(?{ print $i })/ } + my $i = 2; + /$r/; # prints "1" + +Inside a C<(?{...})> block, C<$_> refers to the string the regular expression is matching against. You can also use C to know what is the current position of matching within this string. -The C is properly scoped in the following sense: If the assertion -is backtracked (compare L<"Backtracking">), all changes introduced after -Cization are undone, so that +The code block introduces a new scope from the perspective of lexical +variable declarations, but B from the perspective of C and +similar localizing behaviours. So later code blocks within the same +pattern will still see the values which were localized in earlier blocks. +These accumulated localizations are undone either at the end of a +successful match, or if the assertion is backtracked (compare +L<"Backtracking">). For example, $_ = 'a' x 8; m< - (?{ $cnt = 0 }) # Initialize $cnt. + (?{ $cnt = 0 }) # Initialize $cnt. ( a (?{ - local $cnt = $cnt + 1; # Update $cnt, backtracking-safe. + local $cnt = $cnt + 1; # Update $cnt, + # backtracking-safe. }) )* aaaa - (?{ $res = $cnt }) # On success copy to - # non-localized location. + (?{ $res = $cnt }) # On success copy to + # non-localized location. >x; -will set C<$res = 4>. Note that after the match, C<$cnt> returns to the globally -introduced value, because the scopes that restrict C operators -are unwound. +will initially increment C<$cnt> up to 8; then during backtracking, its +value will be unwound back to 4, which is the value assigned to C<$res>. +At the end of the regex execution, $cnt will be wound back to its initial +value of 0. + +This assertion may be used as the condition in a + + (?(condition)yes-pattern|no-pattern) -This assertion may be used as a C<(?(condition)yes-pattern|no-pattern)> -switch. If I used in this way, the result of evaluation of -C is put into the special variable C<$^R>. This happens -immediately, so C<$^R> can be used from other C<(?{ code })> assertions -inside the same regular expression. +switch. If I used in this way, the result of evaluation of C +is put into the special variable C<$^R>. This happens immediately, so +C<$^R> can be used from other C<(?{ code })> assertions inside the same +regular expression. The assignment to C<$^R> above is properly localized, so the old value of C<$^R> is restored if the assertion is backtracked; compare L<"Backtracking">. -For reasons of security, this construct is forbidden if the regular -expression involves run-time interpolation of variables, unless the -perilous C pragma has been used (see L), or the -variables contain results of C operator (see -Lmsixpo">). +Note that the special variable C<$^N> is particularly useful with code +blocks to capture the results of submatches in variables without having to +keep track of the number of nested parentheses. For example: -This restriction is due to the wide-spread and remarkably convenient -custom of using run-time determined strings as patterns. For example: + $_ = "The brown fox jumps over the lazy dog"; + /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i; + print "color = $color, animal = $animal\n"; - $re = <>; - chomp $re; - $string =~ /$re/; - -Before Perl knew how to execute interpolated code within a pattern, -this operation was completely safe from a security point of view, -although it could raise an exception from an illegal pattern. If -you turn on the C, though, it is no longer secure, -so you should only do so if you are also using taint checking. -Better yet, use the carefully constrained evaluation within a Safe -compartment. See L for details about both these mechanisms. - -B: Use of lexical (C) variables in these blocks is -broken. The result is unpredictable and will make perl unstable. The -workaround is to use global (C) variables. - -B: In perl 5.12.x and earlier, the regex engine -was not re-entrant, so interpolated code could not -safely invoke the regex engine either directly with -C or C), or indirectly with functions such as -C. Invoking the regex engine in these blocks would make perl -unstable. =item C<(??{ code })> X<(??{})> @@ -1011,61 +1381,60 @@ experimental, and may be changed without notice. Code executed that has side effects may not perform identically from version to version due to the effect of future optimisations in the regex engine. -This is a "postponed" regular subexpression. The C is evaluated -at run time, at the moment this subexpression may match. The result -of evaluation is considered as a regular expression and matched as -if it were inserted instead of this construct. Note that this means -that the contents of capture groups defined inside an eval'ed pattern -are not available outside of the pattern, and vice versa, there is no -way for the inner pattern to refer to a capture group defined outside. -Thus, +This is a "postponed" regular subexpression. It behaves in I the +same way as a C<(?{ code })> code block as described above, except that +its return value, rather than being assigned to C<$^R>, is treated as a +pattern, compiled if it's a string (or used as-is if its a qr// object), +then matched as if it were inserted instead of this construct. - ('a' x 100)=~/(??{'(.)' x 100})/ +During the matching of this sub-pattern, it has its own set of +captures which are valid during the sub-match, but are discarded once +control returns to the main pattern. For example, the following matches, +with the inner pattern capturing "B" and matching "BB", while the outer +pattern captures "A"; -B match, it will B set $1. + my $inner = '(.)\1'; + "ABBA" =~ /^(.)(??{ $inner })\1/; + print $1; # prints "A"; -The C is not interpolated. As before, the rules to determine -where the C ends are currently somewhat convoluted. +Note that this means that there is no way for the inner pattern to refer +to a capture group defined outside. (The code block itself can use C<$1>, +etc., to refer to the enclosing pattern's capture groups.) Thus, although + + ('a' x 100)=~/(??{'(.)' x 100})/ + +I match, it will I set $1 on exit. The following pattern matches a parenthesized group: - $re = qr{ - \( - (?: - (?> [^()]+ ) # Non-parens without backtracking - | - (??{ $re }) # Group with matching parens - )* - \) - }x; + $re = qr{ + \( + (?: + (?> [^()]+ ) # Non-parens without backtracking + | + (??{ $re }) # Group with matching parens + )* + \) + }x; See also C<(?PARNO)> for a different, more efficient way to accomplish the same task. -For reasons of security, this construct is forbidden if the regular -expression involves run-time interpolation of variables, unless the -perilous C pragma has been used (see L), or the -variables contain results of C operator (see -LSTRINGEmsixpo">). - -In perl 5.12.x and earlier, because the regex engine was not re-entrant, -delayed code could not safely invoke the regex engine either directly with -C or C), or indirectly with functions such as C. - -Recursing deeper than 50 times without consuming any input string will -result in a fatal error. The maximum depth is compiled into perl, so -changing it requires a custom build. +Executing a postponed regular expression 50 times without consuming any +input string will result in a fatal error. The maximum depth is compiled +into perl, so changing it requires a custom build. =item C<(?PARNO)> C<(?-PARNO)> C<(?+PARNO)> C<(?R)> C<(?0)> X<(?PARNO)> X<(?1)> X<(?R)> X<(?0)> X<(?-1)> X<(?+1)> X<(?-PARNO)> X<(?+PARNO)> X X X X -Similar to C<(??{ code })> except it does not involve compiling any code, -instead it treats the contents of a capture group as an independent -pattern that must match at the current position. Capture groups -contained by the pattern will have the value as determined by the -outermost recursion. +Similar to C<(??{ code })> except that it does not involve executing any +code or potentially compiling a returned pattern string; instead it treats +the part of the current pattern contained within a specified capture group +as an independent pattern that must match at the current position. +Capture groups contained by the pattern will have the value as determined +by the outermost recursion. PARNO is a sequence of digits (not starting with 0) whose value reflects the paren-number of the capture group to recurse to. C<(?R)> recurses to @@ -1081,15 +1450,15 @@ included. The following pattern matches a function foo() which may contain balanced parentheses as the argument. - $re = qr{ ( # paren group 1 (full function) + $re = qr{ ( # paren group 1 (full function) foo - ( # paren group 2 (parens) + ( # paren group 2 (parens) \( - ( # paren group 3 (contents of parens) + ( # paren group 3 (contents of parens) (?: - (?> [^()]+ ) # Non-parens without backtracking + (?> [^()]+ ) # Non-parens without backtracking | - (?2) # Recurse to start of paren group 2 + (?2) # Recurse to start of paren group 2 )* ) \) @@ -1120,7 +1489,7 @@ easier to embed recursive patterns inside of a C construct for later use: my $parens = qr/(\((?:[^()]++|(?-1))*+\))/; - if (/foo $parens \s+ + \s+ bar $parens/x) { + if (/foo $parens \s+ \+ \s+ bar $parens/x) { # do something here... } @@ -1150,11 +1519,15 @@ X<(?()> =item C<(?(condition)yes-pattern)> -Conditional expression. C<(condition)> should be either an integer in +Conditional expression. Matches C if C yields +a true value, matches C otherwise. A missing pattern always +matches. + +C<(condition)> should be one of: 1) an integer in parentheses (which is valid if the corresponding pair of parentheses -matched), a look-ahead/look-behind/evaluate zero-width assertion, a +matched); 2) a look-ahead/look-behind/evaluate zero-width assertion; 3) a name in angle brackets or single quotes (which is valid if a group -with the given name matched), or the special symbol (R) (true when +with the given name matched); or 4) the special symbol (R) (true when evaluated inside of recursion or eval). Additionally the R may be followed by a number, (which will be true when evaluated when recursing inside of the appropriate group), or by C<&NAME>, in which case it will @@ -1172,9 +1545,14 @@ Checks if the numbered capturing group has matched something. Checks if a group with the given name has matched something. +=item (?=...) (?!...) (?<=...) (? predicate, which never executes directly -its yes-pattern, and does not allow a no-pattern. This allows to define -subpatterns which will be executed only by using the recursion mechanism. +A special form is the C<(DEFINE)> predicate, which never executes its +yes-pattern directly, and does not allow a no-pattern. This allows one to +define subpatterns which will be executed only by the recursion mechanism. This way, you can define a set of regular expression rules that can be bundled into any pattern you choose. @@ -1240,6 +1618,19 @@ after the recursion returns, so the extra layer of capturing groups is necessary. Thus C<$+{NAME_PAT}> would not be defined even though C<$+{NAME}> would be. +Finally, keep in mind that subpatterns created inside a DEFINE block +count towards the absolute and relative number of captures, so this: + + my @captures = "a" =~ /(.) # First capture + (?(DEFINE) + (? 1 ) # Second capture + )/x; + say scalar @captures; + +Will output 2, not 1. This is particularly important if you intend to +compile the definitions with the C operator, and later +interpolate them in another pattern. + =item C<< (?>pattern) >> X X X X @@ -1260,9 +1651,13 @@ group C (see L<"Backtracking">). In particular, C inside C will match fewer characters than a standalone C, since this makes the tail match. +C<< (?>pattern) >> does not disable backtracking altogether once it has +matched. It is still possible to backtrack past the construct, but not +into it. So C<< ((?>a*)|(?>b*))ar >> will still match "bar". + An effect similar to C<< (?>pattern) >> may be achieved by writing -C<(?=(pattern))\g1>. This matches the same substring as a standalone -C, and the following C<\g1> eats the matched string; it therefore +C<(?=(pattern))\g{-1}>. This matches the same substring as a standalone +C, and the following C<\g{-1}> eats the matched string; it therefore makes a zero-length assertion into an analogue of C<< (?>...) >>. (The difference between these two constructs is that the second one uses a capturing group, thus shifting ordinals of backreferences @@ -1302,7 +1697,8 @@ hung. However, a tiny change to this pattern which uses C<< (?>...) >> matches exactly when the one above does (verifying this yourself would be a productive exercise), but finishes in a fourth the time when used on a similar string with 1000000 Cs. Be aware, -however, that this pattern currently triggers a warning message under +however, that, when this construct is followed by a +quantifier, it currently triggers a warning message under the C pragma or B<-w> switch saying it C<"matches null string many times in regex">. @@ -1373,14 +1769,14 @@ C<(*MARK:NAME)> pattern executed. See the explanation for the C<(*MARK:NAME)> verb below for more details. B C<$REGERROR> and C<$REGMARK> are not magic variables like C<$1> -and most other regex related variables. They are not local to a scope, nor +and most other regex-related variables. They are not local to a scope, nor readonly, but instead are volatile package variables similar to C<$AUTOLOAD>. Use C to localize changes to them to a specific scope if necessary. If a pattern does not contain a special backtracking verb that allows an argument, then C<$REGERROR> and C<$REGMARK> are not touched at all. -=over 4 +=over 3 =item Verbs that take an argument @@ -1421,7 +1817,7 @@ If we add a C<(*PRUNE)> before the count like the following 'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/; print "Count=$count\n"; -we prevent backtracking and find the count of the longest matching +we prevent backtracking and find the count of the longest matching string at each matching starting point like so: aaab @@ -1437,7 +1833,6 @@ replaced with a C<< (?>pattern) >> with no functional difference; however, C<(*PRUNE)> can be used to handle cases that cannot be expressed using a C<< (?>pattern) >> alone. - =item C<(*SKIP)> C<(*SKIP:NAME)> X<(*SKIP)> @@ -1455,11 +1850,11 @@ encountered, then the C<(*SKIP)> operator has no effect. When used without a name the "skip point" is where the match point was when executing the (*SKIP) pattern. -Compare the following to the examples in C<(*PRUNE)>, note the string +Compare the following to the examples in C<(*PRUNE)>; note the string is twice as long: - 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/; - print "Count=$count\n"; + 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/; + print "Count=$count\n"; outputs @@ -1472,7 +1867,7 @@ executed, the next starting point will be where the cursor was when the C<(*SKIP)> was executed. =item C<(*MARK:NAME)> C<(*:NAME)> -X<(*MARK)> C<(*MARK:NAME)> C<(*:NAME)> +X<(*MARK)> X<(*MARK:NAME)> X<(*:NAME)> This zero-width pattern can be used to mark the point reached in a string when a certain part of the pattern has been successfully matched. This @@ -1500,16 +1895,18 @@ failing the match and has provided its own name to use, the C<$REGERROR> variable will be set to the name of the most recently executed C<(*MARK:NAME)>. -See C<(*SKIP)> for more details. +See L(*SKIP)> for more details. As a shortcut C<(*MARK:NAME)> can be written C<(*:NAME)>. =item C<(*THEN)> C<(*THEN:NAME)> -This is similar to the "cut group" operator C<::> from Perl 6. Like +This is similar to the "cut group" operator C<::> from Perl 6. Like C<(*PRUNE)>, this verb always matches, and when backtracked into on failure, it causes the regex engine to try the next alternation in the -innermost enclosing group (capturing or otherwise). +innermost enclosing group (capturing or otherwise) that has alternations. +The two branches of a C<(?(condition)yes-pattern|no-pattern)> do not +count as an alternation, as far as C<(*THEN)> is concerned. Its name comes from the observation that this operation combined with the alternation operator (C<|>) can be used to create what is essentially a @@ -1528,15 +1925,21 @@ is the same as but - / ( A (*THEN) B | C (*THEN) D ) / + / ( A (*THEN) B | C ) / is not the same as - / ( A (*PRUNE) B | C (*PRUNE) D ) / + / ( A (*PRUNE) B | C ) / as after matching the A but failing on the B the C<(*THEN)> verb will backtrack and try C; but the C<(*PRUNE)> verb will simply fail. +=back + +=item Verbs without an argument + +=over 4 + =item C<(*COMMIT)> X<(*COMMIT)> @@ -1546,8 +1949,8 @@ into on failure it causes the match to fail outright. No further attempts to find a valid match by advancing the start pointer will occur again. For example, - 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/; - print "Count=$count\n"; + 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/; + print "Count=$count\n"; outputs @@ -1558,12 +1961,6 @@ In other words, once the C<(*COMMIT)> has been entered, and if the pattern does not match, the regex engine will not try any further matching on the rest of the string. -=back - -=item Verbs without an argument - -=over 4 - =item C<(*FAIL)> C<(*F)> X<(*FAIL)> X<(*F)> @@ -1592,7 +1989,7 @@ For instance: 'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x; will match, and C<$1> will be C and C<$2> will be C, C<$3> will not -be set. If another branch in the inner parentheses were matched, such as in the +be set. If another branch in the inner parentheses was matched, such as in the string 'ACDE', then the C and C would have to be matched as well. =back @@ -1751,7 +2148,7 @@ let C<\D*> expand to "ABC", this would have caused the whole pattern to fail. The search engine will initially match C<\D*> with "ABC". Then it will -try to match C<(?!123> with "123", which fails. But because +try to match C<(?!123)> with "123", which fails. But because a quantifier (C<\D*>) has been used in the regular expression, the search engine can backtrack and retry the match differently in the hope of matching the complete regular expression. @@ -1799,12 +2196,12 @@ match takes a long time to finish. A powerful tool for optimizing such beasts is what is known as an "independent group", -which does not backtrack (see Lpattern) >>>). Note also that +which does not backtrack (see Lpattern) >>>). Note also that zero-length look-ahead/look-behind assertions will not backtrack to make the tail match, since they are in "logical" context: only whether they match is considered relevant. For an example where side-effects of look-ahead I have influenced the -following match, see Lpattern) >>>. +following match, see Lpattern) >>>. =head2 Version 8 Regular Expressions X X X @@ -1820,7 +2217,7 @@ character; "\\" matches a "\"). This escape mechanism is also required for the character used as the pattern delimiter. A series of characters matches that series of characters in the target -string, so the pattern C would match "blurfl" in the target +string, so the pattern C would match "blurfl" in the target string. You can specify a character class, by enclosing a list of characters @@ -1859,9 +2256,9 @@ You can specify a series of alternatives for a pattern using "|" to separate them, so that C will match any of "fee", "fie", or "foe" in the target string (as would C). The first alternative includes everything from the last pattern delimiter -("(", "[", or the beginning of the pattern) up to the first "|", and +("(", "(?:", etc. or the beginning of the pattern) up to the first "|", and the last alternative contains everything from the last "|" to the next -pattern delimiter. That's why it's common practice to include +closing pattern delimiter. That's why it's common practice to include alternatives in parentheses: to minimize confusion about where they start and end. @@ -1879,7 +2276,7 @@ so if you write C<[fee|fie|foe]> you're really only matching C<[feio|]>. Within a pattern, you may designate subpatterns for later reference by enclosing them in parentheses, and you may refer back to the Ith subpattern later in the pattern using the metacharacter -\I. Subpatterns are numbered based on the left to right order +\I or \gI. Subpatterns are numbered based on the left to right order of their opening parenthesis. A backreference matches whatever actually matched the subpattern in the string being examined, not the rules for that subpattern. Therefore, C<(0|0x)\d*\s\g1\d*> will @@ -1943,7 +2340,7 @@ However, long experience has shown that many programming tasks may be significantly simplified by using repeated subexpressions that may match zero-length substrings. Here's a simple example being: - @chars = split //, $string; # // is not magic in split + @chars = split //, $string; # // is not magic in split ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// / Thus Perl allows such constructs, by I matches a zero-length string, it stops +the C<*>. + +The higher-level loops preserve an additional state between iterations: whether the last match was zero-length. To break the loop, the following match after a zero-length match is prohibited to have a length of zero. This prohibition interacts with backtracking (see L<"Backtracking">), @@ -1995,7 +2414,7 @@ Each of the elementary pieces of regular expressions which were described before (such as C or C<\Z>) could match at most one substring at the given position of the input string. However, in a typical regular expression these elementary pieces are combined into more complicated -patterns using combining operators C, C, C etc +patterns using combining operators C, C, C etc. (in these examples C and C are regular subexpressions). Such combinations can include alternatives, leading to a problem of choice: @@ -2024,11 +2443,11 @@ Consider two possible matches, C and C, C and C are substrings which can be matched by C, C and C are substrings which can be matched by C. -If C is better match for C than C, C is a better +If C is a better match for C than C, C is a better match than C. If C and C coincide: C is a better match than C if -C is better match for C than C. +C is a better match for C than C. =item C @@ -2092,8 +2511,13 @@ than a match at a later position. =head2 Creating Custom RE Engines -Overloaded constants (see L) provide a simple way to extend -the functionality of the RE engine. +As of Perl 5.10.0, one can create custom regular expression engines. This +is not for the faint of heart, as they have to plug in at the C level. See +L for more details. + +As an alternative, overloaded constants (see L) provide a simple +way to extend the functionality of the RE engine, by substituting one +pattern for another. Suppose that we want to enable a new RE escape-sequence C<\Y|> which matches at a boundary between whitespace characters and non-whitespace @@ -2139,11 +2563,11 @@ part of this regular expression needs to be converted explicitly $re = customre::convert $re; /\Y|$re\Y|/; -=head1 PCRE/Python Support +=head2 PCRE/Python Support -As of Perl 5.10.0, Perl supports several Python/PCRE specific extensions +As of Perl 5.10.0, Perl supports several Python/PCRE-specific extensions to the regex syntax. While Perl programmers are encouraged to use the -Perl specific syntax, the following are also accepted: +Perl-specific syntax, the following are also accepted: =over 4 @@ -2163,17 +2587,11 @@ Subroutine call to a named capture group. Equivalent to C<< (?&NAME) >>. =head1 BUGS -There are numerous problems with case insensitive matching of characters -outside the ASCII range, especially with those whose folds are multiple -characters, such as ligatures like C. - -In a bracketed character class with case insensitive matching, ranges only work -for ASCII characters. For example, -C -doesn't match all the Russian upper and lower case letters. - Many regular expression constructs don't work on EBCDIC platforms. +There are a number of issues with regard to case-insensitive matching +in Unicode rules. See C under L above. + This document varies from difficult to understand to completely and utterly opaque. The wandering prose riddled with jargon is hard to fathom in several places.
is not interpolated. Currently, -the rules to determine where the C ends are somewhat convoluted. +This zero-width assertion executes any embedded Perl code. It always +succeeds, and its return value is set as C<$^R>. -This feature can be used together with the special variable C<$^N> to -capture the results of submatches in variables without having to keep -track of the number of nested parentheses. For example: +In literal patterns, the code is parsed at the same time as the +surrounding code. While within the pattern, control is passed temporarily +back to the perl parser, until the logically-balancing closing brace is +encountered. This is similar to the way that an array index expression in +a literal string is handled, for example - $_ = "The brown fox jumps over the lazy dog"; - /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i; - print "color = $color, animal = $animal\n"; + "abc$array[ 1 + f('[') + g()]def" + +In particular, braces do not need to be balanced: + + /abc(?{ f('{'); })/def/ + +Even in a pattern that is interpolated and compiled at run-time, literal +code blocks will be compiled once, at perl compile time; the following +prints "ABCD": -Inside the C<(?{...})> block, C<$_> refers to the string the regular + print "D"; + my $qr = qr/(?{ BEGIN { print "A" } })/; + my $foo = "foo"; + /$foo$qr(?{ BEGIN { print "B" } })/; + BEGIN { print "C" } + +In patterns where the text of the code is derived from run-time +information rather than appearing literally in a source code /pattern/, +the code is compiled at the same time that the pattern is compiled, and +fro reasons of security, C must be in scope. This is to +stop user-supplied patterns containing code snippets from being +executable. + +In situations where you need enable this with C, you should +also have taint checking enabled. Better yet, use the carefully +constrained evaluation within a Safe compartment. See L for +details about both these mechanisms. + +From the viewpoint of parsing, lexical variable scope and closures, + + /AAA(?{ BBB })CCC/ + +behaves approximately like + + /AAA/ && do { BBB } && /CCC/ + +Similarly, + + qr/AAA(?{ BBB })CCC/ + +behaves approximately like + + sub { /AAA/ && do { BBB } && /CCC/ } + +In particular: + + { my $i = 1; $r = qr/(?{ print $i })/ } + my $i = 2; + /$r/; # prints "1" + +Inside a C<(?{...})> block, C<$_> refers to the string the regular expression is matching against. You can also use C to know what is the current position of matching within this string. -The C is properly scoped in the following sense: If the assertion -is backtracked (compare L<"Backtracking">), all changes introduced after -Cization are undone, so that +The code block introduces a new scope from the perspective of lexical +variable declarations, but B from the perspective of C and +similar localizing behaviours. So later code blocks within the same +pattern will still see the values which were localized in earlier blocks. +These accumulated localizations are undone either at the end of a +successful match, or if the assertion is backtracked (compare +L<"Backtracking">). For example, $_ = 'a' x 8; m< - (?{ $cnt = 0 }) # Initialize $cnt. + (?{ $cnt = 0 }) # Initialize $cnt. ( a (?{ - local $cnt = $cnt + 1; # Update $cnt, backtracking-safe. + local $cnt = $cnt + 1; # Update $cnt, + # backtracking-safe. }) )* aaaa - (?{ $res = $cnt }) # On success copy to - # non-localized location. + (?{ $res = $cnt }) # On success copy to + # non-localized location. >x; -will set C<$res = 4>. Note that after the match, C<$cnt> returns to the globally -introduced value, because the scopes that restrict C operators -are unwound. +will initially increment C<$cnt> up to 8; then during backtracking, its +value will be unwound back to 4, which is the value assigned to C<$res>. +At the end of the regex execution, $cnt will be wound back to its initial +value of 0. + +This assertion may be used as the condition in a + + (?(condition)yes-pattern|no-pattern) -This assertion may be used as a C<(?(condition)yes-pattern|no-pattern)> -switch. If I used in this way, the result of evaluation of -C is put into the special variable C<$^R>. This happens -immediately, so C<$^R> can be used from other C<(?{ code })> assertions -inside the same regular expression. +switch. If I used in this way, the result of evaluation of C +is put into the special variable C<$^R>. This happens immediately, so +C<$^R> can be used from other C<(?{ code })> assertions inside the same +regular expression. The assignment to C<$^R> above is properly localized, so the old value of C<$^R> is restored if the assertion is backtracked; compare L<"Backtracking">. -For reasons of security, this construct is forbidden if the regular -expression involves run-time interpolation of variables, unless the -perilous C pragma has been used (see L), or the -variables contain results of C operator (see -Lmsixpo">). +Note that the special variable C<$^N> is particularly useful with code +blocks to capture the results of submatches in variables without having to +keep track of the number of nested parentheses. For example: -This restriction is due to the wide-spread and remarkably convenient -custom of using run-time determined strings as patterns. For example: + $_ = "The brown fox jumps over the lazy dog"; + /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i; + print "color = $color, animal = $animal\n"; - $re = <>; - chomp $re; - $string =~ /$re/; - -Before Perl knew how to execute interpolated code within a pattern, -this operation was completely safe from a security point of view, -although it could raise an exception from an illegal pattern. If -you turn on the C, though, it is no longer secure, -so you should only do so if you are also using taint checking. -Better yet, use the carefully constrained evaluation within a Safe -compartment. See L for details about both these mechanisms. - -B: Use of lexical (C) variables in these blocks is -broken. The result is unpredictable and will make perl unstable. The -workaround is to use global (C) variables. - -B: In perl 5.12.x and earlier, the regex engine -was not re-entrant, so interpolated code could not -safely invoke the regex engine either directly with -C or C), or indirectly with functions such as -C. Invoking the regex engine in these blocks would make perl -unstable. =item C<(??{ code })> X<(??{})> @@ -1011,61 +1381,60 @@ experimental, and may be changed without notice. Code executed that has side effects may not perform identically from version to version due to the effect of future optimisations in the regex engine. -This is a "postponed" regular subexpression. The C is evaluated -at run time, at the moment this subexpression may match. The result -of evaluation is considered as a regular expression and matched as -if it were inserted instead of this construct. Note that this means -that the contents of capture groups defined inside an eval'ed pattern -are not available outside of the pattern, and vice versa, there is no -way for the inner pattern to refer to a capture group defined outside. -Thus, +This is a "postponed" regular subexpression. It behaves in I the +same way as a C<(?{ code })> code block as described above, except that +its return value, rather than being assigned to C<$^R>, is treated as a +pattern, compiled if it's a string (or used as-is if its a qr// object), +then matched as if it were inserted instead of this construct. - ('a' x 100)=~/(??{'(.)' x 100})/ +During the matching of this sub-pattern, it has its own set of +captures which are valid during the sub-match, but are discarded once +control returns to the main pattern. For example, the following matches, +with the inner pattern capturing "B" and matching "BB", while the outer +pattern captures "A"; -B match, it will B set $1. + my $inner = '(.)\1'; + "ABBA" =~ /^(.)(??{ $inner })\1/; + print $1; # prints "A"; -The C is not interpolated. As before, the rules to determine -where the C ends are currently somewhat convoluted. +Note that this means that there is no way for the inner pattern to refer +to a capture group defined outside. (The code block itself can use C<$1>, +etc., to refer to the enclosing pattern's capture groups.) Thus, although + + ('a' x 100)=~/(??{'(.)' x 100})/ + +I match, it will I set $1 on exit. The following pattern matches a parenthesized group: - $re = qr{ - \( - (?: - (?> [^()]+ ) # Non-parens without backtracking - | - (??{ $re }) # Group with matching parens - )* - \) - }x; + $re = qr{ + \( + (?: + (?> [^()]+ ) # Non-parens without backtracking + | + (??{ $re }) # Group with matching parens + )* + \) + }x; See also C<(?PARNO)> for a different, more efficient way to accomplish the same task. -For reasons of security, this construct is forbidden if the regular -expression involves run-time interpolation of variables, unless the -perilous C pragma has been used (see L), or the -variables contain results of C operator (see -LSTRINGEmsixpo">). - -In perl 5.12.x and earlier, because the regex engine was not re-entrant, -delayed code could not safely invoke the regex engine either directly with -C or C), or indirectly with functions such as C. - -Recursing deeper than 50 times without consuming any input string will -result in a fatal error. The maximum depth is compiled into perl, so -changing it requires a custom build. +Executing a postponed regular expression 50 times without consuming any +input string will result in a fatal error. The maximum depth is compiled +into perl, so changing it requires a custom build. =item C<(?PARNO)> C<(?-PARNO)> C<(?+PARNO)> C<(?R)> C<(?0)> X<(?PARNO)> X<(?1)> X<(?R)> X<(?0)> X<(?-1)> X<(?+1)> X<(?-PARNO)> X<(?+PARNO)> X X X X -Similar to C<(??{ code })> except it does not involve compiling any code, -instead it treats the contents of a capture group as an independent -pattern that must match at the current position. Capture groups -contained by the pattern will have the value as determined by the -outermost recursion. +Similar to C<(??{ code })> except that it does not involve executing any +code or potentially compiling a returned pattern string; instead it treats +the part of the current pattern contained within a specified capture group +as an independent pattern that must match at the current position. +Capture groups contained by the pattern will have the value as determined +by the outermost recursion. PARNO is a sequence of digits (not starting with 0) whose value reflects the paren-number of the capture group to recurse to. C<(?R)> recurses to @@ -1081,15 +1450,15 @@ included. The following pattern matches a function foo() which may contain balanced parentheses as the argument. - $re = qr{ ( # paren group 1 (full function) + $re = qr{ ( # paren group 1 (full function) foo - ( # paren group 2 (parens) + ( # paren group 2 (parens) \( - ( # paren group 3 (contents of parens) + ( # paren group 3 (contents of parens) (?: - (?> [^()]+ ) # Non-parens without backtracking + (?> [^()]+ ) # Non-parens without backtracking | - (?2) # Recurse to start of paren group 2 + (?2) # Recurse to start of paren group 2 )* ) \) @@ -1120,7 +1489,7 @@ easier to embed recursive patterns inside of a C construct for later use: my $parens = qr/(\((?:[^()]++|(?-1))*+\))/; - if (/foo $parens \s+ + \s+ bar $parens/x) { + if (/foo $parens \s+ \+ \s+ bar $parens/x) { # do something here... } @@ -1150,11 +1519,15 @@ X<(?()> =item C<(?(condition)yes-pattern)> -Conditional expression. C<(condition)> should be either an integer in +Conditional expression. Matches C if C yields +a true value, matches C otherwise. A missing pattern always +matches. + +C<(condition)> should be one of: 1) an integer in parentheses (which is valid if the corresponding pair of parentheses -matched), a look-ahead/look-behind/evaluate zero-width assertion, a +matched); 2) a look-ahead/look-behind/evaluate zero-width assertion; 3) a name in angle brackets or single quotes (which is valid if a group -with the given name matched), or the special symbol (R) (true when +with the given name matched); or 4) the special symbol (R) (true when evaluated inside of recursion or eval). Additionally the R may be followed by a number, (which will be true when evaluated when recursing inside of the appropriate group), or by C<&NAME>, in which case it will @@ -1172,9 +1545,14 @@ Checks if the numbered capturing group has matched something. Checks if a group with the given name has matched something. +=item (?=...) (?!...) (?<=...) (? predicate, which never executes directly -its yes-pattern, and does not allow a no-pattern. This allows to define -subpatterns which will be executed only by using the recursion mechanism. +A special form is the C<(DEFINE)> predicate, which never executes its +yes-pattern directly, and does not allow a no-pattern. This allows one to +define subpatterns which will be executed only by the recursion mechanism. This way, you can define a set of regular expression rules that can be bundled into any pattern you choose. @@ -1240,6 +1618,19 @@ after the recursion returns, so the extra layer of capturing groups is necessary. Thus C<$+{NAME_PAT}> would not be defined even though C<$+{NAME}> would be. +Finally, keep in mind that subpatterns created inside a DEFINE block +count towards the absolute and relative number of captures, so this: + + my @captures = "a" =~ /(.) # First capture + (?(DEFINE) + (? 1 ) # Second capture + )/x; + say scalar @captures; + +Will output 2, not 1. This is particularly important if you intend to +compile the definitions with the C operator, and later +interpolate them in another pattern. + =item C<< (?>pattern) >> X X X X @@ -1260,9 +1651,13 @@ group C (see L<"Backtracking">). In particular, C inside C will match fewer characters than a standalone C, since this makes the tail match. +C<< (?>pattern) >> does not disable backtracking altogether once it has +matched. It is still possible to backtrack past the construct, but not +into it. So C<< ((?>a*)|(?>b*))ar >> will still match "bar". + An effect similar to C<< (?>pattern) >> may be achieved by writing -C<(?=(pattern))\g1>. This matches the same substring as a standalone -C, and the following C<\g1> eats the matched string; it therefore +C<(?=(pattern))\g{-1}>. This matches the same substring as a standalone +C, and the following C<\g{-1}> eats the matched string; it therefore makes a zero-length assertion into an analogue of C<< (?>...) >>. (The difference between these two constructs is that the second one uses a capturing group, thus shifting ordinals of backreferences @@ -1302,7 +1697,8 @@ hung. However, a tiny change to this pattern which uses C<< (?>...) >> matches exactly when the one above does (verifying this yourself would be a productive exercise), but finishes in a fourth the time when used on a similar string with 1000000 Cs. Be aware, -however, that this pattern currently triggers a warning message under +however, that, when this construct is followed by a +quantifier, it currently triggers a warning message under the C pragma or B<-w> switch saying it C<"matches null string many times in regex">. @@ -1373,14 +1769,14 @@ C<(*MARK:NAME)> pattern executed. See the explanation for the C<(*MARK:NAME)> verb below for more details. B C<$REGERROR> and C<$REGMARK> are not magic variables like C<$1> -and most other regex related variables. They are not local to a scope, nor +and most other regex-related variables. They are not local to a scope, nor readonly, but instead are volatile package variables similar to C<$AUTOLOAD>. Use C to localize changes to them to a specific scope if necessary. If a pattern does not contain a special backtracking verb that allows an argument, then C<$REGERROR> and C<$REGMARK> are not touched at all. -=over 4 +=over 3 =item Verbs that take an argument @@ -1421,7 +1817,7 @@ If we add a C<(*PRUNE)> before the count like the following 'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/; print "Count=$count\n"; -we prevent backtracking and find the count of the longest matching +we prevent backtracking and find the count of the longest matching string at each matching starting point like so: aaab @@ -1437,7 +1833,6 @@ replaced with a C<< (?>pattern) >> with no functional difference; however, C<(*PRUNE)> can be used to handle cases that cannot be expressed using a C<< (?>pattern) >> alone. - =item C<(*SKIP)> C<(*SKIP:NAME)> X<(*SKIP)> @@ -1455,11 +1850,11 @@ encountered, then the C<(*SKIP)> operator has no effect. When used without a name the "skip point" is where the match point was when executing the (*SKIP) pattern. -Compare the following to the examples in C<(*PRUNE)>, note the string +Compare the following to the examples in C<(*PRUNE)>; note the string is twice as long: - 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/; - print "Count=$count\n"; + 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/; + print "Count=$count\n"; outputs @@ -1472,7 +1867,7 @@ executed, the next starting point will be where the cursor was when the C<(*SKIP)> was executed. =item C<(*MARK:NAME)> C<(*:NAME)> -X<(*MARK)> C<(*MARK:NAME)> C<(*:NAME)> +X<(*MARK)> X<(*MARK:NAME)> X<(*:NAME)> This zero-width pattern can be used to mark the point reached in a string when a certain part of the pattern has been successfully matched. This @@ -1500,16 +1895,18 @@ failing the match and has provided its own name to use, the C<$REGERROR> variable will be set to the name of the most recently executed C<(*MARK:NAME)>. -See C<(*SKIP)> for more details. +See L(*SKIP)> for more details. As a shortcut C<(*MARK:NAME)> can be written C<(*:NAME)>. =item C<(*THEN)> C<(*THEN:NAME)> -This is similar to the "cut group" operator C<::> from Perl 6. Like +This is similar to the "cut group" operator C<::> from Perl 6. Like C<(*PRUNE)>, this verb always matches, and when backtracked into on failure, it causes the regex engine to try the next alternation in the -innermost enclosing group (capturing or otherwise). +innermost enclosing group (capturing or otherwise) that has alternations. +The two branches of a C<(?(condition)yes-pattern|no-pattern)> do not +count as an alternation, as far as C<(*THEN)> is concerned. Its name comes from the observation that this operation combined with the alternation operator (C<|>) can be used to create what is essentially a @@ -1528,15 +1925,21 @@ is the same as but - / ( A (*THEN) B | C (*THEN) D ) / + / ( A (*THEN) B | C ) / is not the same as - / ( A (*PRUNE) B | C (*PRUNE) D ) / + / ( A (*PRUNE) B | C ) / as after matching the A but failing on the B the C<(*THEN)> verb will backtrack and try C; but the C<(*PRUNE)> verb will simply fail. +=back + +=item Verbs without an argument + +=over 4 + =item C<(*COMMIT)> X<(*COMMIT)> @@ -1546,8 +1949,8 @@ into on failure it causes the match to fail outright. No further attempts to find a valid match by advancing the start pointer will occur again. For example, - 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/; - print "Count=$count\n"; + 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/; + print "Count=$count\n"; outputs @@ -1558,12 +1961,6 @@ In other words, once the C<(*COMMIT)> has been entered, and if the pattern does not match, the regex engine will not try any further matching on the rest of the string. -=back - -=item Verbs without an argument - -=over 4 - =item C<(*FAIL)> C<(*F)> X<(*FAIL)> X<(*F)> @@ -1592,7 +1989,7 @@ For instance: 'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x; will match, and C<$1> will be C and C<$2> will be C, C<$3> will not -be set. If another branch in the inner parentheses were matched, such as in the +be set. If another branch in the inner parentheses was matched, such as in the string 'ACDE', then the C and C would have to be matched as well. =back @@ -1751,7 +2148,7 @@ let C<\D*> expand to "ABC", this would have caused the whole pattern to fail. The search engine will initially match C<\D*> with "ABC". Then it will -try to match C<(?!123> with "123", which fails. But because +try to match C<(?!123)> with "123", which fails. But because a quantifier (C<\D*>) has been used in the regular expression, the search engine can backtrack and retry the match differently in the hope of matching the complete regular expression. @@ -1799,12 +2196,12 @@ match takes a long time to finish. A powerful tool for optimizing such beasts is what is known as an "independent group", -which does not backtrack (see Lpattern) >>>). Note also that +which does not backtrack (see Lpattern) >>>). Note also that zero-length look-ahead/look-behind assertions will not backtrack to make the tail match, since they are in "logical" context: only whether they match is considered relevant. For an example where side-effects of look-ahead I have influenced the -following match, see Lpattern) >>>. +following match, see Lpattern) >>>. =head2 Version 8 Regular Expressions X X X @@ -1820,7 +2217,7 @@ character; "\\" matches a "\"). This escape mechanism is also required for the character used as the pattern delimiter. A series of characters matches that series of characters in the target -string, so the pattern C would match "blurfl" in the target +string, so the pattern C would match "blurfl" in the target string. You can specify a character class, by enclosing a list of characters @@ -1859,9 +2256,9 @@ You can specify a series of alternatives for a pattern using "|" to separate them, so that C will match any of "fee", "fie", or "foe" in the target string (as would C). The first alternative includes everything from the last pattern delimiter -("(", "[", or the beginning of the pattern) up to the first "|", and +("(", "(?:", etc. or the beginning of the pattern) up to the first "|", and the last alternative contains everything from the last "|" to the next -pattern delimiter. That's why it's common practice to include +closing pattern delimiter. That's why it's common practice to include alternatives in parentheses: to minimize confusion about where they start and end. @@ -1879,7 +2276,7 @@ so if you write C<[fee|fie|foe]> you're really only matching C<[feio|]>. Within a pattern, you may designate subpatterns for later reference by enclosing them in parentheses, and you may refer back to the Ith subpattern later in the pattern using the metacharacter -\I. Subpatterns are numbered based on the left to right order +\I or \gI. Subpatterns are numbered based on the left to right order of their opening parenthesis. A backreference matches whatever actually matched the subpattern in the string being examined, not the rules for that subpattern. Therefore, C<(0|0x)\d*\s\g1\d*> will @@ -1943,7 +2340,7 @@ However, long experience has shown that many programming tasks may be significantly simplified by using repeated subexpressions that may match zero-length substrings. Here's a simple example being: - @chars = split //, $string; # // is not magic in split + @chars = split //, $string; # // is not magic in split ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// / Thus Perl allows such constructs, by I matches a zero-length string, it stops +the C<*>. + +The higher-level loops preserve an additional state between iterations: whether the last match was zero-length. To break the loop, the following match after a zero-length match is prohibited to have a length of zero. This prohibition interacts with backtracking (see L<"Backtracking">), @@ -1995,7 +2414,7 @@ Each of the elementary pieces of regular expressions which were described before (such as C or C<\Z>) could match at most one substring at the given position of the input string. However, in a typical regular expression these elementary pieces are combined into more complicated -patterns using combining operators C, C, C etc +patterns using combining operators C, C, C etc. (in these examples C and C are regular subexpressions). Such combinations can include alternatives, leading to a problem of choice: @@ -2024,11 +2443,11 @@ Consider two possible matches, C and C, C and C are substrings which can be matched by C, C and C are substrings which can be matched by C. -If C is better match for C than C, C is a better +If C is a better match for C than C, C is a better match than C. If C and C coincide: C is a better match than C if -C is better match for C than C. +C is a better match for C than C. =item C @@ -2092,8 +2511,13 @@ than a match at a later position. =head2 Creating Custom RE Engines -Overloaded constants (see L) provide a simple way to extend -the functionality of the RE engine. +As of Perl 5.10.0, one can create custom regular expression engines. This +is not for the faint of heart, as they have to plug in at the C level. See +L for more details. + +As an alternative, overloaded constants (see L) provide a simple +way to extend the functionality of the RE engine, by substituting one +pattern for another. Suppose that we want to enable a new RE escape-sequence C<\Y|> which matches at a boundary between whitespace characters and non-whitespace @@ -2139,11 +2563,11 @@ part of this regular expression needs to be converted explicitly $re = customre::convert $re; /\Y|$re\Y|/; -=head1 PCRE/Python Support +=head2 PCRE/Python Support -As of Perl 5.10.0, Perl supports several Python/PCRE specific extensions +As of Perl 5.10.0, Perl supports several Python/PCRE-specific extensions to the regex syntax. While Perl programmers are encouraged to use the -Perl specific syntax, the following are also accepted: +Perl-specific syntax, the following are also accepted: =over 4 @@ -2163,17 +2587,11 @@ Subroutine call to a named capture group. Equivalent to C<< (?&NAME) >>. =head1 BUGS -There are numerous problems with case insensitive matching of characters -outside the ASCII range, especially with those whose folds are multiple -characters, such as ligatures like C. - -In a bracketed character class with case insensitive matching, ranges only work -for ASCII characters. For example, -C -doesn't match all the Russian upper and lower case letters. - Many regular expression constructs don't work on EBCDIC platforms. +There are a number of issues with regard to case-insensitive matching +in Unicode rules. See C under L above. + This document varies from difficult to understand to completely and utterly opaque. The wandering prose riddled with jargon is hard to fathom in several places.
ends are somewhat convoluted. +This zero-width assertion executes any embedded Perl code. It always +succeeds, and its return value is set as C<$^R>. -This feature can be used together with the special variable C<$^N> to -capture the results of submatches in variables without having to keep -track of the number of nested parentheses. For example: +In literal patterns, the code is parsed at the same time as the +surrounding code. While within the pattern, control is passed temporarily +back to the perl parser, until the logically-balancing closing brace is +encountered. This is similar to the way that an array index expression in +a literal string is handled, for example - $_ = "The brown fox jumps over the lazy dog"; - /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i; - print "color = $color, animal = $animal\n"; + "abc$array[ 1 + f('[') + g()]def" + +In particular, braces do not need to be balanced: + + /abc(?{ f('{'); })/def/ + +Even in a pattern that is interpolated and compiled at run-time, literal +code blocks will be compiled once, at perl compile time; the following +prints "ABCD": -Inside the C<(?{...})> block, C<$_> refers to the string the regular + print "D"; + my $qr = qr/(?{ BEGIN { print "A" } })/; + my $foo = "foo"; + /$foo$qr(?{ BEGIN { print "B" } })/; + BEGIN { print "C" } + +In patterns where the text of the code is derived from run-time +information rather than appearing literally in a source code /pattern/, +the code is compiled at the same time that the pattern is compiled, and +fro reasons of security, C must be in scope. This is to +stop user-supplied patterns containing code snippets from being +executable. + +In situations where you need enable this with C, you should +also have taint checking enabled. Better yet, use the carefully +constrained evaluation within a Safe compartment. See L for +details about both these mechanisms. + +From the viewpoint of parsing, lexical variable scope and closures, + + /AAA(?{ BBB })CCC/ + +behaves approximately like + + /AAA/ && do { BBB } && /CCC/ + +Similarly, + + qr/AAA(?{ BBB })CCC/ + +behaves approximately like + + sub { /AAA/ && do { BBB } && /CCC/ } + +In particular: + + { my $i = 1; $r = qr/(?{ print $i })/ } + my $i = 2; + /$r/; # prints "1" + +Inside a C<(?{...})> block, C<$_> refers to the string the regular expression is matching against. You can also use C to know what is the current position of matching within this string. -The C is properly scoped in the following sense: If the assertion -is backtracked (compare L<"Backtracking">), all changes introduced after -Cization are undone, so that +The code block introduces a new scope from the perspective of lexical +variable declarations, but B from the perspective of C and +similar localizing behaviours. So later code blocks within the same +pattern will still see the values which were localized in earlier blocks. +These accumulated localizations are undone either at the end of a +successful match, or if the assertion is backtracked (compare +L<"Backtracking">). For example, $_ = 'a' x 8; m< - (?{ $cnt = 0 }) # Initialize $cnt. + (?{ $cnt = 0 }) # Initialize $cnt. ( a (?{ - local $cnt = $cnt + 1; # Update $cnt, backtracking-safe. + local $cnt = $cnt + 1; # Update $cnt, + # backtracking-safe. }) )* aaaa - (?{ $res = $cnt }) # On success copy to - # non-localized location. + (?{ $res = $cnt }) # On success copy to + # non-localized location. >x; -will set C<$res = 4>. Note that after the match, C<$cnt> returns to the globally -introduced value, because the scopes that restrict C operators -are unwound. +will initially increment C<$cnt> up to 8; then during backtracking, its +value will be unwound back to 4, which is the value assigned to C<$res>. +At the end of the regex execution, $cnt will be wound back to its initial +value of 0. + +This assertion may be used as the condition in a + + (?(condition)yes-pattern|no-pattern) -This assertion may be used as a C<(?(condition)yes-pattern|no-pattern)> -switch. If I used in this way, the result of evaluation of -C is put into the special variable C<$^R>. This happens -immediately, so C<$^R> can be used from other C<(?{ code })> assertions -inside the same regular expression. +switch. If I used in this way, the result of evaluation of C +is put into the special variable C<$^R>. This happens immediately, so +C<$^R> can be used from other C<(?{ code })> assertions inside the same +regular expression. The assignment to C<$^R> above is properly localized, so the old value of C<$^R> is restored if the assertion is backtracked; compare L<"Backtracking">. -For reasons of security, this construct is forbidden if the regular -expression involves run-time interpolation of variables, unless the -perilous C pragma has been used (see L), or the -variables contain results of C operator (see -Lmsixpo">). +Note that the special variable C<$^N> is particularly useful with code +blocks to capture the results of submatches in variables without having to +keep track of the number of nested parentheses. For example: -This restriction is due to the wide-spread and remarkably convenient -custom of using run-time determined strings as patterns. For example: + $_ = "The brown fox jumps over the lazy dog"; + /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i; + print "color = $color, animal = $animal\n"; - $re = <>; - chomp $re; - $string =~ /$re/; - -Before Perl knew how to execute interpolated code within a pattern, -this operation was completely safe from a security point of view, -although it could raise an exception from an illegal pattern. If -you turn on the C, though, it is no longer secure, -so you should only do so if you are also using taint checking. -Better yet, use the carefully constrained evaluation within a Safe -compartment. See L for details about both these mechanisms. - -B: Use of lexical (C) variables in these blocks is -broken. The result is unpredictable and will make perl unstable. The -workaround is to use global (C) variables. - -B: In perl 5.12.x and earlier, the regex engine -was not re-entrant, so interpolated code could not -safely invoke the regex engine either directly with -C or C), or indirectly with functions such as -C. Invoking the regex engine in these blocks would make perl -unstable. =item C<(??{ code })> X<(??{})> @@ -1011,61 +1381,60 @@ experimental, and may be changed without notice. Code executed that has side effects may not perform identically from version to version due to the effect of future optimisations in the regex engine. -This is a "postponed" regular subexpression. The C is evaluated -at run time, at the moment this subexpression may match. The result -of evaluation is considered as a regular expression and matched as -if it were inserted instead of this construct. Note that this means -that the contents of capture groups defined inside an eval'ed pattern -are not available outside of the pattern, and vice versa, there is no -way for the inner pattern to refer to a capture group defined outside. -Thus, +This is a "postponed" regular subexpression. It behaves in I the +same way as a C<(?{ code })> code block as described above, except that +its return value, rather than being assigned to C<$^R>, is treated as a +pattern, compiled if it's a string (or used as-is if its a qr// object), +then matched as if it were inserted instead of this construct. - ('a' x 100)=~/(??{'(.)' x 100})/ +During the matching of this sub-pattern, it has its own set of +captures which are valid during the sub-match, but are discarded once +control returns to the main pattern. For example, the following matches, +with the inner pattern capturing "B" and matching "BB", while the outer +pattern captures "A"; -B match, it will B set $1. + my $inner = '(.)\1'; + "ABBA" =~ /^(.)(??{ $inner })\1/; + print $1; # prints "A"; -The C is not interpolated. As before, the rules to determine -where the C ends are currently somewhat convoluted. +Note that this means that there is no way for the inner pattern to refer +to a capture group defined outside. (The code block itself can use C<$1>, +etc., to refer to the enclosing pattern's capture groups.) Thus, although + + ('a' x 100)=~/(??{'(.)' x 100})/ + +I match, it will I set $1 on exit. The following pattern matches a parenthesized group: - $re = qr{ - \( - (?: - (?> [^()]+ ) # Non-parens without backtracking - | - (??{ $re }) # Group with matching parens - )* - \) - }x; + $re = qr{ + \( + (?: + (?> [^()]+ ) # Non-parens without backtracking + | + (??{ $re }) # Group with matching parens + )* + \) + }x; See also C<(?PARNO)> for a different, more efficient way to accomplish the same task. -For reasons of security, this construct is forbidden if the regular -expression involves run-time interpolation of variables, unless the -perilous C pragma has been used (see L), or the -variables contain results of C operator (see -LSTRINGEmsixpo">). - -In perl 5.12.x and earlier, because the regex engine was not re-entrant, -delayed code could not safely invoke the regex engine either directly with -C or C), or indirectly with functions such as C. - -Recursing deeper than 50 times without consuming any input string will -result in a fatal error. The maximum depth is compiled into perl, so -changing it requires a custom build. +Executing a postponed regular expression 50 times without consuming any +input string will result in a fatal error. The maximum depth is compiled +into perl, so changing it requires a custom build. =item C<(?PARNO)> C<(?-PARNO)> C<(?+PARNO)> C<(?R)> C<(?0)> X<(?PARNO)> X<(?1)> X<(?R)> X<(?0)> X<(?-1)> X<(?+1)> X<(?-PARNO)> X<(?+PARNO)> X X X X -Similar to C<(??{ code })> except it does not involve compiling any code, -instead it treats the contents of a capture group as an independent -pattern that must match at the current position. Capture groups -contained by the pattern will have the value as determined by the -outermost recursion. +Similar to C<(??{ code })> except that it does not involve executing any +code or potentially compiling a returned pattern string; instead it treats +the part of the current pattern contained within a specified capture group +as an independent pattern that must match at the current position. +Capture groups contained by the pattern will have the value as determined +by the outermost recursion. PARNO is a sequence of digits (not starting with 0) whose value reflects the paren-number of the capture group to recurse to. C<(?R)> recurses to @@ -1081,15 +1450,15 @@ included. The following pattern matches a function foo() which may contain balanced parentheses as the argument. - $re = qr{ ( # paren group 1 (full function) + $re = qr{ ( # paren group 1 (full function) foo - ( # paren group 2 (parens) + ( # paren group 2 (parens) \( - ( # paren group 3 (contents of parens) + ( # paren group 3 (contents of parens) (?: - (?> [^()]+ ) # Non-parens without backtracking + (?> [^()]+ ) # Non-parens without backtracking | - (?2) # Recurse to start of paren group 2 + (?2) # Recurse to start of paren group 2 )* ) \) @@ -1120,7 +1489,7 @@ easier to embed recursive patterns inside of a C construct for later use: my $parens = qr/(\((?:[^()]++|(?-1))*+\))/; - if (/foo $parens \s+ + \s+ bar $parens/x) { + if (/foo $parens \s+ \+ \s+ bar $parens/x) { # do something here... } @@ -1150,11 +1519,15 @@ X<(?()> =item C<(?(condition)yes-pattern)> -Conditional expression. C<(condition)> should be either an integer in +Conditional expression. Matches C if C yields +a true value, matches C otherwise. A missing pattern always +matches. + +C<(condition)> should be one of: 1) an integer in parentheses (which is valid if the corresponding pair of parentheses -matched), a look-ahead/look-behind/evaluate zero-width assertion, a +matched); 2) a look-ahead/look-behind/evaluate zero-width assertion; 3) a name in angle brackets or single quotes (which is valid if a group -with the given name matched), or the special symbol (R) (true when +with the given name matched); or 4) the special symbol (R) (true when evaluated inside of recursion or eval). Additionally the R may be followed by a number, (which will be true when evaluated when recursing inside of the appropriate group), or by C<&NAME>, in which case it will @@ -1172,9 +1545,14 @@ Checks if the numbered capturing group has matched something. Checks if a group with the given name has matched something. +=item (?=...) (?!...) (?<=...) (? predicate, which never executes directly -its yes-pattern, and does not allow a no-pattern. This allows to define -subpatterns which will be executed only by using the recursion mechanism. +A special form is the C<(DEFINE)> predicate, which never executes its +yes-pattern directly, and does not allow a no-pattern. This allows one to +define subpatterns which will be executed only by the recursion mechanism. This way, you can define a set of regular expression rules that can be bundled into any pattern you choose. @@ -1240,6 +1618,19 @@ after the recursion returns, so the extra layer of capturing groups is necessary. Thus C<$+{NAME_PAT}> would not be defined even though C<$+{NAME}> would be. +Finally, keep in mind that subpatterns created inside a DEFINE block +count towards the absolute and relative number of captures, so this: + + my @captures = "a" =~ /(.) # First capture + (?(DEFINE) + (? 1 ) # Second capture + )/x; + say scalar @captures; + +Will output 2, not 1. This is particularly important if you intend to +compile the definitions with the C operator, and later +interpolate them in another pattern. + =item C<< (?>pattern) >> X X X X @@ -1260,9 +1651,13 @@ group C (see L<"Backtracking">). In particular, C inside C will match fewer characters than a standalone C, since this makes the tail match. +C<< (?>pattern) >> does not disable backtracking altogether once it has +matched. It is still possible to backtrack past the construct, but not +into it. So C<< ((?>a*)|(?>b*))ar >> will still match "bar". + An effect similar to C<< (?>pattern) >> may be achieved by writing -C<(?=(pattern))\g1>. This matches the same substring as a standalone -C, and the following C<\g1> eats the matched string; it therefore +C<(?=(pattern))\g{-1}>. This matches the same substring as a standalone +C, and the following C<\g{-1}> eats the matched string; it therefore makes a zero-length assertion into an analogue of C<< (?>...) >>. (The difference between these two constructs is that the second one uses a capturing group, thus shifting ordinals of backreferences @@ -1302,7 +1697,8 @@ hung. However, a tiny change to this pattern which uses C<< (?>...) >> matches exactly when the one above does (verifying this yourself would be a productive exercise), but finishes in a fourth the time when used on a similar string with 1000000 Cs. Be aware, -however, that this pattern currently triggers a warning message under +however, that, when this construct is followed by a +quantifier, it currently triggers a warning message under the C pragma or B<-w> switch saying it C<"matches null string many times in regex">. @@ -1373,14 +1769,14 @@ C<(*MARK:NAME)> pattern executed. See the explanation for the C<(*MARK:NAME)> verb below for more details. B C<$REGERROR> and C<$REGMARK> are not magic variables like C<$1> -and most other regex related variables. They are not local to a scope, nor +and most other regex-related variables. They are not local to a scope, nor readonly, but instead are volatile package variables similar to C<$AUTOLOAD>. Use C to localize changes to them to a specific scope if necessary. If a pattern does not contain a special backtracking verb that allows an argument, then C<$REGERROR> and C<$REGMARK> are not touched at all. -=over 4 +=over 3 =item Verbs that take an argument @@ -1421,7 +1817,7 @@ If we add a C<(*PRUNE)> before the count like the following 'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/; print "Count=$count\n"; -we prevent backtracking and find the count of the longest matching +we prevent backtracking and find the count of the longest matching string at each matching starting point like so: aaab @@ -1437,7 +1833,6 @@ replaced with a C<< (?>pattern) >> with no functional difference; however, C<(*PRUNE)> can be used to handle cases that cannot be expressed using a C<< (?>pattern) >> alone. - =item C<(*SKIP)> C<(*SKIP:NAME)> X<(*SKIP)> @@ -1455,11 +1850,11 @@ encountered, then the C<(*SKIP)> operator has no effect. When used without a name the "skip point" is where the match point was when executing the (*SKIP) pattern. -Compare the following to the examples in C<(*PRUNE)>, note the string +Compare the following to the examples in C<(*PRUNE)>; note the string is twice as long: - 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/; - print "Count=$count\n"; + 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/; + print "Count=$count\n"; outputs @@ -1472,7 +1867,7 @@ executed, the next starting point will be where the cursor was when the C<(*SKIP)> was executed. =item C<(*MARK:NAME)> C<(*:NAME)> -X<(*MARK)> C<(*MARK:NAME)> C<(*:NAME)> +X<(*MARK)> X<(*MARK:NAME)> X<(*:NAME)> This zero-width pattern can be used to mark the point reached in a string when a certain part of the pattern has been successfully matched. This @@ -1500,16 +1895,18 @@ failing the match and has provided its own name to use, the C<$REGERROR> variable will be set to the name of the most recently executed C<(*MARK:NAME)>. -See C<(*SKIP)> for more details. +See L(*SKIP)> for more details. As a shortcut C<(*MARK:NAME)> can be written C<(*:NAME)>. =item C<(*THEN)> C<(*THEN:NAME)> -This is similar to the "cut group" operator C<::> from Perl 6. Like +This is similar to the "cut group" operator C<::> from Perl 6. Like C<(*PRUNE)>, this verb always matches, and when backtracked into on failure, it causes the regex engine to try the next alternation in the -innermost enclosing group (capturing or otherwise). +innermost enclosing group (capturing or otherwise) that has alternations. +The two branches of a C<(?(condition)yes-pattern|no-pattern)> do not +count as an alternation, as far as C<(*THEN)> is concerned. Its name comes from the observation that this operation combined with the alternation operator (C<|>) can be used to create what is essentially a @@ -1528,15 +1925,21 @@ is the same as but - / ( A (*THEN) B | C (*THEN) D ) / + / ( A (*THEN) B | C ) / is not the same as - / ( A (*PRUNE) B | C (*PRUNE) D ) / + / ( A (*PRUNE) B | C ) / as after matching the A but failing on the B the C<(*THEN)> verb will backtrack and try C; but the C<(*PRUNE)> verb will simply fail. +=back + +=item Verbs without an argument + +=over 4 + =item C<(*COMMIT)> X<(*COMMIT)> @@ -1546,8 +1949,8 @@ into on failure it causes the match to fail outright. No further attempts to find a valid match by advancing the start pointer will occur again. For example, - 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/; - print "Count=$count\n"; + 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/; + print "Count=$count\n"; outputs @@ -1558,12 +1961,6 @@ In other words, once the C<(*COMMIT)> has been entered, and if the pattern does not match, the regex engine will not try any further matching on the rest of the string. -=back - -=item Verbs without an argument - -=over 4 - =item C<(*FAIL)> C<(*F)> X<(*FAIL)> X<(*F)> @@ -1592,7 +1989,7 @@ For instance: 'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x; will match, and C<$1> will be C and C<$2> will be C, C<$3> will not -be set. If another branch in the inner parentheses were matched, such as in the +be set. If another branch in the inner parentheses was matched, such as in the string 'ACDE', then the C and C would have to be matched as well. =back @@ -1751,7 +2148,7 @@ let C<\D*> expand to "ABC", this would have caused the whole pattern to fail. The search engine will initially match C<\D*> with "ABC". Then it will -try to match C<(?!123> with "123", which fails. But because +try to match C<(?!123)> with "123", which fails. But because a quantifier (C<\D*>) has been used in the regular expression, the search engine can backtrack and retry the match differently in the hope of matching the complete regular expression. @@ -1799,12 +2196,12 @@ match takes a long time to finish. A powerful tool for optimizing such beasts is what is known as an "independent group", -which does not backtrack (see Lpattern) >>>). Note also that +which does not backtrack (see Lpattern) >>>). Note also that zero-length look-ahead/look-behind assertions will not backtrack to make the tail match, since they are in "logical" context: only whether they match is considered relevant. For an example where side-effects of look-ahead I have influenced the -following match, see Lpattern) >>>. +following match, see Lpattern) >>>. =head2 Version 8 Regular Expressions X X X @@ -1820,7 +2217,7 @@ character; "\\" matches a "\"). This escape mechanism is also required for the character used as the pattern delimiter. A series of characters matches that series of characters in the target -string, so the pattern C would match "blurfl" in the target +string, so the pattern C would match "blurfl" in the target string. You can specify a character class, by enclosing a list of characters @@ -1859,9 +2256,9 @@ You can specify a series of alternatives for a pattern using "|" to separate them, so that C will match any of "fee", "fie", or "foe" in the target string (as would C). The first alternative includes everything from the last pattern delimiter -("(", "[", or the beginning of the pattern) up to the first "|", and +("(", "(?:", etc. or the beginning of the pattern) up to the first "|", and the last alternative contains everything from the last "|" to the next -pattern delimiter. That's why it's common practice to include +closing pattern delimiter. That's why it's common practice to include alternatives in parentheses: to minimize confusion about where they start and end. @@ -1879,7 +2276,7 @@ so if you write C<[fee|fie|foe]> you're really only matching C<[feio|]>. Within a pattern, you may designate subpatterns for later reference by enclosing them in parentheses, and you may refer back to the Ith subpattern later in the pattern using the metacharacter -\I. Subpatterns are numbered based on the left to right order +\I or \gI. Subpatterns are numbered based on the left to right order of their opening parenthesis. A backreference matches whatever actually matched the subpattern in the string being examined, not the rules for that subpattern. Therefore, C<(0|0x)\d*\s\g1\d*> will @@ -1943,7 +2340,7 @@ However, long experience has shown that many programming tasks may be significantly simplified by using repeated subexpressions that may match zero-length substrings. Here's a simple example being: - @chars = split //, $string; # // is not magic in split + @chars = split //, $string; # // is not magic in split ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// / Thus Perl allows such constructs, by I matches a zero-length string, it stops +the C<*>. + +The higher-level loops preserve an additional state between iterations: whether the last match was zero-length. To break the loop, the following match after a zero-length match is prohibited to have a length of zero. This prohibition interacts with backtracking (see L<"Backtracking">), @@ -1995,7 +2414,7 @@ Each of the elementary pieces of regular expressions which were described before (such as C or C<\Z>) could match at most one substring at the given position of the input string. However, in a typical regular expression these elementary pieces are combined into more complicated -patterns using combining operators C, C, C etc +patterns using combining operators C, C, C etc. (in these examples C and C are regular subexpressions). Such combinations can include alternatives, leading to a problem of choice: @@ -2024,11 +2443,11 @@ Consider two possible matches, C and C, C and C are substrings which can be matched by C, C and C are substrings which can be matched by C. -If C is better match for C than C, C is a better +If C is a better match for C than C, C is a better match than C. If C and C coincide: C is a better match than C if -C is better match for C than C. +C is a better match for C than C. =item C @@ -2092,8 +2511,13 @@ than a match at a later position. =head2 Creating Custom RE Engines -Overloaded constants (see L) provide a simple way to extend -the functionality of the RE engine. +As of Perl 5.10.0, one can create custom regular expression engines. This +is not for the faint of heart, as they have to plug in at the C level. See +L for more details. + +As an alternative, overloaded constants (see L) provide a simple +way to extend the functionality of the RE engine, by substituting one +pattern for another. Suppose that we want to enable a new RE escape-sequence C<\Y|> which matches at a boundary between whitespace characters and non-whitespace @@ -2139,11 +2563,11 @@ part of this regular expression needs to be converted explicitly $re = customre::convert $re; /\Y|$re\Y|/; -=head1 PCRE/Python Support +=head2 PCRE/Python Support -As of Perl 5.10.0, Perl supports several Python/PCRE specific extensions +As of Perl 5.10.0, Perl supports several Python/PCRE-specific extensions to the regex syntax. While Perl programmers are encouraged to use the -Perl specific syntax, the following are also accepted: +Perl-specific syntax, the following are also accepted: =over 4 @@ -2163,17 +2587,11 @@ Subroutine call to a named capture group. Equivalent to C<< (?&NAME) >>. =head1 BUGS -There are numerous problems with case insensitive matching of characters -outside the ASCII range, especially with those whose folds are multiple -characters, such as ligatures like C. - -In a bracketed character class with case insensitive matching, ranges only work -for ASCII characters. For example, -C -doesn't match all the Russian upper and lower case letters. - Many regular expression constructs don't work on EBCDIC platforms. +There are a number of issues with regard to case-insensitive matching +in Unicode rules. See C under L above. + This document varies from difficult to understand to completely and utterly opaque. The wandering prose riddled with jargon is hard to fathom in several places.
is properly scoped in the following sense: If the assertion -is backtracked (compare L<"Backtracking">), all changes introduced after -Cization are undone, so that +The code block introduces a new scope from the perspective of lexical +variable declarations, but B from the perspective of C and +similar localizing behaviours. So later code blocks within the same +pattern will still see the values which were localized in earlier blocks. +These accumulated localizations are undone either at the end of a +successful match, or if the assertion is backtracked (compare +L<"Backtracking">). For example, $_ = 'a' x 8; m< - (?{ $cnt = 0 }) # Initialize $cnt. + (?{ $cnt = 0 }) # Initialize $cnt. ( a (?{ - local $cnt = $cnt + 1; # Update $cnt, backtracking-safe. + local $cnt = $cnt + 1; # Update $cnt, + # backtracking-safe. }) )* aaaa - (?{ $res = $cnt }) # On success copy to - # non-localized location. + (?{ $res = $cnt }) # On success copy to + # non-localized location. >x; -will set C<$res = 4>. Note that after the match, C<$cnt> returns to the globally -introduced value, because the scopes that restrict C operators -are unwound. +will initially increment C<$cnt> up to 8; then during backtracking, its +value will be unwound back to 4, which is the value assigned to C<$res>. +At the end of the regex execution, $cnt will be wound back to its initial +value of 0. + +This assertion may be used as the condition in a + + (?(condition)yes-pattern|no-pattern) -This assertion may be used as a C<(?(condition)yes-pattern|no-pattern)> -switch. If I used in this way, the result of evaluation of -C is put into the special variable C<$^R>. This happens -immediately, so C<$^R> can be used from other C<(?{ code })> assertions -inside the same regular expression. +switch. If I used in this way, the result of evaluation of C +is put into the special variable C<$^R>. This happens immediately, so +C<$^R> can be used from other C<(?{ code })> assertions inside the same +regular expression. The assignment to C<$^R> above is properly localized, so the old value of C<$^R> is restored if the assertion is backtracked; compare L<"Backtracking">. -For reasons of security, this construct is forbidden if the regular -expression involves run-time interpolation of variables, unless the -perilous C pragma has been used (see L), or the -variables contain results of C operator (see -Lmsixpo">). +Note that the special variable C<$^N> is particularly useful with code +blocks to capture the results of submatches in variables without having to +keep track of the number of nested parentheses. For example: -This restriction is due to the wide-spread and remarkably convenient -custom of using run-time determined strings as patterns. For example: + $_ = "The brown fox jumps over the lazy dog"; + /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i; + print "color = $color, animal = $animal\n"; - $re = <>; - chomp $re; - $string =~ /$re/; - -Before Perl knew how to execute interpolated code within a pattern, -this operation was completely safe from a security point of view, -although it could raise an exception from an illegal pattern. If -you turn on the C, though, it is no longer secure, -so you should only do so if you are also using taint checking. -Better yet, use the carefully constrained evaluation within a Safe -compartment. See L for details about both these mechanisms. - -B: Use of lexical (C) variables in these blocks is -broken. The result is unpredictable and will make perl unstable. The -workaround is to use global (C) variables. - -B: In perl 5.12.x and earlier, the regex engine -was not re-entrant, so interpolated code could not -safely invoke the regex engine either directly with -C or C), or indirectly with functions such as -C. Invoking the regex engine in these blocks would make perl -unstable. =item C<(??{ code })> X<(??{})> @@ -1011,61 +1381,60 @@ experimental, and may be changed without notice. Code executed that has side effects may not perform identically from version to version due to the effect of future optimisations in the regex engine. -This is a "postponed" regular subexpression. The C is evaluated -at run time, at the moment this subexpression may match. The result -of evaluation is considered as a regular expression and matched as -if it were inserted instead of this construct. Note that this means -that the contents of capture groups defined inside an eval'ed pattern -are not available outside of the pattern, and vice versa, there is no -way for the inner pattern to refer to a capture group defined outside. -Thus, +This is a "postponed" regular subexpression. It behaves in I the +same way as a C<(?{ code })> code block as described above, except that +its return value, rather than being assigned to C<$^R>, is treated as a +pattern, compiled if it's a string (or used as-is if its a qr// object), +then matched as if it were inserted instead of this construct. - ('a' x 100)=~/(??{'(.)' x 100})/ +During the matching of this sub-pattern, it has its own set of +captures which are valid during the sub-match, but are discarded once +control returns to the main pattern. For example, the following matches, +with the inner pattern capturing "B" and matching "BB", while the outer +pattern captures "A"; -B match, it will B set $1. + my $inner = '(.)\1'; + "ABBA" =~ /^(.)(??{ $inner })\1/; + print $1; # prints "A"; -The C is not interpolated. As before, the rules to determine -where the C ends are currently somewhat convoluted. +Note that this means that there is no way for the inner pattern to refer +to a capture group defined outside. (The code block itself can use C<$1>, +etc., to refer to the enclosing pattern's capture groups.) Thus, although + + ('a' x 100)=~/(??{'(.)' x 100})/ + +I match, it will I set $1 on exit. The following pattern matches a parenthesized group: - $re = qr{ - \( - (?: - (?> [^()]+ ) # Non-parens without backtracking - | - (??{ $re }) # Group with matching parens - )* - \) - }x; + $re = qr{ + \( + (?: + (?> [^()]+ ) # Non-parens without backtracking + | + (??{ $re }) # Group with matching parens + )* + \) + }x; See also C<(?PARNO)> for a different, more efficient way to accomplish the same task. -For reasons of security, this construct is forbidden if the regular -expression involves run-time interpolation of variables, unless the -perilous C pragma has been used (see L), or the -variables contain results of C operator (see -LSTRINGEmsixpo">). - -In perl 5.12.x and earlier, because the regex engine was not re-entrant, -delayed code could not safely invoke the regex engine either directly with -C or C), or indirectly with functions such as C. - -Recursing deeper than 50 times without consuming any input string will -result in a fatal error. The maximum depth is compiled into perl, so -changing it requires a custom build. +Executing a postponed regular expression 50 times without consuming any +input string will result in a fatal error. The maximum depth is compiled +into perl, so changing it requires a custom build. =item C<(?PARNO)> C<(?-PARNO)> C<(?+PARNO)> C<(?R)> C<(?0)> X<(?PARNO)> X<(?1)> X<(?R)> X<(?0)> X<(?-1)> X<(?+1)> X<(?-PARNO)> X<(?+PARNO)> X X X X -Similar to C<(??{ code })> except it does not involve compiling any code, -instead it treats the contents of a capture group as an independent -pattern that must match at the current position. Capture groups -contained by the pattern will have the value as determined by the -outermost recursion. +Similar to C<(??{ code })> except that it does not involve executing any +code or potentially compiling a returned pattern string; instead it treats +the part of the current pattern contained within a specified capture group +as an independent pattern that must match at the current position. +Capture groups contained by the pattern will have the value as determined +by the outermost recursion. PARNO is a sequence of digits (not starting with 0) whose value reflects the paren-number of the capture group to recurse to. C<(?R)> recurses to @@ -1081,15 +1450,15 @@ included. The following pattern matches a function foo() which may contain balanced parentheses as the argument. - $re = qr{ ( # paren group 1 (full function) + $re = qr{ ( # paren group 1 (full function) foo - ( # paren group 2 (parens) + ( # paren group 2 (parens) \( - ( # paren group 3 (contents of parens) + ( # paren group 3 (contents of parens) (?: - (?> [^()]+ ) # Non-parens without backtracking + (?> [^()]+ ) # Non-parens without backtracking | - (?2) # Recurse to start of paren group 2 + (?2) # Recurse to start of paren group 2 )* ) \) @@ -1120,7 +1489,7 @@ easier to embed recursive patterns inside of a C construct for later use: my $parens = qr/(\((?:[^()]++|(?-1))*+\))/; - if (/foo $parens \s+ + \s+ bar $parens/x) { + if (/foo $parens \s+ \+ \s+ bar $parens/x) { # do something here... } @@ -1150,11 +1519,15 @@ X<(?()> =item C<(?(condition)yes-pattern)> -Conditional expression. C<(condition)> should be either an integer in +Conditional expression. Matches C if C yields +a true value, matches C otherwise. A missing pattern always +matches. + +C<(condition)> should be one of: 1) an integer in parentheses (which is valid if the corresponding pair of parentheses -matched), a look-ahead/look-behind/evaluate zero-width assertion, a +matched); 2) a look-ahead/look-behind/evaluate zero-width assertion; 3) a name in angle brackets or single quotes (which is valid if a group -with the given name matched), or the special symbol (R) (true when +with the given name matched); or 4) the special symbol (R) (true when evaluated inside of recursion or eval). Additionally the R may be followed by a number, (which will be true when evaluated when recursing inside of the appropriate group), or by C<&NAME>, in which case it will @@ -1172,9 +1545,14 @@ Checks if the numbered capturing group has matched something. Checks if a group with the given name has matched something. +=item (?=...) (?!...) (?<=...) (? predicate, which never executes directly -its yes-pattern, and does not allow a no-pattern. This allows to define -subpatterns which will be executed only by using the recursion mechanism. +A special form is the C<(DEFINE)> predicate, which never executes its +yes-pattern directly, and does not allow a no-pattern. This allows one to +define subpatterns which will be executed only by the recursion mechanism. This way, you can define a set of regular expression rules that can be bundled into any pattern you choose. @@ -1240,6 +1618,19 @@ after the recursion returns, so the extra layer of capturing groups is necessary. Thus C<$+{NAME_PAT}> would not be defined even though C<$+{NAME}> would be. +Finally, keep in mind that subpatterns created inside a DEFINE block +count towards the absolute and relative number of captures, so this: + + my @captures = "a" =~ /(.) # First capture + (?(DEFINE) + (? 1 ) # Second capture + )/x; + say scalar @captures; + +Will output 2, not 1. This is particularly important if you intend to +compile the definitions with the C operator, and later +interpolate them in another pattern. + =item C<< (?>pattern) >> X X X X @@ -1260,9 +1651,13 @@ group C (see L<"Backtracking">). In particular, C inside C will match fewer characters than a standalone C, since this makes the tail match. +C<< (?>pattern) >> does not disable backtracking altogether once it has +matched. It is still possible to backtrack past the construct, but not +into it. So C<< ((?>a*)|(?>b*))ar >> will still match "bar". + An effect similar to C<< (?>pattern) >> may be achieved by writing -C<(?=(pattern))\g1>. This matches the same substring as a standalone -C, and the following C<\g1> eats the matched string; it therefore +C<(?=(pattern))\g{-1}>. This matches the same substring as a standalone +C, and the following C<\g{-1}> eats the matched string; it therefore makes a zero-length assertion into an analogue of C<< (?>...) >>. (The difference between these two constructs is that the second one uses a capturing group, thus shifting ordinals of backreferences @@ -1302,7 +1697,8 @@ hung. However, a tiny change to this pattern which uses C<< (?>...) >> matches exactly when the one above does (verifying this yourself would be a productive exercise), but finishes in a fourth the time when used on a similar string with 1000000 Cs. Be aware, -however, that this pattern currently triggers a warning message under +however, that, when this construct is followed by a +quantifier, it currently triggers a warning message under the C pragma or B<-w> switch saying it C<"matches null string many times in regex">. @@ -1373,14 +1769,14 @@ C<(*MARK:NAME)> pattern executed. See the explanation for the C<(*MARK:NAME)> verb below for more details. B C<$REGERROR> and C<$REGMARK> are not magic variables like C<$1> -and most other regex related variables. They are not local to a scope, nor +and most other regex-related variables. They are not local to a scope, nor readonly, but instead are volatile package variables similar to C<$AUTOLOAD>. Use C to localize changes to them to a specific scope if necessary. If a pattern does not contain a special backtracking verb that allows an argument, then C<$REGERROR> and C<$REGMARK> are not touched at all. -=over 4 +=over 3 =item Verbs that take an argument @@ -1421,7 +1817,7 @@ If we add a C<(*PRUNE)> before the count like the following 'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/; print "Count=$count\n"; -we prevent backtracking and find the count of the longest matching +we prevent backtracking and find the count of the longest matching string at each matching starting point like so: aaab @@ -1437,7 +1833,6 @@ replaced with a C<< (?>pattern) >> with no functional difference; however, C<(*PRUNE)> can be used to handle cases that cannot be expressed using a C<< (?>pattern) >> alone. - =item C<(*SKIP)> C<(*SKIP:NAME)> X<(*SKIP)> @@ -1455,11 +1850,11 @@ encountered, then the C<(*SKIP)> operator has no effect. When used without a name the "skip point" is where the match point was when executing the (*SKIP) pattern. -Compare the following to the examples in C<(*PRUNE)>, note the string +Compare the following to the examples in C<(*PRUNE)>; note the string is twice as long: - 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/; - print "Count=$count\n"; + 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/; + print "Count=$count\n"; outputs @@ -1472,7 +1867,7 @@ executed, the next starting point will be where the cursor was when the C<(*SKIP)> was executed. =item C<(*MARK:NAME)> C<(*:NAME)> -X<(*MARK)> C<(*MARK:NAME)> C<(*:NAME)> +X<(*MARK)> X<(*MARK:NAME)> X<(*:NAME)> This zero-width pattern can be used to mark the point reached in a string when a certain part of the pattern has been successfully matched. This @@ -1500,16 +1895,18 @@ failing the match and has provided its own name to use, the C<$REGERROR> variable will be set to the name of the most recently executed C<(*MARK:NAME)>. -See C<(*SKIP)> for more details. +See L(*SKIP)> for more details. As a shortcut C<(*MARK:NAME)> can be written C<(*:NAME)>. =item C<(*THEN)> C<(*THEN:NAME)> -This is similar to the "cut group" operator C<::> from Perl 6. Like +This is similar to the "cut group" operator C<::> from Perl 6. Like C<(*PRUNE)>, this verb always matches, and when backtracked into on failure, it causes the regex engine to try the next alternation in the -innermost enclosing group (capturing or otherwise). +innermost enclosing group (capturing or otherwise) that has alternations. +The two branches of a C<(?(condition)yes-pattern|no-pattern)> do not +count as an alternation, as far as C<(*THEN)> is concerned. Its name comes from the observation that this operation combined with the alternation operator (C<|>) can be used to create what is essentially a @@ -1528,15 +1925,21 @@ is the same as but - / ( A (*THEN) B | C (*THEN) D ) / + / ( A (*THEN) B | C ) / is not the same as - / ( A (*PRUNE) B | C (*PRUNE) D ) / + / ( A (*PRUNE) B | C ) / as after matching the A but failing on the B the C<(*THEN)> verb will backtrack and try C; but the C<(*PRUNE)> verb will simply fail. +=back + +=item Verbs without an argument + +=over 4 + =item C<(*COMMIT)> X<(*COMMIT)> @@ -1546,8 +1949,8 @@ into on failure it causes the match to fail outright. No further attempts to find a valid match by advancing the start pointer will occur again. For example, - 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/; - print "Count=$count\n"; + 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/; + print "Count=$count\n"; outputs @@ -1558,12 +1961,6 @@ In other words, once the C<(*COMMIT)> has been entered, and if the pattern does not match, the regex engine will not try any further matching on the rest of the string. -=back - -=item Verbs without an argument - -=over 4 - =item C<(*FAIL)> C<(*F)> X<(*FAIL)> X<(*F)> @@ -1592,7 +1989,7 @@ For instance: 'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x; will match, and C<$1> will be C and C<$2> will be C, C<$3> will not -be set. If another branch in the inner parentheses were matched, such as in the +be set. If another branch in the inner parentheses was matched, such as in the string 'ACDE', then the C and C would have to be matched as well. =back @@ -1751,7 +2148,7 @@ let C<\D*> expand to "ABC", this would have caused the whole pattern to fail. The search engine will initially match C<\D*> with "ABC". Then it will -try to match C<(?!123> with "123", which fails. But because +try to match C<(?!123)> with "123", which fails. But because a quantifier (C<\D*>) has been used in the regular expression, the search engine can backtrack and retry the match differently in the hope of matching the complete regular expression. @@ -1799,12 +2196,12 @@ match takes a long time to finish. A powerful tool for optimizing such beasts is what is known as an "independent group", -which does not backtrack (see Lpattern) >>>). Note also that +which does not backtrack (see Lpattern) >>>). Note also that zero-length look-ahead/look-behind assertions will not backtrack to make the tail match, since they are in "logical" context: only whether they match is considered relevant. For an example where side-effects of look-ahead I have influenced the -following match, see Lpattern) >>>. +following match, see Lpattern) >>>. =head2 Version 8 Regular Expressions X X X @@ -1820,7 +2217,7 @@ character; "\\" matches a "\"). This escape mechanism is also required for the character used as the pattern delimiter. A series of characters matches that series of characters in the target -string, so the pattern C would match "blurfl" in the target +string, so the pattern C would match "blurfl" in the target string. You can specify a character class, by enclosing a list of characters @@ -1859,9 +2256,9 @@ You can specify a series of alternatives for a pattern using "|" to separate them, so that C will match any of "fee", "fie", or "foe" in the target string (as would C). The first alternative includes everything from the last pattern delimiter -("(", "[", or the beginning of the pattern) up to the first "|", and +("(", "(?:", etc. or the beginning of the pattern) up to the first "|", and the last alternative contains everything from the last "|" to the next -pattern delimiter. That's why it's common practice to include +closing pattern delimiter. That's why it's common practice to include alternatives in parentheses: to minimize confusion about where they start and end. @@ -1879,7 +2276,7 @@ so if you write C<[fee|fie|foe]> you're really only matching C<[feio|]>. Within a pattern, you may designate subpatterns for later reference by enclosing them in parentheses, and you may refer back to the Ith subpattern later in the pattern using the metacharacter -\I. Subpatterns are numbered based on the left to right order +\I or \gI. Subpatterns are numbered based on the left to right order of their opening parenthesis. A backreference matches whatever actually matched the subpattern in the string being examined, not the rules for that subpattern. Therefore, C<(0|0x)\d*\s\g1\d*> will @@ -1943,7 +2340,7 @@ However, long experience has shown that many programming tasks may be significantly simplified by using repeated subexpressions that may match zero-length substrings. Here's a simple example being: - @chars = split //, $string; # // is not magic in split + @chars = split //, $string; # // is not magic in split ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// / Thus Perl allows such constructs, by I matches a zero-length string, it stops +the C<*>. + +The higher-level loops preserve an additional state between iterations: whether the last match was zero-length. To break the loop, the following match after a zero-length match is prohibited to have a length of zero. This prohibition interacts with backtracking (see L<"Backtracking">), @@ -1995,7 +2414,7 @@ Each of the elementary pieces of regular expressions which were described before (such as C or C<\Z>) could match at most one substring at the given position of the input string. However, in a typical regular expression these elementary pieces are combined into more complicated -patterns using combining operators C, C, C etc +patterns using combining operators C, C, C etc. (in these examples C and C are regular subexpressions). Such combinations can include alternatives, leading to a problem of choice: @@ -2024,11 +2443,11 @@ Consider two possible matches, C and C, C and C are substrings which can be matched by C, C and C are substrings which can be matched by C. -If C is better match for C than C, C is a better +If C is a better match for C than C, C is a better match than C. If C and C coincide: C is a better match than C if -C is better match for C than C. +C is a better match for C than C. =item C @@ -2092,8 +2511,13 @@ than a match at a later position. =head2 Creating Custom RE Engines -Overloaded constants (see L) provide a simple way to extend -the functionality of the RE engine. +As of Perl 5.10.0, one can create custom regular expression engines. This +is not for the faint of heart, as they have to plug in at the C level. See +L for more details. + +As an alternative, overloaded constants (see L) provide a simple +way to extend the functionality of the RE engine, by substituting one +pattern for another. Suppose that we want to enable a new RE escape-sequence C<\Y|> which matches at a boundary between whitespace characters and non-whitespace @@ -2139,11 +2563,11 @@ part of this regular expression needs to be converted explicitly $re = customre::convert $re; /\Y|$re\Y|/; -=head1 PCRE/Python Support +=head2 PCRE/Python Support -As of Perl 5.10.0, Perl supports several Python/PCRE specific extensions +As of Perl 5.10.0, Perl supports several Python/PCRE-specific extensions to the regex syntax. While Perl programmers are encouraged to use the -Perl specific syntax, the following are also accepted: +Perl-specific syntax, the following are also accepted: =over 4 @@ -2163,17 +2587,11 @@ Subroutine call to a named capture group. Equivalent to C<< (?&NAME) >>. =head1 BUGS -There are numerous problems with case insensitive matching of characters -outside the ASCII range, especially with those whose folds are multiple -characters, such as ligatures like C. - -In a bracketed character class with case insensitive matching, ranges only work -for ASCII characters. For example, -C -doesn't match all the Russian upper and lower case letters. - Many regular expression constructs don't work on EBCDIC platforms. +There are a number of issues with regard to case-insensitive matching +in Unicode rules. See C under L above. + This document varies from difficult to understand to completely and utterly opaque. The wandering prose riddled with jargon is hard to fathom in several places.
is put into the special variable C<$^R>. This happens -immediately, so C<$^R> can be used from other C<(?{ code })> assertions -inside the same regular expression. +switch. If I used in this way, the result of evaluation of C +is put into the special variable C<$^R>. This happens immediately, so +C<$^R> can be used from other C<(?{ code })> assertions inside the same +regular expression. The assignment to C<$^R> above is properly localized, so the old value of C<$^R> is restored if the assertion is backtracked; compare L<"Backtracking">. -For reasons of security, this construct is forbidden if the regular -expression involves run-time interpolation of variables, unless the -perilous C pragma has been used (see L), or the -variables contain results of C operator (see -Lmsixpo">). +Note that the special variable C<$^N> is particularly useful with code +blocks to capture the results of submatches in variables without having to +keep track of the number of nested parentheses. For example: -This restriction is due to the wide-spread and remarkably convenient -custom of using run-time determined strings as patterns. For example: + $_ = "The brown fox jumps over the lazy dog"; + /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i; + print "color = $color, animal = $animal\n"; - $re = <>; - chomp $re; - $string =~ /$re/; - -Before Perl knew how to execute interpolated code within a pattern, -this operation was completely safe from a security point of view, -although it could raise an exception from an illegal pattern. If -you turn on the C, though, it is no longer secure, -so you should only do so if you are also using taint checking. -Better yet, use the carefully constrained evaluation within a Safe -compartment. See L for details about both these mechanisms. - -B: Use of lexical (C) variables in these blocks is -broken. The result is unpredictable and will make perl unstable. The -workaround is to use global (C) variables. - -B: In perl 5.12.x and earlier, the regex engine -was not re-entrant, so interpolated code could not -safely invoke the regex engine either directly with -C or C), or indirectly with functions such as -C. Invoking the regex engine in these blocks would make perl -unstable. =item C<(??{ code })> X<(??{})> @@ -1011,61 +1381,60 @@ experimental, and may be changed without notice. Code executed that has side effects may not perform identically from version to version due to the effect of future optimisations in the regex engine. -This is a "postponed" regular subexpression. The C is evaluated -at run time, at the moment this subexpression may match. The result -of evaluation is considered as a regular expression and matched as -if it were inserted instead of this construct. Note that this means -that the contents of capture groups defined inside an eval'ed pattern -are not available outside of the pattern, and vice versa, there is no -way for the inner pattern to refer to a capture group defined outside. -Thus, +This is a "postponed" regular subexpression. It behaves in I the +same way as a C<(?{ code })> code block as described above, except that +its return value, rather than being assigned to C<$^R>, is treated as a +pattern, compiled if it's a string (or used as-is if its a qr// object), +then matched as if it were inserted instead of this construct. - ('a' x 100)=~/(??{'(.)' x 100})/ +During the matching of this sub-pattern, it has its own set of +captures which are valid during the sub-match, but are discarded once +control returns to the main pattern. For example, the following matches, +with the inner pattern capturing "B" and matching "BB", while the outer +pattern captures "A"; -B match, it will B set $1. + my $inner = '(.)\1'; + "ABBA" =~ /^(.)(??{ $inner })\1/; + print $1; # prints "A"; -The C is not interpolated. As before, the rules to determine -where the C ends are currently somewhat convoluted. +Note that this means that there is no way for the inner pattern to refer +to a capture group defined outside. (The code block itself can use C<$1>, +etc., to refer to the enclosing pattern's capture groups.) Thus, although + + ('a' x 100)=~/(??{'(.)' x 100})/ + +I match, it will I set $1 on exit. The following pattern matches a parenthesized group: - $re = qr{ - \( - (?: - (?> [^()]+ ) # Non-parens without backtracking - | - (??{ $re }) # Group with matching parens - )* - \) - }x; + $re = qr{ + \( + (?: + (?> [^()]+ ) # Non-parens without backtracking + | + (??{ $re }) # Group with matching parens + )* + \) + }x; See also C<(?PARNO)> for a different, more efficient way to accomplish the same task. -For reasons of security, this construct is forbidden if the regular -expression involves run-time interpolation of variables, unless the -perilous C pragma has been used (see L), or the -variables contain results of C operator (see -LSTRINGEmsixpo">). - -In perl 5.12.x and earlier, because the regex engine was not re-entrant, -delayed code could not safely invoke the regex engine either directly with -C or C), or indirectly with functions such as C. - -Recursing deeper than 50 times without consuming any input string will -result in a fatal error. The maximum depth is compiled into perl, so -changing it requires a custom build. +Executing a postponed regular expression 50 times without consuming any +input string will result in a fatal error. The maximum depth is compiled +into perl, so changing it requires a custom build. =item C<(?PARNO)> C<(?-PARNO)> C<(?+PARNO)> C<(?R)> C<(?0)> X<(?PARNO)> X<(?1)> X<(?R)> X<(?0)> X<(?-1)> X<(?+1)> X<(?-PARNO)> X<(?+PARNO)> X X X X -Similar to C<(??{ code })> except it does not involve compiling any code, -instead it treats the contents of a capture group as an independent -pattern that must match at the current position. Capture groups -contained by the pattern will have the value as determined by the -outermost recursion. +Similar to C<(??{ code })> except that it does not involve executing any +code or potentially compiling a returned pattern string; instead it treats +the part of the current pattern contained within a specified capture group +as an independent pattern that must match at the current position. +Capture groups contained by the pattern will have the value as determined +by the outermost recursion. PARNO is a sequence of digits (not starting with 0) whose value reflects the paren-number of the capture group to recurse to. C<(?R)> recurses to @@ -1081,15 +1450,15 @@ included. The following pattern matches a function foo() which may contain balanced parentheses as the argument. - $re = qr{ ( # paren group 1 (full function) + $re = qr{ ( # paren group 1 (full function) foo - ( # paren group 2 (parens) + ( # paren group 2 (parens) \( - ( # paren group 3 (contents of parens) + ( # paren group 3 (contents of parens) (?: - (?> [^()]+ ) # Non-parens without backtracking + (?> [^()]+ ) # Non-parens without backtracking | - (?2) # Recurse to start of paren group 2 + (?2) # Recurse to start of paren group 2 )* ) \) @@ -1120,7 +1489,7 @@ easier to embed recursive patterns inside of a C construct for later use: my $parens = qr/(\((?:[^()]++|(?-1))*+\))/; - if (/foo $parens \s+ + \s+ bar $parens/x) { + if (/foo $parens \s+ \+ \s+ bar $parens/x) { # do something here... } @@ -1150,11 +1519,15 @@ X<(?()> =item C<(?(condition)yes-pattern)> -Conditional expression. C<(condition)> should be either an integer in +Conditional expression. Matches C if C yields +a true value, matches C otherwise. A missing pattern always +matches. + +C<(condition)> should be one of: 1) an integer in parentheses (which is valid if the corresponding pair of parentheses -matched), a look-ahead/look-behind/evaluate zero-width assertion, a +matched); 2) a look-ahead/look-behind/evaluate zero-width assertion; 3) a name in angle brackets or single quotes (which is valid if a group -with the given name matched), or the special symbol (R) (true when +with the given name matched); or 4) the special symbol (R) (true when evaluated inside of recursion or eval). Additionally the R may be followed by a number, (which will be true when evaluated when recursing inside of the appropriate group), or by C<&NAME>, in which case it will @@ -1172,9 +1545,14 @@ Checks if the numbered capturing group has matched something. Checks if a group with the given name has matched something. +=item (?=...) (?!...) (?<=...) (? predicate, which never executes directly -its yes-pattern, and does not allow a no-pattern. This allows to define -subpatterns which will be executed only by using the recursion mechanism. +A special form is the C<(DEFINE)> predicate, which never executes its +yes-pattern directly, and does not allow a no-pattern. This allows one to +define subpatterns which will be executed only by the recursion mechanism. This way, you can define a set of regular expression rules that can be bundled into any pattern you choose. @@ -1240,6 +1618,19 @@ after the recursion returns, so the extra layer of capturing groups is necessary. Thus C<$+{NAME_PAT}> would not be defined even though C<$+{NAME}> would be. +Finally, keep in mind that subpatterns created inside a DEFINE block +count towards the absolute and relative number of captures, so this: + + my @captures = "a" =~ /(.) # First capture + (?(DEFINE) + (? 1 ) # Second capture + )/x; + say scalar @captures; + +Will output 2, not 1. This is particularly important if you intend to +compile the definitions with the C operator, and later +interpolate them in another pattern. + =item C<< (?>pattern) >> X X X X @@ -1260,9 +1651,13 @@ group C (see L<"Backtracking">). In particular, C inside C will match fewer characters than a standalone C, since this makes the tail match. +C<< (?>pattern) >> does not disable backtracking altogether once it has +matched. It is still possible to backtrack past the construct, but not +into it. So C<< ((?>a*)|(?>b*))ar >> will still match "bar". + An effect similar to C<< (?>pattern) >> may be achieved by writing -C<(?=(pattern))\g1>. This matches the same substring as a standalone -C, and the following C<\g1> eats the matched string; it therefore +C<(?=(pattern))\g{-1}>. This matches the same substring as a standalone +C, and the following C<\g{-1}> eats the matched string; it therefore makes a zero-length assertion into an analogue of C<< (?>...) >>. (The difference between these two constructs is that the second one uses a capturing group, thus shifting ordinals of backreferences @@ -1302,7 +1697,8 @@ hung. However, a tiny change to this pattern which uses C<< (?>...) >> matches exactly when the one above does (verifying this yourself would be a productive exercise), but finishes in a fourth the time when used on a similar string with 1000000 Cs. Be aware, -however, that this pattern currently triggers a warning message under +however, that, when this construct is followed by a +quantifier, it currently triggers a warning message under the C pragma or B<-w> switch saying it C<"matches null string many times in regex">. @@ -1373,14 +1769,14 @@ C<(*MARK:NAME)> pattern executed. See the explanation for the C<(*MARK:NAME)> verb below for more details. B C<$REGERROR> and C<$REGMARK> are not magic variables like C<$1> -and most other regex related variables. They are not local to a scope, nor +and most other regex-related variables. They are not local to a scope, nor readonly, but instead are volatile package variables similar to C<$AUTOLOAD>. Use C to localize changes to them to a specific scope if necessary. If a pattern does not contain a special backtracking verb that allows an argument, then C<$REGERROR> and C<$REGMARK> are not touched at all. -=over 4 +=over 3 =item Verbs that take an argument @@ -1421,7 +1817,7 @@ If we add a C<(*PRUNE)> before the count like the following 'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/; print "Count=$count\n"; -we prevent backtracking and find the count of the longest matching +we prevent backtracking and find the count of the longest matching string at each matching starting point like so: aaab @@ -1437,7 +1833,6 @@ replaced with a C<< (?>pattern) >> with no functional difference; however, C<(*PRUNE)> can be used to handle cases that cannot be expressed using a C<< (?>pattern) >> alone. - =item C<(*SKIP)> C<(*SKIP:NAME)> X<(*SKIP)> @@ -1455,11 +1850,11 @@ encountered, then the C<(*SKIP)> operator has no effect. When used without a name the "skip point" is where the match point was when executing the (*SKIP) pattern. -Compare the following to the examples in C<(*PRUNE)>, note the string +Compare the following to the examples in C<(*PRUNE)>; note the string is twice as long: - 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/; - print "Count=$count\n"; + 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/; + print "Count=$count\n"; outputs @@ -1472,7 +1867,7 @@ executed, the next starting point will be where the cursor was when the C<(*SKIP)> was executed. =item C<(*MARK:NAME)> C<(*:NAME)> -X<(*MARK)> C<(*MARK:NAME)> C<(*:NAME)> +X<(*MARK)> X<(*MARK:NAME)> X<(*:NAME)> This zero-width pattern can be used to mark the point reached in a string when a certain part of the pattern has been successfully matched. This @@ -1500,16 +1895,18 @@ failing the match and has provided its own name to use, the C<$REGERROR> variable will be set to the name of the most recently executed C<(*MARK:NAME)>. -See C<(*SKIP)> for more details. +See L(*SKIP)> for more details. As a shortcut C<(*MARK:NAME)> can be written C<(*:NAME)>. =item C<(*THEN)> C<(*THEN:NAME)> -This is similar to the "cut group" operator C<::> from Perl 6. Like +This is similar to the "cut group" operator C<::> from Perl 6. Like C<(*PRUNE)>, this verb always matches, and when backtracked into on failure, it causes the regex engine to try the next alternation in the -innermost enclosing group (capturing or otherwise). +innermost enclosing group (capturing or otherwise) that has alternations. +The two branches of a C<(?(condition)yes-pattern|no-pattern)> do not +count as an alternation, as far as C<(*THEN)> is concerned. Its name comes from the observation that this operation combined with the alternation operator (C<|>) can be used to create what is essentially a @@ -1528,15 +1925,21 @@ is the same as but - / ( A (*THEN) B | C (*THEN) D ) / + / ( A (*THEN) B | C ) / is not the same as - / ( A (*PRUNE) B | C (*PRUNE) D ) / + / ( A (*PRUNE) B | C ) / as after matching the A but failing on the B the C<(*THEN)> verb will backtrack and try C; but the C<(*PRUNE)> verb will simply fail. +=back + +=item Verbs without an argument + +=over 4 + =item C<(*COMMIT)> X<(*COMMIT)> @@ -1546,8 +1949,8 @@ into on failure it causes the match to fail outright. No further attempts to find a valid match by advancing the start pointer will occur again. For example, - 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/; - print "Count=$count\n"; + 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/; + print "Count=$count\n"; outputs @@ -1558,12 +1961,6 @@ In other words, once the C<(*COMMIT)> has been entered, and if the pattern does not match, the regex engine will not try any further matching on the rest of the string. -=back - -=item Verbs without an argument - -=over 4 - =item C<(*FAIL)> C<(*F)> X<(*FAIL)> X<(*F)> @@ -1592,7 +1989,7 @@ For instance: 'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x; will match, and C<$1> will be C and C<$2> will be C, C<$3> will not -be set. If another branch in the inner parentheses were matched, such as in the +be set. If another branch in the inner parentheses was matched, such as in the string 'ACDE', then the C and C would have to be matched as well. =back @@ -1751,7 +2148,7 @@ let C<\D*> expand to "ABC", this would have caused the whole pattern to fail. The search engine will initially match C<\D*> with "ABC". Then it will -try to match C<(?!123> with "123", which fails. But because +try to match C<(?!123)> with "123", which fails. But because a quantifier (C<\D*>) has been used in the regular expression, the search engine can backtrack and retry the match differently in the hope of matching the complete regular expression. @@ -1799,12 +2196,12 @@ match takes a long time to finish. A powerful tool for optimizing such beasts is what is known as an "independent group", -which does not backtrack (see Lpattern) >>>). Note also that +which does not backtrack (see Lpattern) >>>). Note also that zero-length look-ahead/look-behind assertions will not backtrack to make the tail match, since they are in "logical" context: only whether they match is considered relevant. For an example where side-effects of look-ahead I have influenced the -following match, see Lpattern) >>>. +following match, see Lpattern) >>>. =head2 Version 8 Regular Expressions X X X @@ -1820,7 +2217,7 @@ character; "\\" matches a "\"). This escape mechanism is also required for the character used as the pattern delimiter. A series of characters matches that series of characters in the target -string, so the pattern C would match "blurfl" in the target +string, so the pattern C would match "blurfl" in the target string. You can specify a character class, by enclosing a list of characters @@ -1859,9 +2256,9 @@ You can specify a series of alternatives for a pattern using "|" to separate them, so that C will match any of "fee", "fie", or "foe" in the target string (as would C). The first alternative includes everything from the last pattern delimiter -("(", "[", or the beginning of the pattern) up to the first "|", and +("(", "(?:", etc. or the beginning of the pattern) up to the first "|", and the last alternative contains everything from the last "|" to the next -pattern delimiter. That's why it's common practice to include +closing pattern delimiter. That's why it's common practice to include alternatives in parentheses: to minimize confusion about where they start and end. @@ -1879,7 +2276,7 @@ so if you write C<[fee|fie|foe]> you're really only matching C<[feio|]>. Within a pattern, you may designate subpatterns for later reference by enclosing them in parentheses, and you may refer back to the Ith subpattern later in the pattern using the metacharacter -\I. Subpatterns are numbered based on the left to right order +\I or \gI. Subpatterns are numbered based on the left to right order of their opening parenthesis. A backreference matches whatever actually matched the subpattern in the string being examined, not the rules for that subpattern. Therefore, C<(0|0x)\d*\s\g1\d*> will @@ -1943,7 +2340,7 @@ However, long experience has shown that many programming tasks may be significantly simplified by using repeated subexpressions that may match zero-length substrings. Here's a simple example being: - @chars = split //, $string; # // is not magic in split + @chars = split //, $string; # // is not magic in split ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// / Thus Perl allows such constructs, by I matches a zero-length string, it stops +the C<*>. + +The higher-level loops preserve an additional state between iterations: whether the last match was zero-length. To break the loop, the following match after a zero-length match is prohibited to have a length of zero. This prohibition interacts with backtracking (see L<"Backtracking">), @@ -1995,7 +2414,7 @@ Each of the elementary pieces of regular expressions which were described before (such as C or C<\Z>) could match at most one substring at the given position of the input string. However, in a typical regular expression these elementary pieces are combined into more complicated -patterns using combining operators C, C, C etc +patterns using combining operators C, C, C etc. (in these examples C and C are regular subexpressions). Such combinations can include alternatives, leading to a problem of choice: @@ -2024,11 +2443,11 @@ Consider two possible matches, C and C, C and C are substrings which can be matched by C, C and C are substrings which can be matched by C. -If C is better match for C than C, C is a better +If C is a better match for C than C, C is a better match than C. If C and C coincide: C is a better match than C if -C is better match for C than C. +C is a better match for C than C. =item C @@ -2092,8 +2511,13 @@ than a match at a later position. =head2 Creating Custom RE Engines -Overloaded constants (see L) provide a simple way to extend -the functionality of the RE engine. +As of Perl 5.10.0, one can create custom regular expression engines. This +is not for the faint of heart, as they have to plug in at the C level. See +L for more details. + +As an alternative, overloaded constants (see L) provide a simple +way to extend the functionality of the RE engine, by substituting one +pattern for another. Suppose that we want to enable a new RE escape-sequence C<\Y|> which matches at a boundary between whitespace characters and non-whitespace @@ -2139,11 +2563,11 @@ part of this regular expression needs to be converted explicitly $re = customre::convert $re; /\Y|$re\Y|/; -=head1 PCRE/Python Support +=head2 PCRE/Python Support -As of Perl 5.10.0, Perl supports several Python/PCRE specific extensions +As of Perl 5.10.0, Perl supports several Python/PCRE-specific extensions to the regex syntax. While Perl programmers are encouraged to use the -Perl specific syntax, the following are also accepted: +Perl-specific syntax, the following are also accepted: =over 4 @@ -2163,17 +2587,11 @@ Subroutine call to a named capture group. Equivalent to C<< (?&NAME) >>. =head1 BUGS -There are numerous problems with case insensitive matching of characters -outside the ASCII range, especially with those whose folds are multiple -characters, such as ligatures like C. - -In a bracketed character class with case insensitive matching, ranges only work -for ASCII characters. For example, -C -doesn't match all the Russian upper and lower case letters. - Many regular expression constructs don't work on EBCDIC platforms. +There are a number of issues with regard to case-insensitive matching +in Unicode rules. See C under L above. + This document varies from difficult to understand to completely and utterly opaque. The wandering prose riddled with jargon is hard to fathom in several places.
+is put into the special variable C<$^R>. This happens immediately, so +C<$^R> can be used from other C<(?{ code })> assertions inside the same +regular expression. The assignment to C<$^R> above is properly localized, so the old value of C<$^R> is restored if the assertion is backtracked; compare L<"Backtracking">. -For reasons of security, this construct is forbidden if the regular -expression involves run-time interpolation of variables, unless the -perilous C pragma has been used (see L), or the -variables contain results of C operator (see -Lmsixpo">). +Note that the special variable C<$^N> is particularly useful with code +blocks to capture the results of submatches in variables without having to +keep track of the number of nested parentheses. For example: -This restriction is due to the wide-spread and remarkably convenient -custom of using run-time determined strings as patterns. For example: + $_ = "The brown fox jumps over the lazy dog"; + /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i; + print "color = $color, animal = $animal\n"; - $re = <>; - chomp $re; - $string =~ /$re/; - -Before Perl knew how to execute interpolated code within a pattern, -this operation was completely safe from a security point of view, -although it could raise an exception from an illegal pattern. If -you turn on the C, though, it is no longer secure, -so you should only do so if you are also using taint checking. -Better yet, use the carefully constrained evaluation within a Safe -compartment. See L for details about both these mechanisms. - -B: Use of lexical (C) variables in these blocks is -broken. The result is unpredictable and will make perl unstable. The -workaround is to use global (C) variables. - -B: In perl 5.12.x and earlier, the regex engine -was not re-entrant, so interpolated code could not -safely invoke the regex engine either directly with -C or C), or indirectly with functions such as -C. Invoking the regex engine in these blocks would make perl -unstable. =item C<(??{ code })> X<(??{})> @@ -1011,61 +1381,60 @@ experimental, and may be changed without notice. Code executed that has side effects may not perform identically from version to version due to the effect of future optimisations in the regex engine. -This is a "postponed" regular subexpression. The C is evaluated -at run time, at the moment this subexpression may match. The result -of evaluation is considered as a regular expression and matched as -if it were inserted instead of this construct. Note that this means -that the contents of capture groups defined inside an eval'ed pattern -are not available outside of the pattern, and vice versa, there is no -way for the inner pattern to refer to a capture group defined outside. -Thus, +This is a "postponed" regular subexpression. It behaves in I the +same way as a C<(?{ code })> code block as described above, except that +its return value, rather than being assigned to C<$^R>, is treated as a +pattern, compiled if it's a string (or used as-is if its a qr// object), +then matched as if it were inserted instead of this construct. - ('a' x 100)=~/(??{'(.)' x 100})/ +During the matching of this sub-pattern, it has its own set of +captures which are valid during the sub-match, but are discarded once +control returns to the main pattern. For example, the following matches, +with the inner pattern capturing "B" and matching "BB", while the outer +pattern captures "A"; -B match, it will B set $1. + my $inner = '(.)\1'; + "ABBA" =~ /^(.)(??{ $inner })\1/; + print $1; # prints "A"; -The C is not interpolated. As before, the rules to determine -where the C ends are currently somewhat convoluted. +Note that this means that there is no way for the inner pattern to refer +to a capture group defined outside. (The code block itself can use C<$1>, +etc., to refer to the enclosing pattern's capture groups.) Thus, although + + ('a' x 100)=~/(??{'(.)' x 100})/ + +I match, it will I set $1 on exit. The following pattern matches a parenthesized group: - $re = qr{ - \( - (?: - (?> [^()]+ ) # Non-parens without backtracking - | - (??{ $re }) # Group with matching parens - )* - \) - }x; + $re = qr{ + \( + (?: + (?> [^()]+ ) # Non-parens without backtracking + | + (??{ $re }) # Group with matching parens + )* + \) + }x; See also C<(?PARNO)> for a different, more efficient way to accomplish the same task. -For reasons of security, this construct is forbidden if the regular -expression involves run-time interpolation of variables, unless the -perilous C pragma has been used (see L), or the -variables contain results of C operator (see -LSTRINGEmsixpo">). - -In perl 5.12.x and earlier, because the regex engine was not re-entrant, -delayed code could not safely invoke the regex engine either directly with -C or C), or indirectly with functions such as C. - -Recursing deeper than 50 times without consuming any input string will -result in a fatal error. The maximum depth is compiled into perl, so -changing it requires a custom build. +Executing a postponed regular expression 50 times without consuming any +input string will result in a fatal error. The maximum depth is compiled +into perl, so changing it requires a custom build. =item C<(?PARNO)> C<(?-PARNO)> C<(?+PARNO)> C<(?R)> C<(?0)> X<(?PARNO)> X<(?1)> X<(?R)> X<(?0)> X<(?-1)> X<(?+1)> X<(?-PARNO)> X<(?+PARNO)> X X X X -Similar to C<(??{ code })> except it does not involve compiling any code, -instead it treats the contents of a capture group as an independent -pattern that must match at the current position. Capture groups -contained by the pattern will have the value as determined by the -outermost recursion. +Similar to C<(??{ code })> except that it does not involve executing any +code or potentially compiling a returned pattern string; instead it treats +the part of the current pattern contained within a specified capture group +as an independent pattern that must match at the current position. +Capture groups contained by the pattern will have the value as determined +by the outermost recursion. PARNO is a sequence of digits (not starting with 0) whose value reflects the paren-number of the capture group to recurse to. C<(?R)> recurses to @@ -1081,15 +1450,15 @@ included. The following pattern matches a function foo() which may contain balanced parentheses as the argument. - $re = qr{ ( # paren group 1 (full function) + $re = qr{ ( # paren group 1 (full function) foo - ( # paren group 2 (parens) + ( # paren group 2 (parens) \( - ( # paren group 3 (contents of parens) + ( # paren group 3 (contents of parens) (?: - (?> [^()]+ ) # Non-parens without backtracking + (?> [^()]+ ) # Non-parens without backtracking | - (?2) # Recurse to start of paren group 2 + (?2) # Recurse to start of paren group 2 )* ) \) @@ -1120,7 +1489,7 @@ easier to embed recursive patterns inside of a C construct for later use: my $parens = qr/(\((?:[^()]++|(?-1))*+\))/; - if (/foo $parens \s+ + \s+ bar $parens/x) { + if (/foo $parens \s+ \+ \s+ bar $parens/x) { # do something here... } @@ -1150,11 +1519,15 @@ X<(?()> =item C<(?(condition)yes-pattern)> -Conditional expression. C<(condition)> should be either an integer in +Conditional expression. Matches C if C yields +a true value, matches C otherwise. A missing pattern always +matches. + +C<(condition)> should be one of: 1) an integer in parentheses (which is valid if the corresponding pair of parentheses -matched), a look-ahead/look-behind/evaluate zero-width assertion, a +matched); 2) a look-ahead/look-behind/evaluate zero-width assertion; 3) a name in angle brackets or single quotes (which is valid if a group -with the given name matched), or the special symbol (R) (true when +with the given name matched); or 4) the special symbol (R) (true when evaluated inside of recursion or eval). Additionally the R may be followed by a number, (which will be true when evaluated when recursing inside of the appropriate group), or by C<&NAME>, in which case it will @@ -1172,9 +1545,14 @@ Checks if the numbered capturing group has matched something. Checks if a group with the given name has matched something. +=item (?=...) (?!...) (?<=...) (? predicate, which never executes directly -its yes-pattern, and does not allow a no-pattern. This allows to define -subpatterns which will be executed only by using the recursion mechanism. +A special form is the C<(DEFINE)> predicate, which never executes its +yes-pattern directly, and does not allow a no-pattern. This allows one to +define subpatterns which will be executed only by the recursion mechanism. This way, you can define a set of regular expression rules that can be bundled into any pattern you choose. @@ -1240,6 +1618,19 @@ after the recursion returns, so the extra layer of capturing groups is necessary. Thus C<$+{NAME_PAT}> would not be defined even though C<$+{NAME}> would be. +Finally, keep in mind that subpatterns created inside a DEFINE block +count towards the absolute and relative number of captures, so this: + + my @captures = "a" =~ /(.) # First capture + (?(DEFINE) + (? 1 ) # Second capture + )/x; + say scalar @captures; + +Will output 2, not 1. This is particularly important if you intend to +compile the definitions with the C operator, and later +interpolate them in another pattern. + =item C<< (?>pattern) >> X X X X @@ -1260,9 +1651,13 @@ group C (see L<"Backtracking">). In particular, C inside C will match fewer characters than a standalone C, since this makes the tail match. +C<< (?>pattern) >> does not disable backtracking altogether once it has +matched. It is still possible to backtrack past the construct, but not +into it. So C<< ((?>a*)|(?>b*))ar >> will still match "bar". + An effect similar to C<< (?>pattern) >> may be achieved by writing -C<(?=(pattern))\g1>. This matches the same substring as a standalone -C, and the following C<\g1> eats the matched string; it therefore +C<(?=(pattern))\g{-1}>. This matches the same substring as a standalone +C, and the following C<\g{-1}> eats the matched string; it therefore makes a zero-length assertion into an analogue of C<< (?>...) >>. (The difference between these two constructs is that the second one uses a capturing group, thus shifting ordinals of backreferences @@ -1302,7 +1697,8 @@ hung. However, a tiny change to this pattern which uses C<< (?>...) >> matches exactly when the one above does (verifying this yourself would be a productive exercise), but finishes in a fourth the time when used on a similar string with 1000000 Cs. Be aware, -however, that this pattern currently triggers a warning message under +however, that, when this construct is followed by a +quantifier, it currently triggers a warning message under the C pragma or B<-w> switch saying it C<"matches null string many times in regex">. @@ -1373,14 +1769,14 @@ C<(*MARK:NAME)> pattern executed. See the explanation for the C<(*MARK:NAME)> verb below for more details. B C<$REGERROR> and C<$REGMARK> are not magic variables like C<$1> -and most other regex related variables. They are not local to a scope, nor +and most other regex-related variables. They are not local to a scope, nor readonly, but instead are volatile package variables similar to C<$AUTOLOAD>. Use C to localize changes to them to a specific scope if necessary. If a pattern does not contain a special backtracking verb that allows an argument, then C<$REGERROR> and C<$REGMARK> are not touched at all. -=over 4 +=over 3 =item Verbs that take an argument @@ -1421,7 +1817,7 @@ If we add a C<(*PRUNE)> before the count like the following 'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/; print "Count=$count\n"; -we prevent backtracking and find the count of the longest matching +we prevent backtracking and find the count of the longest matching string at each matching starting point like so: aaab @@ -1437,7 +1833,6 @@ replaced with a C<< (?>pattern) >> with no functional difference; however, C<(*PRUNE)> can be used to handle cases that cannot be expressed using a C<< (?>pattern) >> alone. - =item C<(*SKIP)> C<(*SKIP:NAME)> X<(*SKIP)> @@ -1455,11 +1850,11 @@ encountered, then the C<(*SKIP)> operator has no effect. When used without a name the "skip point" is where the match point was when executing the (*SKIP) pattern. -Compare the following to the examples in C<(*PRUNE)>, note the string +Compare the following to the examples in C<(*PRUNE)>; note the string is twice as long: - 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/; - print "Count=$count\n"; + 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/; + print "Count=$count\n"; outputs @@ -1472,7 +1867,7 @@ executed, the next starting point will be where the cursor was when the C<(*SKIP)> was executed. =item C<(*MARK:NAME)> C<(*:NAME)> -X<(*MARK)> C<(*MARK:NAME)> C<(*:NAME)> +X<(*MARK)> X<(*MARK:NAME)> X<(*:NAME)> This zero-width pattern can be used to mark the point reached in a string when a certain part of the pattern has been successfully matched. This @@ -1500,16 +1895,18 @@ failing the match and has provided its own name to use, the C<$REGERROR> variable will be set to the name of the most recently executed C<(*MARK:NAME)>. -See C<(*SKIP)> for more details. +See L(*SKIP)> for more details. As a shortcut C<(*MARK:NAME)> can be written C<(*:NAME)>. =item C<(*THEN)> C<(*THEN:NAME)> -This is similar to the "cut group" operator C<::> from Perl 6. Like +This is similar to the "cut group" operator C<::> from Perl 6. Like C<(*PRUNE)>, this verb always matches, and when backtracked into on failure, it causes the regex engine to try the next alternation in the -innermost enclosing group (capturing or otherwise). +innermost enclosing group (capturing or otherwise) that has alternations. +The two branches of a C<(?(condition)yes-pattern|no-pattern)> do not +count as an alternation, as far as C<(*THEN)> is concerned. Its name comes from the observation that this operation combined with the alternation operator (C<|>) can be used to create what is essentially a @@ -1528,15 +1925,21 @@ is the same as but - / ( A (*THEN) B | C (*THEN) D ) / + / ( A (*THEN) B | C ) / is not the same as - / ( A (*PRUNE) B | C (*PRUNE) D ) / + / ( A (*PRUNE) B | C ) / as after matching the A but failing on the B the C<(*THEN)> verb will backtrack and try C; but the C<(*PRUNE)> verb will simply fail. +=back + +=item Verbs without an argument + +=over 4 + =item C<(*COMMIT)> X<(*COMMIT)> @@ -1546,8 +1949,8 @@ into on failure it causes the match to fail outright. No further attempts to find a valid match by advancing the start pointer will occur again. For example, - 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/; - print "Count=$count\n"; + 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/; + print "Count=$count\n"; outputs @@ -1558,12 +1961,6 @@ In other words, once the C<(*COMMIT)> has been entered, and if the pattern does not match, the regex engine will not try any further matching on the rest of the string. -=back - -=item Verbs without an argument - -=over 4 - =item C<(*FAIL)> C<(*F)> X<(*FAIL)> X<(*F)> @@ -1592,7 +1989,7 @@ For instance: 'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x; will match, and C<$1> will be C and C<$2> will be C, C<$3> will not -be set. If another branch in the inner parentheses were matched, such as in the +be set. If another branch in the inner parentheses was matched, such as in the string 'ACDE', then the C and C would have to be matched as well. =back @@ -1751,7 +2148,7 @@ let C<\D*> expand to "ABC", this would have caused the whole pattern to fail. The search engine will initially match C<\D*> with "ABC". Then it will -try to match C<(?!123> with "123", which fails. But because +try to match C<(?!123)> with "123", which fails. But because a quantifier (C<\D*>) has been used in the regular expression, the search engine can backtrack and retry the match differently in the hope of matching the complete regular expression. @@ -1799,12 +2196,12 @@ match takes a long time to finish. A powerful tool for optimizing such beasts is what is known as an "independent group", -which does not backtrack (see Lpattern) >>>). Note also that +which does not backtrack (see Lpattern) >>>). Note also that zero-length look-ahead/look-behind assertions will not backtrack to make the tail match, since they are in "logical" context: only whether they match is considered relevant. For an example where side-effects of look-ahead I have influenced the -following match, see Lpattern) >>>. +following match, see Lpattern) >>>. =head2 Version 8 Regular Expressions X X X @@ -1820,7 +2217,7 @@ character; "\\" matches a "\"). This escape mechanism is also required for the character used as the pattern delimiter. A series of characters matches that series of characters in the target -string, so the pattern C would match "blurfl" in the target +string, so the pattern C would match "blurfl" in the target string. You can specify a character class, by enclosing a list of characters @@ -1859,9 +2256,9 @@ You can specify a series of alternatives for a pattern using "|" to separate them, so that C will match any of "fee", "fie", or "foe" in the target string (as would C). The first alternative includes everything from the last pattern delimiter -("(", "[", or the beginning of the pattern) up to the first "|", and +("(", "(?:", etc. or the beginning of the pattern) up to the first "|", and the last alternative contains everything from the last "|" to the next -pattern delimiter. That's why it's common practice to include +closing pattern delimiter. That's why it's common practice to include alternatives in parentheses: to minimize confusion about where they start and end. @@ -1879,7 +2276,7 @@ so if you write C<[fee|fie|foe]> you're really only matching C<[feio|]>. Within a pattern, you may designate subpatterns for later reference by enclosing them in parentheses, and you may refer back to the Ith subpattern later in the pattern using the metacharacter -\I. Subpatterns are numbered based on the left to right order +\I or \gI. Subpatterns are numbered based on the left to right order of their opening parenthesis. A backreference matches whatever actually matched the subpattern in the string being examined, not the rules for that subpattern. Therefore, C<(0|0x)\d*\s\g1\d*> will @@ -1943,7 +2340,7 @@ However, long experience has shown that many programming tasks may be significantly simplified by using repeated subexpressions that may match zero-length substrings. Here's a simple example being: - @chars = split //, $string; # // is not magic in split + @chars = split //, $string; # // is not magic in split ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// / Thus Perl allows such constructs, by I matches a zero-length string, it stops +the C<*>. + +The higher-level loops preserve an additional state between iterations: whether the last match was zero-length. To break the loop, the following match after a zero-length match is prohibited to have a length of zero. This prohibition interacts with backtracking (see L<"Backtracking">), @@ -1995,7 +2414,7 @@ Each of the elementary pieces of regular expressions which were described before (such as C or C<\Z>) could match at most one substring at the given position of the input string. However, in a typical regular expression these elementary pieces are combined into more complicated -patterns using combining operators C, C, C etc +patterns using combining operators C, C, C etc. (in these examples C and C are regular subexpressions). Such combinations can include alternatives, leading to a problem of choice: @@ -2024,11 +2443,11 @@ Consider two possible matches, C and C, C and C are substrings which can be matched by C, C and C are substrings which can be matched by C. -If C is better match for C than C, C is a better +If C is a better match for C than C, C is a better match than C. If C and C coincide: C is a better match than C if -C is better match for C than C. +C is a better match for C than C. =item C @@ -2092,8 +2511,13 @@ than a match at a later position. =head2 Creating Custom RE Engines -Overloaded constants (see L) provide a simple way to extend -the functionality of the RE engine. +As of Perl 5.10.0, one can create custom regular expression engines. This +is not for the faint of heart, as they have to plug in at the C level. See +L for more details. + +As an alternative, overloaded constants (see L) provide a simple +way to extend the functionality of the RE engine, by substituting one +pattern for another. Suppose that we want to enable a new RE escape-sequence C<\Y|> which matches at a boundary between whitespace characters and non-whitespace @@ -2139,11 +2563,11 @@ part of this regular expression needs to be converted explicitly $re = customre::convert $re; /\Y|$re\Y|/; -=head1 PCRE/Python Support +=head2 PCRE/Python Support -As of Perl 5.10.0, Perl supports several Python/PCRE specific extensions +As of Perl 5.10.0, Perl supports several Python/PCRE-specific extensions to the regex syntax. While Perl programmers are encouraged to use the -Perl specific syntax, the following are also accepted: +Perl-specific syntax, the following are also accepted: =over 4 @@ -2163,17 +2587,11 @@ Subroutine call to a named capture group. Equivalent to C<< (?&NAME) >>. =head1 BUGS -There are numerous problems with case insensitive matching of characters -outside the ASCII range, especially with those whose folds are multiple -characters, such as ligatures like C. - -In a bracketed character class with case insensitive matching, ranges only work -for ASCII characters. For example, -C -doesn't match all the Russian upper and lower case letters. - Many regular expression constructs don't work on EBCDIC platforms. +There are a number of issues with regard to case-insensitive matching +in Unicode rules. See C under L above. + This document varies from difficult to understand to completely and utterly opaque. The wandering prose riddled with jargon is hard to fathom in several places.
is evaluated -at run time, at the moment this subexpression may match. The result -of evaluation is considered as a regular expression and matched as -if it were inserted instead of this construct. Note that this means -that the contents of capture groups defined inside an eval'ed pattern -are not available outside of the pattern, and vice versa, there is no -way for the inner pattern to refer to a capture group defined outside. -Thus, +This is a "postponed" regular subexpression. It behaves in I the +same way as a C<(?{ code })> code block as described above, except that +its return value, rather than being assigned to C<$^R>, is treated as a +pattern, compiled if it's a string (or used as-is if its a qr// object), +then matched as if it were inserted instead of this construct. - ('a' x 100)=~/(??{'(.)' x 100})/ +During the matching of this sub-pattern, it has its own set of +captures which are valid during the sub-match, but are discarded once +control returns to the main pattern. For example, the following matches, +with the inner pattern capturing "B" and matching "BB", while the outer +pattern captures "A"; -B match, it will B set $1. + my $inner = '(.)\1'; + "ABBA" =~ /^(.)(??{ $inner })\1/; + print $1; # prints "A"; -The C is not interpolated. As before, the rules to determine -where the C ends are currently somewhat convoluted. +Note that this means that there is no way for the inner pattern to refer +to a capture group defined outside. (The code block itself can use C<$1>, +etc., to refer to the enclosing pattern's capture groups.) Thus, although + + ('a' x 100)=~/(??{'(.)' x 100})/ + +I match, it will I set $1 on exit. The following pattern matches a parenthesized group: - $re = qr{ - \( - (?: - (?> [^()]+ ) # Non-parens without backtracking - | - (??{ $re }) # Group with matching parens - )* - \) - }x; + $re = qr{ + \( + (?: + (?> [^()]+ ) # Non-parens without backtracking + | + (??{ $re }) # Group with matching parens + )* + \) + }x; See also C<(?PARNO)> for a different, more efficient way to accomplish the same task. -For reasons of security, this construct is forbidden if the regular -expression involves run-time interpolation of variables, unless the -perilous C pragma has been used (see L), or the -variables contain results of C operator (see -LSTRINGEmsixpo">). - -In perl 5.12.x and earlier, because the regex engine was not re-entrant, -delayed code could not safely invoke the regex engine either directly with -C or C), or indirectly with functions such as C. - -Recursing deeper than 50 times without consuming any input string will -result in a fatal error. The maximum depth is compiled into perl, so -changing it requires a custom build. +Executing a postponed regular expression 50 times without consuming any +input string will result in a fatal error. The maximum depth is compiled +into perl, so changing it requires a custom build. =item C<(?PARNO)> C<(?-PARNO)> C<(?+PARNO)> C<(?R)> C<(?0)> X<(?PARNO)> X<(?1)> X<(?R)> X<(?0)> X<(?-1)> X<(?+1)> X<(?-PARNO)> X<(?+PARNO)> X X X X -Similar to C<(??{ code })> except it does not involve compiling any code, -instead it treats the contents of a capture group as an independent -pattern that must match at the current position. Capture groups -contained by the pattern will have the value as determined by the -outermost recursion. +Similar to C<(??{ code })> except that it does not involve executing any +code or potentially compiling a returned pattern string; instead it treats +the part of the current pattern contained within a specified capture group +as an independent pattern that must match at the current position. +Capture groups contained by the pattern will have the value as determined +by the outermost recursion. PARNO is a sequence of digits (not starting with 0) whose value reflects the paren-number of the capture group to recurse to. C<(?R)> recurses to @@ -1081,15 +1450,15 @@ included. The following pattern matches a function foo() which may contain balanced parentheses as the argument. - $re = qr{ ( # paren group 1 (full function) + $re = qr{ ( # paren group 1 (full function) foo - ( # paren group 2 (parens) + ( # paren group 2 (parens) \( - ( # paren group 3 (contents of parens) + ( # paren group 3 (contents of parens) (?: - (?> [^()]+ ) # Non-parens without backtracking + (?> [^()]+ ) # Non-parens without backtracking | - (?2) # Recurse to start of paren group 2 + (?2) # Recurse to start of paren group 2 )* ) \) @@ -1120,7 +1489,7 @@ easier to embed recursive patterns inside of a C construct for later use: my $parens = qr/(\((?:[^()]++|(?-1))*+\))/; - if (/foo $parens \s+ + \s+ bar $parens/x) { + if (/foo $parens \s+ \+ \s+ bar $parens/x) { # do something here... } @@ -1150,11 +1519,15 @@ X<(?()> =item C<(?(condition)yes-pattern)> -Conditional expression. C<(condition)> should be either an integer in +Conditional expression. Matches C if C yields +a true value, matches C otherwise. A missing pattern always +matches. + +C<(condition)> should be one of: 1) an integer in parentheses (which is valid if the corresponding pair of parentheses -matched), a look-ahead/look-behind/evaluate zero-width assertion, a +matched); 2) a look-ahead/look-behind/evaluate zero-width assertion; 3) a name in angle brackets or single quotes (which is valid if a group -with the given name matched), or the special symbol (R) (true when +with the given name matched); or 4) the special symbol (R) (true when evaluated inside of recursion or eval). Additionally the R may be followed by a number, (which will be true when evaluated when recursing inside of the appropriate group), or by C<&NAME>, in which case it will @@ -1172,9 +1545,14 @@ Checks if the numbered capturing group has matched something. Checks if a group with the given name has matched something. +=item (?=...) (?!...) (?<=...) (? predicate, which never executes directly -its yes-pattern, and does not allow a no-pattern. This allows to define -subpatterns which will be executed only by using the recursion mechanism. +A special form is the C<(DEFINE)> predicate, which never executes its +yes-pattern directly, and does not allow a no-pattern. This allows one to +define subpatterns which will be executed only by the recursion mechanism. This way, you can define a set of regular expression rules that can be bundled into any pattern you choose. @@ -1240,6 +1618,19 @@ after the recursion returns, so the extra layer of capturing groups is necessary. Thus C<$+{NAME_PAT}> would not be defined even though C<$+{NAME}> would be. +Finally, keep in mind that subpatterns created inside a DEFINE block +count towards the absolute and relative number of captures, so this: + + my @captures = "a" =~ /(.) # First capture + (?(DEFINE) + (? 1 ) # Second capture + )/x; + say scalar @captures; + +Will output 2, not 1. This is particularly important if you intend to +compile the definitions with the C operator, and later +interpolate them in another pattern. + =item C<< (?>pattern) >> X X X X @@ -1260,9 +1651,13 @@ group C (see L<"Backtracking">). In particular, C inside C will match fewer characters than a standalone C, since this makes the tail match. +C<< (?>pattern) >> does not disable backtracking altogether once it has +matched. It is still possible to backtrack past the construct, but not +into it. So C<< ((?>a*)|(?>b*))ar >> will still match "bar". + An effect similar to C<< (?>pattern) >> may be achieved by writing -C<(?=(pattern))\g1>. This matches the same substring as a standalone -C, and the following C<\g1> eats the matched string; it therefore +C<(?=(pattern))\g{-1}>. This matches the same substring as a standalone +C, and the following C<\g{-1}> eats the matched string; it therefore makes a zero-length assertion into an analogue of C<< (?>...) >>. (The difference between these two constructs is that the second one uses a capturing group, thus shifting ordinals of backreferences @@ -1302,7 +1697,8 @@ hung. However, a tiny change to this pattern which uses C<< (?>...) >> matches exactly when the one above does (verifying this yourself would be a productive exercise), but finishes in a fourth the time when used on a similar string with 1000000 Cs. Be aware, -however, that this pattern currently triggers a warning message under +however, that, when this construct is followed by a +quantifier, it currently triggers a warning message under the C pragma or B<-w> switch saying it C<"matches null string many times in regex">. @@ -1373,14 +1769,14 @@ C<(*MARK:NAME)> pattern executed. See the explanation for the C<(*MARK:NAME)> verb below for more details. B C<$REGERROR> and C<$REGMARK> are not magic variables like C<$1> -and most other regex related variables. They are not local to a scope, nor +and most other regex-related variables. They are not local to a scope, nor readonly, but instead are volatile package variables similar to C<$AUTOLOAD>. Use C to localize changes to them to a specific scope if necessary. If a pattern does not contain a special backtracking verb that allows an argument, then C<$REGERROR> and C<$REGMARK> are not touched at all. -=over 4 +=over 3 =item Verbs that take an argument @@ -1421,7 +1817,7 @@ If we add a C<(*PRUNE)> before the count like the following 'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/; print "Count=$count\n"; -we prevent backtracking and find the count of the longest matching +we prevent backtracking and find the count of the longest matching string at each matching starting point like so: aaab @@ -1437,7 +1833,6 @@ replaced with a C<< (?>pattern) >> with no functional difference; however, C<(*PRUNE)> can be used to handle cases that cannot be expressed using a C<< (?>pattern) >> alone. - =item C<(*SKIP)> C<(*SKIP:NAME)> X<(*SKIP)> @@ -1455,11 +1850,11 @@ encountered, then the C<(*SKIP)> operator has no effect. When used without a name the "skip point" is where the match point was when executing the (*SKIP) pattern. -Compare the following to the examples in C<(*PRUNE)>, note the string +Compare the following to the examples in C<(*PRUNE)>; note the string is twice as long: - 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/; - print "Count=$count\n"; + 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/; + print "Count=$count\n"; outputs @@ -1472,7 +1867,7 @@ executed, the next starting point will be where the cursor was when the C<(*SKIP)> was executed. =item C<(*MARK:NAME)> C<(*:NAME)> -X<(*MARK)> C<(*MARK:NAME)> C<(*:NAME)> +X<(*MARK)> X<(*MARK:NAME)> X<(*:NAME)> This zero-width pattern can be used to mark the point reached in a string when a certain part of the pattern has been successfully matched. This @@ -1500,16 +1895,18 @@ failing the match and has provided its own name to use, the C<$REGERROR> variable will be set to the name of the most recently executed C<(*MARK:NAME)>. -See C<(*SKIP)> for more details. +See L(*SKIP)> for more details. As a shortcut C<(*MARK:NAME)> can be written C<(*:NAME)>. =item C<(*THEN)> C<(*THEN:NAME)> -This is similar to the "cut group" operator C<::> from Perl 6. Like +This is similar to the "cut group" operator C<::> from Perl 6. Like C<(*PRUNE)>, this verb always matches, and when backtracked into on failure, it causes the regex engine to try the next alternation in the -innermost enclosing group (capturing or otherwise). +innermost enclosing group (capturing or otherwise) that has alternations. +The two branches of a C<(?(condition)yes-pattern|no-pattern)> do not +count as an alternation, as far as C<(*THEN)> is concerned. Its name comes from the observation that this operation combined with the alternation operator (C<|>) can be used to create what is essentially a @@ -1528,15 +1925,21 @@ is the same as but - / ( A (*THEN) B | C (*THEN) D ) / + / ( A (*THEN) B | C ) / is not the same as - / ( A (*PRUNE) B | C (*PRUNE) D ) / + / ( A (*PRUNE) B | C ) / as after matching the A but failing on the B the C<(*THEN)> verb will backtrack and try C; but the C<(*PRUNE)> verb will simply fail. +=back + +=item Verbs without an argument + +=over 4 + =item C<(*COMMIT)> X<(*COMMIT)> @@ -1546,8 +1949,8 @@ into on failure it causes the match to fail outright. No further attempts to find a valid match by advancing the start pointer will occur again. For example, - 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/; - print "Count=$count\n"; + 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/; + print "Count=$count\n"; outputs @@ -1558,12 +1961,6 @@ In other words, once the C<(*COMMIT)> has been entered, and if the pattern does not match, the regex engine will not try any further matching on the rest of the string. -=back - -=item Verbs without an argument - -=over 4 - =item C<(*FAIL)> C<(*F)> X<(*FAIL)> X<(*F)> @@ -1592,7 +1989,7 @@ For instance: 'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x; will match, and C<$1> will be C and C<$2> will be C, C<$3> will not -be set. If another branch in the inner parentheses were matched, such as in the +be set. If another branch in the inner parentheses was matched, such as in the string 'ACDE', then the C and C would have to be matched as well. =back @@ -1751,7 +2148,7 @@ let C<\D*> expand to "ABC", this would have caused the whole pattern to fail. The search engine will initially match C<\D*> with "ABC". Then it will -try to match C<(?!123> with "123", which fails. But because +try to match C<(?!123)> with "123", which fails. But because a quantifier (C<\D*>) has been used in the regular expression, the search engine can backtrack and retry the match differently in the hope of matching the complete regular expression. @@ -1799,12 +2196,12 @@ match takes a long time to finish. A powerful tool for optimizing such beasts is what is known as an "independent group", -which does not backtrack (see Lpattern) >>>). Note also that +which does not backtrack (see Lpattern) >>>). Note also that zero-length look-ahead/look-behind assertions will not backtrack to make the tail match, since they are in "logical" context: only whether they match is considered relevant. For an example where side-effects of look-ahead I have influenced the -following match, see Lpattern) >>>. +following match, see Lpattern) >>>. =head2 Version 8 Regular Expressions X X X @@ -1820,7 +2217,7 @@ character; "\\" matches a "\"). This escape mechanism is also required for the character used as the pattern delimiter. A series of characters matches that series of characters in the target -string, so the pattern C would match "blurfl" in the target +string, so the pattern C would match "blurfl" in the target string. You can specify a character class, by enclosing a list of characters @@ -1859,9 +2256,9 @@ You can specify a series of alternatives for a pattern using "|" to separate them, so that C will match any of "fee", "fie", or "foe" in the target string (as would C). The first alternative includes everything from the last pattern delimiter -("(", "[", or the beginning of the pattern) up to the first "|", and +("(", "(?:", etc. or the beginning of the pattern) up to the first "|", and the last alternative contains everything from the last "|" to the next -pattern delimiter. That's why it's common practice to include +closing pattern delimiter. That's why it's common practice to include alternatives in parentheses: to minimize confusion about where they start and end. @@ -1879,7 +2276,7 @@ so if you write C<[fee|fie|foe]> you're really only matching C<[feio|]>. Within a pattern, you may designate subpatterns for later reference by enclosing them in parentheses, and you may refer back to the Ith subpattern later in the pattern using the metacharacter -\I. Subpatterns are numbered based on the left to right order +\I or \gI. Subpatterns are numbered based on the left to right order of their opening parenthesis. A backreference matches whatever actually matched the subpattern in the string being examined, not the rules for that subpattern. Therefore, C<(0|0x)\d*\s\g1\d*> will @@ -1943,7 +2340,7 @@ However, long experience has shown that many programming tasks may be significantly simplified by using repeated subexpressions that may match zero-length substrings. Here's a simple example being: - @chars = split //, $string; # // is not magic in split + @chars = split //, $string; # // is not magic in split ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// / Thus Perl allows such constructs, by I matches a zero-length string, it stops +the C<*>. + +The higher-level loops preserve an additional state between iterations: whether the last match was zero-length. To break the loop, the following match after a zero-length match is prohibited to have a length of zero. This prohibition interacts with backtracking (see L<"Backtracking">), @@ -1995,7 +2414,7 @@ Each of the elementary pieces of regular expressions which were described before (such as C or C<\Z>) could match at most one substring at the given position of the input string. However, in a typical regular expression these elementary pieces are combined into more complicated -patterns using combining operators C, C, C etc +patterns using combining operators C, C, C etc. (in these examples C and C are regular subexpressions). Such combinations can include alternatives, leading to a problem of choice: @@ -2024,11 +2443,11 @@ Consider two possible matches, C and C, C and C are substrings which can be matched by C, C and C are substrings which can be matched by C. -If C is better match for C than C, C is a better +If C is a better match for C than C, C is a better match than C. If C and C coincide: C is a better match than C if -C is better match for C than C. +C is a better match for C than C. =item C @@ -2092,8 +2511,13 @@ than a match at a later position. =head2 Creating Custom RE Engines -Overloaded constants (see L) provide a simple way to extend -the functionality of the RE engine. +As of Perl 5.10.0, one can create custom regular expression engines. This +is not for the faint of heart, as they have to plug in at the C level. See +L for more details. + +As an alternative, overloaded constants (see L) provide a simple +way to extend the functionality of the RE engine, by substituting one +pattern for another. Suppose that we want to enable a new RE escape-sequence C<\Y|> which matches at a boundary between whitespace characters and non-whitespace @@ -2139,11 +2563,11 @@ part of this regular expression needs to be converted explicitly $re = customre::convert $re; /\Y|$re\Y|/; -=head1 PCRE/Python Support +=head2 PCRE/Python Support -As of Perl 5.10.0, Perl supports several Python/PCRE specific extensions +As of Perl 5.10.0, Perl supports several Python/PCRE-specific extensions to the regex syntax. While Perl programmers are encouraged to use the -Perl specific syntax, the following are also accepted: +Perl-specific syntax, the following are also accepted: =over 4 @@ -2163,17 +2587,11 @@ Subroutine call to a named capture group. Equivalent to C<< (?&NAME) >>. =head1 BUGS -There are numerous problems with case insensitive matching of characters -outside the ASCII range, especially with those whose folds are multiple -characters, such as ligatures like C. - -In a bracketed character class with case insensitive matching, ranges only work -for ASCII characters. For example, -C -doesn't match all the Russian upper and lower case letters. - Many regular expression constructs don't work on EBCDIC platforms. +There are a number of issues with regard to case-insensitive matching +in Unicode rules. See C under L above. + This document varies from difficult to understand to completely and utterly opaque. The wandering prose riddled with jargon is hard to fathom in several places.
is not interpolated. As before, the rules to determine -where the C ends are currently somewhat convoluted. +Note that this means that there is no way for the inner pattern to refer +to a capture group defined outside. (The code block itself can use C<$1>, +etc., to refer to the enclosing pattern's capture groups.) Thus, although + + ('a' x 100)=~/(??{'(.)' x 100})/ + +I match, it will I set $1 on exit. The following pattern matches a parenthesized group: - $re = qr{ - \( - (?: - (?> [^()]+ ) # Non-parens without backtracking - | - (??{ $re }) # Group with matching parens - )* - \) - }x; + $re = qr{ + \( + (?: + (?> [^()]+ ) # Non-parens without backtracking + | + (??{ $re }) # Group with matching parens + )* + \) + }x; See also C<(?PARNO)> for a different, more efficient way to accomplish the same task. -For reasons of security, this construct is forbidden if the regular -expression involves run-time interpolation of variables, unless the -perilous C pragma has been used (see L), or the -variables contain results of C operator (see -LSTRINGEmsixpo">). - -In perl 5.12.x and earlier, because the regex engine was not re-entrant, -delayed code could not safely invoke the regex engine either directly with -C or C), or indirectly with functions such as C. - -Recursing deeper than 50 times without consuming any input string will -result in a fatal error. The maximum depth is compiled into perl, so -changing it requires a custom build. +Executing a postponed regular expression 50 times without consuming any +input string will result in a fatal error. The maximum depth is compiled +into perl, so changing it requires a custom build. =item C<(?PARNO)> C<(?-PARNO)> C<(?+PARNO)> C<(?R)> C<(?0)> X<(?PARNO)> X<(?1)> X<(?R)> X<(?0)> X<(?-1)> X<(?+1)> X<(?-PARNO)> X<(?+PARNO)> X X X X -Similar to C<(??{ code })> except it does not involve compiling any code, -instead it treats the contents of a capture group as an independent -pattern that must match at the current position. Capture groups -contained by the pattern will have the value as determined by the -outermost recursion. +Similar to C<(??{ code })> except that it does not involve executing any +code or potentially compiling a returned pattern string; instead it treats +the part of the current pattern contained within a specified capture group +as an independent pattern that must match at the current position. +Capture groups contained by the pattern will have the value as determined +by the outermost recursion. PARNO is a sequence of digits (not starting with 0) whose value reflects the paren-number of the capture group to recurse to. C<(?R)> recurses to @@ -1081,15 +1450,15 @@ included. The following pattern matches a function foo() which may contain balanced parentheses as the argument. - $re = qr{ ( # paren group 1 (full function) + $re = qr{ ( # paren group 1 (full function) foo - ( # paren group 2 (parens) + ( # paren group 2 (parens) \( - ( # paren group 3 (contents of parens) + ( # paren group 3 (contents of parens) (?: - (?> [^()]+ ) # Non-parens without backtracking + (?> [^()]+ ) # Non-parens without backtracking | - (?2) # Recurse to start of paren group 2 + (?2) # Recurse to start of paren group 2 )* ) \) @@ -1120,7 +1489,7 @@ easier to embed recursive patterns inside of a C construct for later use: my $parens = qr/(\((?:[^()]++|(?-1))*+\))/; - if (/foo $parens \s+ + \s+ bar $parens/x) { + if (/foo $parens \s+ \+ \s+ bar $parens/x) { # do something here... } @@ -1150,11 +1519,15 @@ X<(?()> =item C<(?(condition)yes-pattern)> -Conditional expression. C<(condition)> should be either an integer in +Conditional expression. Matches C if C yields +a true value, matches C otherwise. A missing pattern always +matches. + +C<(condition)> should be one of: 1) an integer in parentheses (which is valid if the corresponding pair of parentheses -matched), a look-ahead/look-behind/evaluate zero-width assertion, a +matched); 2) a look-ahead/look-behind/evaluate zero-width assertion; 3) a name in angle brackets or single quotes (which is valid if a group -with the given name matched), or the special symbol (R) (true when +with the given name matched); or 4) the special symbol (R) (true when evaluated inside of recursion or eval). Additionally the R may be followed by a number, (which will be true when evaluated when recursing inside of the appropriate group), or by C<&NAME>, in which case it will @@ -1172,9 +1545,14 @@ Checks if the numbered capturing group has matched something. Checks if a group with the given name has matched something. +=item (?=...) (?!...) (?<=...) (? predicate, which never executes directly -its yes-pattern, and does not allow a no-pattern. This allows to define -subpatterns which will be executed only by using the recursion mechanism. +A special form is the C<(DEFINE)> predicate, which never executes its +yes-pattern directly, and does not allow a no-pattern. This allows one to +define subpatterns which will be executed only by the recursion mechanism. This way, you can define a set of regular expression rules that can be bundled into any pattern you choose. @@ -1240,6 +1618,19 @@ after the recursion returns, so the extra layer of capturing groups is necessary. Thus C<$+{NAME_PAT}> would not be defined even though C<$+{NAME}> would be. +Finally, keep in mind that subpatterns created inside a DEFINE block +count towards the absolute and relative number of captures, so this: + + my @captures = "a" =~ /(.) # First capture + (?(DEFINE) + (? 1 ) # Second capture + )/x; + say scalar @captures; + +Will output 2, not 1. This is particularly important if you intend to +compile the definitions with the C operator, and later +interpolate them in another pattern. + =item C<< (?>pattern) >> X X X X @@ -1260,9 +1651,13 @@ group C (see L<"Backtracking">). In particular, C inside C will match fewer characters than a standalone C, since this makes the tail match. +C<< (?>pattern) >> does not disable backtracking altogether once it has +matched. It is still possible to backtrack past the construct, but not +into it. So C<< ((?>a*)|(?>b*))ar >> will still match "bar". + An effect similar to C<< (?>pattern) >> may be achieved by writing -C<(?=(pattern))\g1>. This matches the same substring as a standalone -C, and the following C<\g1> eats the matched string; it therefore +C<(?=(pattern))\g{-1}>. This matches the same substring as a standalone +C, and the following C<\g{-1}> eats the matched string; it therefore makes a zero-length assertion into an analogue of C<< (?>...) >>. (The difference between these two constructs is that the second one uses a capturing group, thus shifting ordinals of backreferences @@ -1302,7 +1697,8 @@ hung. However, a tiny change to this pattern which uses C<< (?>...) >> matches exactly when the one above does (verifying this yourself would be a productive exercise), but finishes in a fourth the time when used on a similar string with 1000000 Cs. Be aware, -however, that this pattern currently triggers a warning message under +however, that, when this construct is followed by a +quantifier, it currently triggers a warning message under the C pragma or B<-w> switch saying it C<"matches null string many times in regex">. @@ -1373,14 +1769,14 @@ C<(*MARK:NAME)> pattern executed. See the explanation for the C<(*MARK:NAME)> verb below for more details. B C<$REGERROR> and C<$REGMARK> are not magic variables like C<$1> -and most other regex related variables. They are not local to a scope, nor +and most other regex-related variables. They are not local to a scope, nor readonly, but instead are volatile package variables similar to C<$AUTOLOAD>. Use C to localize changes to them to a specific scope if necessary. If a pattern does not contain a special backtracking verb that allows an argument, then C<$REGERROR> and C<$REGMARK> are not touched at all. -=over 4 +=over 3 =item Verbs that take an argument @@ -1421,7 +1817,7 @@ If we add a C<(*PRUNE)> before the count like the following 'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/; print "Count=$count\n"; -we prevent backtracking and find the count of the longest matching +we prevent backtracking and find the count of the longest matching string at each matching starting point like so: aaab @@ -1437,7 +1833,6 @@ replaced with a C<< (?>pattern) >> with no functional difference; however, C<(*PRUNE)> can be used to handle cases that cannot be expressed using a C<< (?>pattern) >> alone. - =item C<(*SKIP)> C<(*SKIP:NAME)> X<(*SKIP)> @@ -1455,11 +1850,11 @@ encountered, then the C<(*SKIP)> operator has no effect. When used without a name the "skip point" is where the match point was when executing the (*SKIP) pattern. -Compare the following to the examples in C<(*PRUNE)>, note the string +Compare the following to the examples in C<(*PRUNE)>; note the string is twice as long: - 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/; - print "Count=$count\n"; + 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/; + print "Count=$count\n"; outputs @@ -1472,7 +1867,7 @@ executed, the next starting point will be where the cursor was when the C<(*SKIP)> was executed. =item C<(*MARK:NAME)> C<(*:NAME)> -X<(*MARK)> C<(*MARK:NAME)> C<(*:NAME)> +X<(*MARK)> X<(*MARK:NAME)> X<(*:NAME)> This zero-width pattern can be used to mark the point reached in a string when a certain part of the pattern has been successfully matched. This @@ -1500,16 +1895,18 @@ failing the match and has provided its own name to use, the C<$REGERROR> variable will be set to the name of the most recently executed C<(*MARK:NAME)>. -See C<(*SKIP)> for more details. +See L(*SKIP)> for more details. As a shortcut C<(*MARK:NAME)> can be written C<(*:NAME)>. =item C<(*THEN)> C<(*THEN:NAME)> -This is similar to the "cut group" operator C<::> from Perl 6. Like +This is similar to the "cut group" operator C<::> from Perl 6. Like C<(*PRUNE)>, this verb always matches, and when backtracked into on failure, it causes the regex engine to try the next alternation in the -innermost enclosing group (capturing or otherwise). +innermost enclosing group (capturing or otherwise) that has alternations. +The two branches of a C<(?(condition)yes-pattern|no-pattern)> do not +count as an alternation, as far as C<(*THEN)> is concerned. Its name comes from the observation that this operation combined with the alternation operator (C<|>) can be used to create what is essentially a @@ -1528,15 +1925,21 @@ is the same as but - / ( A (*THEN) B | C (*THEN) D ) / + / ( A (*THEN) B | C ) / is not the same as - / ( A (*PRUNE) B | C (*PRUNE) D ) / + / ( A (*PRUNE) B | C ) / as after matching the A but failing on the B the C<(*THEN)> verb will backtrack and try C; but the C<(*PRUNE)> verb will simply fail. +=back + +=item Verbs without an argument + +=over 4 + =item C<(*COMMIT)> X<(*COMMIT)> @@ -1546,8 +1949,8 @@ into on failure it causes the match to fail outright. No further attempts to find a valid match by advancing the start pointer will occur again. For example, - 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/; - print "Count=$count\n"; + 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/; + print "Count=$count\n"; outputs @@ -1558,12 +1961,6 @@ In other words, once the C<(*COMMIT)> has been entered, and if the pattern does not match, the regex engine will not try any further matching on the rest of the string. -=back - -=item Verbs without an argument - -=over 4 - =item C<(*FAIL)> C<(*F)> X<(*FAIL)> X<(*F)> @@ -1592,7 +1989,7 @@ For instance: 'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x; will match, and C<$1> will be C and C<$2> will be C, C<$3> will not -be set. If another branch in the inner parentheses were matched, such as in the +be set. If another branch in the inner parentheses was matched, such as in the string 'ACDE', then the C and C would have to be matched as well. =back @@ -1751,7 +2148,7 @@ let C<\D*> expand to "ABC", this would have caused the whole pattern to fail. The search engine will initially match C<\D*> with "ABC". Then it will -try to match C<(?!123> with "123", which fails. But because +try to match C<(?!123)> with "123", which fails. But because a quantifier (C<\D*>) has been used in the regular expression, the search engine can backtrack and retry the match differently in the hope of matching the complete regular expression. @@ -1799,12 +2196,12 @@ match takes a long time to finish. A powerful tool for optimizing such beasts is what is known as an "independent group", -which does not backtrack (see Lpattern) >>>). Note also that +which does not backtrack (see Lpattern) >>>). Note also that zero-length look-ahead/look-behind assertions will not backtrack to make the tail match, since they are in "logical" context: only whether they match is considered relevant. For an example where side-effects of look-ahead I have influenced the -following match, see Lpattern) >>>. +following match, see Lpattern) >>>. =head2 Version 8 Regular Expressions X X X @@ -1820,7 +2217,7 @@ character; "\\" matches a "\"). This escape mechanism is also required for the character used as the pattern delimiter. A series of characters matches that series of characters in the target -string, so the pattern C would match "blurfl" in the target +string, so the pattern C would match "blurfl" in the target string. You can specify a character class, by enclosing a list of characters @@ -1859,9 +2256,9 @@ You can specify a series of alternatives for a pattern using "|" to separate them, so that C will match any of "fee", "fie", or "foe" in the target string (as would C). The first alternative includes everything from the last pattern delimiter -("(", "[", or the beginning of the pattern) up to the first "|", and +("(", "(?:", etc. or the beginning of the pattern) up to the first "|", and the last alternative contains everything from the last "|" to the next -pattern delimiter. That's why it's common practice to include +closing pattern delimiter. That's why it's common practice to include alternatives in parentheses: to minimize confusion about where they start and end. @@ -1879,7 +2276,7 @@ so if you write C<[fee|fie|foe]> you're really only matching C<[feio|]>. Within a pattern, you may designate subpatterns for later reference by enclosing them in parentheses, and you may refer back to the Ith subpattern later in the pattern using the metacharacter -\I. Subpatterns are numbered based on the left to right order +\I or \gI. Subpatterns are numbered based on the left to right order of their opening parenthesis. A backreference matches whatever actually matched the subpattern in the string being examined, not the rules for that subpattern. Therefore, C<(0|0x)\d*\s\g1\d*> will @@ -1943,7 +2340,7 @@ However, long experience has shown that many programming tasks may be significantly simplified by using repeated subexpressions that may match zero-length substrings. Here's a simple example being: - @chars = split //, $string; # // is not magic in split + @chars = split //, $string; # // is not magic in split ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// / Thus Perl allows such constructs, by I matches a zero-length string, it stops +the C<*>. + +The higher-level loops preserve an additional state between iterations: whether the last match was zero-length. To break the loop, the following match after a zero-length match is prohibited to have a length of zero. This prohibition interacts with backtracking (see L<"Backtracking">), @@ -1995,7 +2414,7 @@ Each of the elementary pieces of regular expressions which were described before (such as C or C<\Z>) could match at most one substring at the given position of the input string. However, in a typical regular expression these elementary pieces are combined into more complicated -patterns using combining operators C, C, C etc +patterns using combining operators C, C, C etc. (in these examples C and C are regular subexpressions). Such combinations can include alternatives, leading to a problem of choice: @@ -2024,11 +2443,11 @@ Consider two possible matches, C and C, C and C are substrings which can be matched by C, C and C are substrings which can be matched by C. -If C is better match for C than C, C is a better +If C is a better match for C than C, C is a better match than C. If C and C coincide: C is a better match than C if -C is better match for C than C. +C is a better match for C than C. =item C @@ -2092,8 +2511,13 @@ than a match at a later position. =head2 Creating Custom RE Engines -Overloaded constants (see L) provide a simple way to extend -the functionality of the RE engine. +As of Perl 5.10.0, one can create custom regular expression engines. This +is not for the faint of heart, as they have to plug in at the C level. See +L for more details. + +As an alternative, overloaded constants (see L) provide a simple +way to extend the functionality of the RE engine, by substituting one +pattern for another. Suppose that we want to enable a new RE escape-sequence C<\Y|> which matches at a boundary between whitespace characters and non-whitespace @@ -2139,11 +2563,11 @@ part of this regular expression needs to be converted explicitly $re = customre::convert $re; /\Y|$re\Y|/; -=head1 PCRE/Python Support +=head2 PCRE/Python Support -As of Perl 5.10.0, Perl supports several Python/PCRE specific extensions +As of Perl 5.10.0, Perl supports several Python/PCRE-specific extensions to the regex syntax. While Perl programmers are encouraged to use the -Perl specific syntax, the following are also accepted: +Perl-specific syntax, the following are also accepted: =over 4 @@ -2163,17 +2587,11 @@ Subroutine call to a named capture group. Equivalent to C<< (?&NAME) >>. =head1 BUGS -There are numerous problems with case insensitive matching of characters -outside the ASCII range, especially with those whose folds are multiple -characters, such as ligatures like C. - -In a bracketed character class with case insensitive matching, ranges only work -for ASCII characters. For example, -C -doesn't match all the Russian upper and lower case letters. - Many regular expression constructs don't work on EBCDIC platforms. +There are a number of issues with regard to case-insensitive matching +in Unicode rules. See C under L above. + This document varies from difficult to understand to completely and utterly opaque. The wandering prose riddled with jargon is hard to fathom in several places.
ends are currently somewhat convoluted. +Note that this means that there is no way for the inner pattern to refer +to a capture group defined outside. (The code block itself can use C<$1>, +etc., to refer to the enclosing pattern's capture groups.) Thus, although + + ('a' x 100)=~/(??{'(.)' x 100})/ + +I match, it will I set $1 on exit. The following pattern matches a parenthesized group: - $re = qr{ - \( - (?: - (?> [^()]+ ) # Non-parens without backtracking - | - (??{ $re }) # Group with matching parens - )* - \) - }x; + $re = qr{ + \( + (?: + (?> [^()]+ ) # Non-parens without backtracking + | + (??{ $re }) # Group with matching parens + )* + \) + }x; See also C<(?PARNO)> for a different, more efficient way to accomplish the same task. -For reasons of security, this construct is forbidden if the regular -expression involves run-time interpolation of variables, unless the -perilous C pragma has been used (see L), or the -variables contain results of C operator (see -LSTRINGEmsixpo">). - -In perl 5.12.x and earlier, because the regex engine was not re-entrant, -delayed code could not safely invoke the regex engine either directly with -C or C), or indirectly with functions such as C. - -Recursing deeper than 50 times without consuming any input string will -result in a fatal error. The maximum depth is compiled into perl, so -changing it requires a custom build. +Executing a postponed regular expression 50 times without consuming any +input string will result in a fatal error. The maximum depth is compiled +into perl, so changing it requires a custom build. =item C<(?PARNO)> C<(?-PARNO)> C<(?+PARNO)> C<(?R)> C<(?0)> X<(?PARNO)> X<(?1)> X<(?R)> X<(?0)> X<(?-1)> X<(?+1)> X<(?-PARNO)> X<(?+PARNO)> X X X X -Similar to C<(??{ code })> except it does not involve compiling any code, -instead it treats the contents of a capture group as an independent -pattern that must match at the current position. Capture groups -contained by the pattern will have the value as determined by the -outermost recursion. +Similar to C<(??{ code })> except that it does not involve executing any +code or potentially compiling a returned pattern string; instead it treats +the part of the current pattern contained within a specified capture group +as an independent pattern that must match at the current position. +Capture groups contained by the pattern will have the value as determined +by the outermost recursion. PARNO is a sequence of digits (not starting with 0) whose value reflects the paren-number of the capture group to recurse to. C<(?R)> recurses to @@ -1081,15 +1450,15 @@ included. The following pattern matches a function foo() which may contain balanced parentheses as the argument. - $re = qr{ ( # paren group 1 (full function) + $re = qr{ ( # paren group 1 (full function) foo - ( # paren group 2 (parens) + ( # paren group 2 (parens) \( - ( # paren group 3 (contents of parens) + ( # paren group 3 (contents of parens) (?: - (?> [^()]+ ) # Non-parens without backtracking + (?> [^()]+ ) # Non-parens without backtracking | - (?2) # Recurse to start of paren group 2 + (?2) # Recurse to start of paren group 2 )* ) \) @@ -1120,7 +1489,7 @@ easier to embed recursive patterns inside of a C construct for later use: my $parens = qr/(\((?:[^()]++|(?-1))*+\))/; - if (/foo $parens \s+ + \s+ bar $parens/x) { + if (/foo $parens \s+ \+ \s+ bar $parens/x) { # do something here... } @@ -1150,11 +1519,15 @@ X<(?()> =item C<(?(condition)yes-pattern)> -Conditional expression. C<(condition)> should be either an integer in +Conditional expression. Matches C if C yields +a true value, matches C otherwise. A missing pattern always +matches. + +C<(condition)> should be one of: 1) an integer in parentheses (which is valid if the corresponding pair of parentheses -matched), a look-ahead/look-behind/evaluate zero-width assertion, a +matched); 2) a look-ahead/look-behind/evaluate zero-width assertion; 3) a name in angle brackets or single quotes (which is valid if a group -with the given name matched), or the special symbol (R) (true when +with the given name matched); or 4) the special symbol (R) (true when evaluated inside of recursion or eval). Additionally the R may be followed by a number, (which will be true when evaluated when recursing inside of the appropriate group), or by C<&NAME>, in which case it will @@ -1172,9 +1545,14 @@ Checks if the numbered capturing group has matched something. Checks if a group with the given name has matched something. +=item (?=...) (?!...) (?<=...) (? predicate, which never executes directly -its yes-pattern, and does not allow a no-pattern. This allows to define -subpatterns which will be executed only by using the recursion mechanism. +A special form is the C<(DEFINE)> predicate, which never executes its +yes-pattern directly, and does not allow a no-pattern. This allows one to +define subpatterns which will be executed only by the recursion mechanism. This way, you can define a set of regular expression rules that can be bundled into any pattern you choose. @@ -1240,6 +1618,19 @@ after the recursion returns, so the extra layer of capturing groups is necessary. Thus C<$+{NAME_PAT}> would not be defined even though C<$+{NAME}> would be. +Finally, keep in mind that subpatterns created inside a DEFINE block +count towards the absolute and relative number of captures, so this: + + my @captures = "a" =~ /(.) # First capture + (?(DEFINE) + (? 1 ) # Second capture + )/x; + say scalar @captures; + +Will output 2, not 1. This is particularly important if you intend to +compile the definitions with the C operator, and later +interpolate them in another pattern. + =item C<< (?>pattern) >> X X X X @@ -1260,9 +1651,13 @@ group C (see L<"Backtracking">). In particular, C inside C will match fewer characters than a standalone C, since this makes the tail match. +C<< (?>pattern) >> does not disable backtracking altogether once it has +matched. It is still possible to backtrack past the construct, but not +into it. So C<< ((?>a*)|(?>b*))ar >> will still match "bar". + An effect similar to C<< (?>pattern) >> may be achieved by writing -C<(?=(pattern))\g1>. This matches the same substring as a standalone -C, and the following C<\g1> eats the matched string; it therefore +C<(?=(pattern))\g{-1}>. This matches the same substring as a standalone +C, and the following C<\g{-1}> eats the matched string; it therefore makes a zero-length assertion into an analogue of C<< (?>...) >>. (The difference between these two constructs is that the second one uses a capturing group, thus shifting ordinals of backreferences @@ -1302,7 +1697,8 @@ hung. However, a tiny change to this pattern which uses C<< (?>...) >> matches exactly when the one above does (verifying this yourself would be a productive exercise), but finishes in a fourth the time when used on a similar string with 1000000 Cs. Be aware, -however, that this pattern currently triggers a warning message under +however, that, when this construct is followed by a +quantifier, it currently triggers a warning message under the C pragma or B<-w> switch saying it C<"matches null string many times in regex">. @@ -1373,14 +1769,14 @@ C<(*MARK:NAME)> pattern executed. See the explanation for the C<(*MARK:NAME)> verb below for more details. B C<$REGERROR> and C<$REGMARK> are not magic variables like C<$1> -and most other regex related variables. They are not local to a scope, nor +and most other regex-related variables. They are not local to a scope, nor readonly, but instead are volatile package variables similar to C<$AUTOLOAD>. Use C to localize changes to them to a specific scope if necessary. If a pattern does not contain a special backtracking verb that allows an argument, then C<$REGERROR> and C<$REGMARK> are not touched at all. -=over 4 +=over 3 =item Verbs that take an argument @@ -1421,7 +1817,7 @@ If we add a C<(*PRUNE)> before the count like the following 'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/; print "Count=$count\n"; -we prevent backtracking and find the count of the longest matching +we prevent backtracking and find the count of the longest matching string at each matching starting point like so: aaab @@ -1437,7 +1833,6 @@ replaced with a C<< (?>pattern) >> with no functional difference; however, C<(*PRUNE)> can be used to handle cases that cannot be expressed using a C<< (?>pattern) >> alone. - =item C<(*SKIP)> C<(*SKIP:NAME)> X<(*SKIP)> @@ -1455,11 +1850,11 @@ encountered, then the C<(*SKIP)> operator has no effect. When used without a name the "skip point" is where the match point was when executing the (*SKIP) pattern. -Compare the following to the examples in C<(*PRUNE)>, note the string +Compare the following to the examples in C<(*PRUNE)>; note the string is twice as long: - 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/; - print "Count=$count\n"; + 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/; + print "Count=$count\n"; outputs @@ -1472,7 +1867,7 @@ executed, the next starting point will be where the cursor was when the C<(*SKIP)> was executed. =item C<(*MARK:NAME)> C<(*:NAME)> -X<(*MARK)> C<(*MARK:NAME)> C<(*:NAME)> +X<(*MARK)> X<(*MARK:NAME)> X<(*:NAME)> This zero-width pattern can be used to mark the point reached in a string when a certain part of the pattern has been successfully matched. This @@ -1500,16 +1895,18 @@ failing the match and has provided its own name to use, the C<$REGERROR> variable will be set to the name of the most recently executed C<(*MARK:NAME)>. -See C<(*SKIP)> for more details. +See L(*SKIP)> for more details. As a shortcut C<(*MARK:NAME)> can be written C<(*:NAME)>. =item C<(*THEN)> C<(*THEN:NAME)> -This is similar to the "cut group" operator C<::> from Perl 6. Like +This is similar to the "cut group" operator C<::> from Perl 6. Like C<(*PRUNE)>, this verb always matches, and when backtracked into on failure, it causes the regex engine to try the next alternation in the -innermost enclosing group (capturing or otherwise). +innermost enclosing group (capturing or otherwise) that has alternations. +The two branches of a C<(?(condition)yes-pattern|no-pattern)> do not +count as an alternation, as far as C<(*THEN)> is concerned. Its name comes from the observation that this operation combined with the alternation operator (C<|>) can be used to create what is essentially a @@ -1528,15 +1925,21 @@ is the same as but - / ( A (*THEN) B | C (*THEN) D ) / + / ( A (*THEN) B | C ) / is not the same as - / ( A (*PRUNE) B | C (*PRUNE) D ) / + / ( A (*PRUNE) B | C ) / as after matching the A but failing on the B the C<(*THEN)> verb will backtrack and try C; but the C<(*PRUNE)> verb will simply fail. +=back + +=item Verbs without an argument + +=over 4 + =item C<(*COMMIT)> X<(*COMMIT)> @@ -1546,8 +1949,8 @@ into on failure it causes the match to fail outright. No further attempts to find a valid match by advancing the start pointer will occur again. For example, - 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/; - print "Count=$count\n"; + 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/; + print "Count=$count\n"; outputs @@ -1558,12 +1961,6 @@ In other words, once the C<(*COMMIT)> has been entered, and if the pattern does not match, the regex engine will not try any further matching on the rest of the string. -=back - -=item Verbs without an argument - -=over 4 - =item C<(*FAIL)> C<(*F)> X<(*FAIL)> X<(*F)> @@ -1592,7 +1989,7 @@ For instance: 'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x; will match, and C<$1> will be C and C<$2> will be C, C<$3> will not -be set. If another branch in the inner parentheses were matched, such as in the +be set. If another branch in the inner parentheses was matched, such as in the string 'ACDE', then the C and C would have to be matched as well. =back @@ -1751,7 +2148,7 @@ let C<\D*> expand to "ABC", this would have caused the whole pattern to fail. The search engine will initially match C<\D*> with "ABC". Then it will -try to match C<(?!123> with "123", which fails. But because +try to match C<(?!123)> with "123", which fails. But because a quantifier (C<\D*>) has been used in the regular expression, the search engine can backtrack and retry the match differently in the hope of matching the complete regular expression. @@ -1799,12 +2196,12 @@ match takes a long time to finish. A powerful tool for optimizing such beasts is what is known as an "independent group", -which does not backtrack (see Lpattern) >>>). Note also that +which does not backtrack (see Lpattern) >>>). Note also that zero-length look-ahead/look-behind assertions will not backtrack to make the tail match, since they are in "logical" context: only whether they match is considered relevant. For an example where side-effects of look-ahead I have influenced the -following match, see Lpattern) >>>. +following match, see Lpattern) >>>. =head2 Version 8 Regular Expressions X X X @@ -1820,7 +2217,7 @@ character; "\\" matches a "\"). This escape mechanism is also required for the character used as the pattern delimiter. A series of characters matches that series of characters in the target -string, so the pattern C would match "blurfl" in the target +string, so the pattern C would match "blurfl" in the target string. You can specify a character class, by enclosing a list of characters @@ -1859,9 +2256,9 @@ You can specify a series of alternatives for a pattern using "|" to separate them, so that C will match any of "fee", "fie", or "foe" in the target string (as would C). The first alternative includes everything from the last pattern delimiter -("(", "[", or the beginning of the pattern) up to the first "|", and +("(", "(?:", etc. or the beginning of the pattern) up to the first "|", and the last alternative contains everything from the last "|" to the next -pattern delimiter. That's why it's common practice to include +closing pattern delimiter. That's why it's common practice to include alternatives in parentheses: to minimize confusion about where they start and end. @@ -1879,7 +2276,7 @@ so if you write C<[fee|fie|foe]> you're really only matching C<[feio|]>. Within a pattern, you may designate subpatterns for later reference by enclosing them in parentheses, and you may refer back to the Ith subpattern later in the pattern using the metacharacter -\I. Subpatterns are numbered based on the left to right order +\I or \gI. Subpatterns are numbered based on the left to right order of their opening parenthesis. A backreference matches whatever actually matched the subpattern in the string being examined, not the rules for that subpattern. Therefore, C<(0|0x)\d*\s\g1\d*> will @@ -1943,7 +2340,7 @@ However, long experience has shown that many programming tasks may be significantly simplified by using repeated subexpressions that may match zero-length substrings. Here's a simple example being: - @chars = split //, $string; # // is not magic in split + @chars = split //, $string; # // is not magic in split ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// / Thus Perl allows such constructs, by I matches a zero-length string, it stops +the C<*>. + +The higher-level loops preserve an additional state between iterations: whether the last match was zero-length. To break the loop, the following match after a zero-length match is prohibited to have a length of zero. This prohibition interacts with backtracking (see L<"Backtracking">), @@ -1995,7 +2414,7 @@ Each of the elementary pieces of regular expressions which were described before (such as C or C<\Z>) could match at most one substring at the given position of the input string. However, in a typical regular expression these elementary pieces are combined into more complicated -patterns using combining operators C, C, C etc +patterns using combining operators C, C, C etc. (in these examples C and C are regular subexpressions). Such combinations can include alternatives, leading to a problem of choice: @@ -2024,11 +2443,11 @@ Consider two possible matches, C and C, C and C are substrings which can be matched by C, C and C are substrings which can be matched by C. -If C is better match for C than C, C is a better +If C is a better match for C than C, C is a better match than C. If C and C coincide: C is a better match than C if -C is better match for C than C. +C is a better match for C than C. =item C @@ -2092,8 +2511,13 @@ than a match at a later position. =head2 Creating Custom RE Engines -Overloaded constants (see L) provide a simple way to extend -the functionality of the RE engine. +As of Perl 5.10.0, one can create custom regular expression engines. This +is not for the faint of heart, as they have to plug in at the C level. See +L for more details. + +As an alternative, overloaded constants (see L) provide a simple +way to extend the functionality of the RE engine, by substituting one +pattern for another. Suppose that we want to enable a new RE escape-sequence C<\Y|> which matches at a boundary between whitespace characters and non-whitespace @@ -2139,11 +2563,11 @@ part of this regular expression needs to be converted explicitly $re = customre::convert $re; /\Y|$re\Y|/; -=head1 PCRE/Python Support +=head2 PCRE/Python Support -As of Perl 5.10.0, Perl supports several Python/PCRE specific extensions +As of Perl 5.10.0, Perl supports several Python/PCRE-specific extensions to the regex syntax. While Perl programmers are encouraged to use the -Perl specific syntax, the following are also accepted: +Perl-specific syntax, the following are also accepted: =over 4 @@ -2163,17 +2587,11 @@ Subroutine call to a named capture group. Equivalent to C<< (?&NAME) >>. =head1 BUGS -There are numerous problems with case insensitive matching of characters -outside the ASCII range, especially with those whose folds are multiple -characters, such as ligatures like C. - -In a bracketed character class with case insensitive matching, ranges only work -for ASCII characters. For example, -C -doesn't match all the Russian upper and lower case letters. - Many regular expression constructs don't work on EBCDIC platforms. +There are a number of issues with regard to case-insensitive matching +in Unicode rules. See C under L above. + This document varies from difficult to understand to completely and utterly opaque. The wandering prose riddled with jargon is hard to fathom in several places.