X-Git-Url: https://perl5.git.perl.org/perl5.git/blobdiff_plain/4375e838ae24b385ae79fa7b6918e613bedaaee6..542fa716385f08c64e1dc5ef1d9ceacf2ee69d29:/pod/perlre.pod diff --git a/pod/perlre.pod b/pod/perlre.pod index 2db4139..c2b9680 100644 --- a/pod/perlre.pod +++ b/pod/perlre.pod @@ -1,23 +1,32 @@ =head1 NAME +X X X perlre - Perl regular expressions =head1 DESCRIPTION -This page describes the syntax of regular expressions in Perl. For a -description of how to I regular expressions in matching -operations, plus various examples of the same, see discussions -of C, C, C and C in L. +This page describes the syntax of regular expressions in Perl. + +If you haven't used regular expressions before, a quick-start +introduction is available in L, and a longer tutorial +introduction is available in L. + +For reference on how regular expressions are used in matching +operations, plus various examples of the same, see discussions of +C, C, C and C in L. Matching operations can have various modifiers. Modifiers that relate to the interpretation of the regular expression inside are listed below. Modifiers that alter the way a regular expression -is used by Perl are detailed in L and +is used by Perl are detailed in L and L. =over 4 =item i +X X X +X Do case-insensitive pattern matching. @@ -25,25 +34,25 @@ If C is in effect, the case map is taken from the current locale. See L. =item m +X X X X Treat string as multiple lines. That is, change "^" and "$" from matching the start or end of the string to matching the start or end of any line anywhere within the string. =item s +X X X +X Treat string as single line. That is, change "." to match any character whatsoever, even a newline, which normally it would not match. -The C and C modifiers both override the C<$*> setting. That -is, no matter what C<$*> contains, C without C will force -"^" to match only at the beginning of the string and "$" to match -only at the end (or just before a newline at the end) of the string. -Together, as /ms, they let the "." match any character whatsoever, -while yet allowing "^" and "$" to match, respectively, just after +Used together, as /ms, they let the "." match any character whatsoever, +while still allowing "^" and "$" to match, respectively, just after and just before newlines within the string. =item x +X Extend your pattern's legibility by permitting whitespace and comments. @@ -61,16 +70,20 @@ your regular expression into (slightly) more readable parts. The C<#> character is also treated as a metacharacter introducing a comment, just as in ordinary Perl code. This also means that if you want real whitespace or C<#> characters in the pattern (outside a character -class, where they are unaffected by C), that you'll either have to -escape them or encode them using octal or hex escapes. Taken together, -these features go a long way towards making Perl's regular expressions -more readable. Note that you have to be careful not to include the -pattern delimiter in the comment--perl has no way of knowing you did -not intend to close the pattern early. See the C-comment deletion code -in L. +class, where they are unaffected by C), then you'll either have to +escape them (using backslashes or C<\Q...\E>) or encode them using octal +or hex escapes. Taken together, these features go a long way towards +making Perl's regular expressions more readable. Note that you have to +be careful not to include the pattern delimiter in the comment--perl has +no way of knowing you did not intend to close the pattern early. See +the C-comment deletion code in L. Also note that anything inside +a C<\Q...\E> stays unaffected by C. +X =head2 Regular Expressions +=head3 Metacharacters + The patterns used in Perl pattern matching derive from supplied in the Version 8 regex routines. (The routines are derived (distantly) from Henry Spencer's freely redistributable reimplementation @@ -79,6 +92,9 @@ details. In particular the following metacharacters have their standard I-ish meanings: +X +X<\> X<^> X<.> X<$> X<|> X<(> X<()> X<[> X<[]> + \ Quote the next metacharacter ^ Match the beginning of the line @@ -94,18 +110,22 @@ newline at the end), and Perl does certain optimizations with the assumption that the string contains only one line. Embedded newlines will not be matched by "^" or "$". You may, however, wish to treat a string as a multi-line buffer, such that the "^" will match after any -newline within the string, and "$" will match before any newline. At the +newline within the string (except if the newline is the last character in +the string), and "$" will match before any newline. At the cost of a little more overhead, you can do this by using the /m modifier on the pattern match operator. (Older programs did this by setting C<$*>, -but this practice is now deprecated.) +but this practice has been removed in perl 5.9.) +X<^> X<$> X To simplify multi-line substitutions, the "." character never matches a newline unless you use the C modifier, which in effect tells Perl to pretend -the string is a single line--even if it isn't. The C modifier also -overrides the setting of C<$*>, in case you have some (badly behaved) older -code that sets it in another module. +the string is a single line--even if it isn't. +X<.> X + +=head3 Quantifiers The following standard quantifiers are recognized: +X X X<*> X<+> X X<{n}> X<{n,}> X<{n,m}> * Match 0 or more times + Match 1 or more times @@ -115,7 +135,8 @@ The following standard quantifiers are recognized: {n,m} Match at least n but not more than m times (If a curly bracket occurs in any other context, it is treated -as a regular character.) The "*" modifier is equivalent to C<{0,}>, the "+" +as a regular character. In particular, the lower bound +is not optional.) The "*" modifier is equivalent to C<{0,}>, the "+" modifier to C<{1,}>, and the "?" modifier to C<{0,1}>. n and m are limited to integral values less than a preset limit defined when perl is built. This is usually 32766 on the most common platforms. The actual limit can @@ -128,6 +149,8 @@ many times as possible (given a particular starting location) while still allowing the rest of the pattern to match. If you want it to match the minimum number of times possible, follow the quantifier with a "?". Note that the meanings don't change, just the "greediness": +X X X +X X<*?> X<+?> X X<{n}?> X<{n,}?> X<{n,m}?> *? Match 0 or more times +? Match 1 or more times @@ -136,8 +159,43 @@ that the meanings don't change, just the "greediness": {n,}? Match at least n times {n,m}? Match at least n but not more than m times +By default, when a quantified subpattern does not allow the rest of the +overall pattern to match, Perl will backtrack. However, this behaviour is +sometimes undesirable. Thus Perl provides the "possesive" quantifier form +as well. + + *+ Match 0 or more times and give nothing back + ++ Match 1 or more times and give nothing back + ?+ Match 0 or 1 time and give nothing back + {n}+ Match exactly n times and give nothing back (redundant) + {n,}+ Match at least n times and give nothing back + {n,m}+ Match at least n but not more than m times and give nothing back + +For instance, + + 'aaaa' =~ /a++a/ + +will never match, as the C will gobble up all the C's in the +string and won't leave any for the remaining part of the pattern. This +feature can be extremely useful to give perl hints about where it +shouldn't backtrack. For instance, the typical "match a double-quoted +string" problem can be most efficiently performed when written as: + + /"(?:[^"\\]++|\\.)*+"/ + +as we know that if the final quote does not match, bactracking will not +help. See the independent subexpression C<< (?>...) >> for more details; +possessive quantifiers are just syntactic sugar for that construct. For +instance the above example could also be written as follows: + + /"(?>(?:(?>[^"\\]+)|\\.)*)"/ + +=head3 Escape sequences + Because patterns are processed as double quoted strings, the following also work: +X<\t> X<\n> X<\r> X<\f> X<\a> X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q> +X<\0> X<\c> X<\N> X<\x> \t tab (HT, TAB) \n newline (LF, NL) @@ -166,124 +224,195 @@ An unescaped C<$> or C<@> interpolates the corresponding variable, while escaping will cause the literal string C<\$> to be matched. You'll need to write something like C. -In addition, Perl defines the following: +=head3 Character classes - \w Match a "word" character (alphanumeric plus "_") - \W Match a non-word character - \s Match a whitespace character - \S Match a non-whitespace character - \d Match a digit character - \D Match a non-digit character - \pP Match P, named property. Use \p{Prop} for longer names. - \PP Match non-P - \X Match eXtended Unicode "combining character sequence", - equivalent to C<(?:\PM\pM*)> - \C Match a single C char (octet) even under utf8. - -A C<\w> matches a single alphanumeric character, not a whole word. -Use C<\w+> to match a string of Perl-identifier characters (which isn't -the same as matching an English word). If C is in effect, the -list of alphabetic characters generated by C<\w> is taken from the -current locale. See L. You may use C<\w>, C<\W>, C<\s>, C<\S>, +In addition, Perl defines the following: +X +X<\w> X<\W> X<\s> X<\S> X<\d> X<\D> X<\X> X<\p> X<\P> X<\C> +X X + + \w Match a "word" character (alphanumeric plus "_") + \W Match a non-"word" character + \s Match a whitespace character + \S Match a non-whitespace character + \d Match a digit character + \D Match a non-digit character + \pP Match P, named property. Use \p{Prop} for longer names. + \PP Match non-P + \X Match eXtended Unicode "combining character sequence", + equivalent to (?:\PM\pM*) + \C Match a single C char (octet) even under Unicode. + NOTE: breaks up characters into their UTF-8 bytes, + so you may end up with malformed pieces of UTF-8. + Unsupported in lookbehind. + \1 Backreference to a specific group. + '1' may actually be any positive integer. + \k Named backreference + \N{name} Named unicode character, or unicode escape + \x12 Hexadecimal escape sequence + \x{1234} Long hexadecimal escape sequence + +A C<\w> matches a single alphanumeric character (an alphabetic +character, or a decimal digit) or C<_>, not a whole word. Use C<\w+> +to match a string of Perl-identifier characters (which isn't the same +as matching an English word). If C is in effect, the list +of alphabetic characters generated by C<\w> is taken from the current +locale. See L. You may use C<\w>, C<\W>, C<\s>, C<\S>, C<\d>, and C<\D> within character classes, but if you try to use them -as endpoints of a range, that's not a range, the "-" is understood literally. -See L for details about C<\pP>, C<\PP>, and C<\X>. +as endpoints of a range, that's not a range, the "-" is understood +literally. If Unicode is in effect, C<\s> matches also "\x{85}", +"\x{2028}, and "\x{2029}", see L for more details about +C<\pP>, C<\PP>, and C<\X>, and L about Unicode in general. +You can define your own C<\p> and C<\P> properties, see L. +X<\w> X<\W> X The POSIX character class syntax +X [:class:] -is also available. The available classes and their backslash -equivalents (if available) are as follows: +is also available. Note that the C<[> and C<]> braces are I; +they must always be used within a character class expression. + + # this is correct: + $string =~ /[[:alpha:]]/; + + # this is not, and will generate a warning: + $string =~ /[:alpha:]/; + +The available classes and their backslash equivalents (if available) are +as follows: +X +X X X X X X X +X X X X X X X alpha alnum ascii + blank [1] cntrl digit \d graph lower print punct - space \s + space \s [2] upper - word \w + word \w [3] xdigit +=over + +=item [1] + +A GNU extension equivalent to C<[ \t]>, "all horizontal whitespace". + +=item [2] + +Not exactly equivalent to C<\s> since the C<[[:space:]]> includes +also the (very rare) "vertical tabulator", "\ck", chr(11). + +=item [3] + +A Perl extension, see above. + +=back + For example use C<[:upper:]> to match all the uppercase characters. -Note that the C<[]> are part of the C<[::]> construct, not part of the whole -character class. For example: +Note that the C<[]> are part of the C<[::]> construct, not part of the +whole character class. For example: [01[:alpha:]%] -matches one, zero, any alphabetic character, and the percentage sign. +matches zero, one, any alphabetic character, and the percentage sign. + +The following equivalences to Unicode \p{} constructs and equivalent +backslash character classes (if available), will hold: +X X<\p> X<\p{}> -If the C pragma is used, the following equivalences to Unicode -\p{} constructs hold: + [[:...:]] \p{...} backslash alpha IsAlpha alnum IsAlnum ascii IsASCII + blank IsSpace cntrl IsCntrl - digit IsDigit + digit IsDigit \d graph IsGraph lower IsLower print IsPrint punct IsPunct space IsSpace + IsSpacePerl \s upper IsUpper word IsWord xdigit IsXDigit -For example C<[:lower:]> and C<\p{IsLower}> are equivalent. +For example C<[[:lower:]]> and C<\p{IsLower}> are equivalent. If the C pragma is not used but the C pragma is, the -classes correlate with the isalpha(3) interface (except for `word', -which is a Perl extension, mirroring C<\w>). +classes correlate with the usual isalpha(3) interface (except for +"word" and "blank"). The assumedly non-obviously named classes are: =over 4 =item cntrl +X Any control character. Usually characters that don't produce output as such but instead control the terminal somehow: for example newline and backspace are control characters. All characters with ord() less than -32 are most often classified as control characters. +32 are most often classified as control characters (assuming ASCII, +the ISO Latin character sets, and Unicode), as is the character with +the ord() value of 127 (C). =item graph +X -Any alphanumeric or punctuation character. +Any alphanumeric or punctuation (special) character. =item print +X -Any alphanumeric or punctuation character or space. +Any alphanumeric or punctuation (special) character or the space character. =item punct +X -Any punctuation character. +Any punctuation (special) character. =item xdigit +X -Any hexadecimal digit. Though this may feel silly (/0-9a-f/i would +Any hexadecimal digit. Though this may feel silly ([0-9A-Fa-f] would work just fine) it is included for completeness. =back You can negate the [::] character classes by prefixing the class name with a '^'. This is a Perl extension. For example: +X + + POSIX traditional Unicode - POSIX trad. Perl utf8 Perl + [[:^digit:]] \D \P{IsDigit} + [[:^space:]] \S \P{IsSpace} + [[:^word:]] \W \P{IsWord} - [:^digit:] \D \P{IsDigit} - [:^space:] \S \P{IsSpace} - [:^word:] \W \P{IsWord} +Perl respects the POSIX standard in that POSIX character classes are +only supported within a character class. The POSIX character classes +[.cc.] and [=cc=] are recognized but B supported and trying to +use them will cause an error. -The POSIX character classes [.cc.] and [=cc=] are recognized but -B supported and trying to use them will cause an error. +=head3 Assertions Perl defines the following zero-width assertions: +X X X +X +X +X<\b> X<\B> X<\A> X<\Z> X<\z> X<\G> \b Match a word boundary \B Match a non-(word boundary) @@ -304,6 +433,7 @@ won't match multiple times when the C modifier is used, while "^" and "$" will match at every internal line boundary. To match the actual end of the string and not ignore an optional trailing newline, use C<\z>. +X<\b> X<\A> X<\Z> X<\z> X The C<\G> assertion can be used to chain global matches (using C), as described in L. @@ -311,32 +441,61 @@ It is also useful when writing C-like scanners, when you have several patterns that you want to match against consequent substrings of your string, see the previous reference. The actual location where C<\G> will match can also be influenced by using C as -an lvalue. See L. +an lvalue: see L. Currently C<\G> is only fully +supported when anchored to the start of the pattern; while it +is permitted to use it elsewhere, as in C, some +such uses (C, for example) currently cause problems, and +it is recommended that you avoid such usage for now. +X<\G> + +=head3 Capture buffers The bracketing construct C<( ... )> creates capture buffers. To refer to the digit'th buffer use \ within the match. Outside the match use "$" instead of "\". (The -\ notation works in certain circumstances outside +\ notation works in certain circumstances outside the match. See the warning below about \1 vs $1 for details.) Referring back to another part of the match is called a I. +X X +X X There is no limit to the number of captured substrings that you may use. However Perl also uses \10, \11, etc. as aliases for \010, -\011, etc. (Recall that 0 means octal, so \011 is the 9'th ASCII -character, a tab.) Perl resolves this ambiguity by interpreting -\10 as a backreference only if at least 10 left parentheses have -opened before it. Likewise \11 is a backreference only if at least -11 left parentheses have opened before it. And so on. \1 through -\9 are always interpreted as backreferences." +\011, etc. (Recall that 0 means octal, so \011 is the character at +number 9 in your coded character set; which would be the 10th character, +a horizontal tab under ASCII.) Perl resolves this +ambiguity by interpreting \10 as a backreference only if at least 10 +left parentheses have opened before it. Likewise \11 is a +backreference only if at least 11 left parentheses have opened +before it. And so on. \1 through \9 are always interpreted as +backreferences. + +Additionally, as of Perl 5.10 you may use named capture buffers and named +backreferences. The notation is C<< (?...) >> and C<< \k >> +(you may also use single quotes instead of angle brackets to quote the +name). The only difference with named capture buffers and unnamed ones is +that multiple buffers may have the same name and that the contents of +named capture buffers is available via the C<%+> hash. When multiple +groups share the same name C<$+{name}> and C<< \k >> refer to the +leftmost defined group, thus it's possible to do things with named capture +buffers that would otherwise require C<(??{})> code to accomplish. Named +capture buffers are numbered just as normal capture buffers are and may be +referenced via the magic numeric variables or via numeric backreferences +as well as by name. Examples: s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words - if (/(.)\1/) { # find first doubled char - print "'$1' is the first doubled character\n"; - } + /(.)\1/ # find first doubled char + and print "'$1' is the first doubled character\n"; + + /(?.)\k/ # ... a different way + and print "'$+{char}' is the first doubled character\n"; + + /(?.)\1/ # ... mix and match + and print "'$1' is the first doubled character\n"; if (/Time: (..):(..):(..)/) { # parse out values $hours = $1; @@ -348,13 +507,24 @@ Several special variables also refer back to portions of the previous match. C<$+> returns whatever the last bracket match matched. C<$&> returns the entire matched string. (At one point C<$0> did also, but now it returns the name of the program.) C<$`> returns -everything before the matched string. And C<$'> returns everything -after the matched string. - -The numbered variables ($1, $2, $3, etc.) and the related punctuation -set (C<<$+>, C<$&>, C<$`>, and C<$'>) are all dynamically scoped +everything before the matched string. C<$'> returns everything +after the matched string. And C<$^N> contains whatever was matched by +the most-recently closed group (submatch). C<$^N> can be used in +extended patterns (see below), for example to assign a submatch to a +variable. +X<$+> X<$^N> X<$&> X<$`> X<$'> + +The numbered match variables ($1, $2, $3, etc.) and the related punctuation +set (C<$+>, C<$&>, C<$`>, C<$'>, and C<$^N>) are all dynamically scoped until the end of the enclosing block or until the next successful match, whichever comes first. (See L.) +X<$+> X<$^N> X<$&> X<$`> X<$'> +X<$1> X<$2> X<$3> X<$4> X<$5> X<$6> X<$7> X<$8> X<$9> + + +B: failed matches in Perl do not reset the match variables, +which makes it easier to write code that tests for a series of more +specific cases and remembers the best match. B: Once Perl sees that you need one of C<$&>, C<$`>, or C<$'> anywhere in the program, it has to provide them for every @@ -369,6 +539,7 @@ if you can, but if you can't (and some algorithms really appreciate them), once you've used them once, use them at will, because you've already paid the price. As of 5.005, C<$&> is not so costly as the other two. +X<$&> X<$`> X<$'> Backslashed metacharacters in Perl are alphanumeric, such as C<\b>, C<\w>, C<\n>. Unlike some other regular expression languages, there @@ -377,10 +548,11 @@ that looks like \\, \(, \), \<, \>, \{, or \} is always interpreted as a literal character, not a metacharacter. This was once used in a common idiom to disable or quote the special meanings of regular expression metacharacters in a string that you want to -use for a pattern. Simply quote all non-alphanumeric characters: +use for a pattern. Simply quote all non-"word" characters: $pattern =~ s/(\W)/\\$1/g; +(If C is set, then this depends on the current locale.) Today it is more common to use the quotemeta() function or the C<\Q> metaquoting escape sequence to disable all metacharacters' special meanings like this: @@ -415,6 +587,7 @@ expressions, and 2) whenever you see one, you should stop and =over 10 =item C<(?#text)> +X<(?#)> A comment. The text is ignored. If the C modifier enables whitespace formatting, a simple C<#> will suffice. Note that Perl closes @@ -422,24 +595,26 @@ the comment as soon as it sees a C<)>, so there is no way to put a literal C<)> in the comment. =item C<(?imsx-imsx)> +X<(?)> -One or more embedded pattern-match modifiers. This is particularly -useful for dynamic patterns, such as those read in from a configuration -file, read in as an argument, are specified in a table somewhere, -etc. Consider the case that some of which want to be case sensitive -and some do not. The case insensitive ones need to include merely -C<(?i)> at the front of the pattern. For example: +One or more embedded pattern-match modifiers, to be turned on (or +turned off, if preceded by C<->) for the remainder of the pattern or +the remainder of the enclosing pattern group (if any). This is +particularly useful for dynamic patterns, such as those read in from a +configuration file, read in as an argument, are specified in a table +somewhere, etc. Consider the case that some of which want to be case +sensitive and some do not. The case insensitive ones need to include +merely C<(?i)> at the front of the pattern. For example: $pattern = "foobar"; - if ( /$pattern/i ) { } + if ( /$pattern/i ) { } # more flexible: $pattern = "(?i)foobar"; - if ( /$pattern/ ) { } + if ( /$pattern/ ) { } -Letters after a C<-> turn those modifiers off. These modifiers are -localized inside an enclosing group (if any). For example, +These modifiers are restored at the end of the enclosing group. For example, ( (?i) blah ) \s+ \1 @@ -448,6 +623,7 @@ case, assuming C modifier, and no C modifier outside this group. =item C<(?:pattern)> +X<(?:)> =item C<(?imsx-imsx:pattern)> @@ -464,7 +640,7 @@ but doesn't spit out extra fields. It's also cheaper not to capture characters if you don't need to. Any letters between C and C<:> act as flags modifiers as with -C<(?imsx-imsx)>. For example, +C<(?imsx-imsx)>. For example, /(?s-i:more.*than).*million/i @@ -473,11 +649,13 @@ is equivalent to the more verbose /(?:(?s-i)more.*than).*million/i =item C<(?=pattern)> +X<(?=)> X X A zero-width positive look-ahead assertion. For example, C matches a word followed by a tab, without including the tab in C<$&>. =item C<(?!pattern)> +X<(?!)> X X A zero-width negative look-ahead assertion. For example C matches any occurrence of "foo" that isn't followed by "bar". Note @@ -497,39 +675,98 @@ Sometimes it's still easier just to say: For look-behind see below. =item C<(?<=pattern)> +X<(?<=)> X X A zero-width positive look-behind assertion. For example, C matches a word that follows a tab, without including the tab in C<$&>. Works only for fixed-width look-behind. =item C<(? +X<(? X X A zero-width negative look-behind assertion. For example C matches any occurrence of "foo" that does not follow "bar". Works only for fixed-width look-behind. +=item C<(?'NAME'pattern)> + +=item C<< (?pattern) >> +X<< (?) >> X<(?'NAME')> X X + +A named capture buffer. Identical in every respect to normal capturing +parens C<()> but for the additional fact that C<%+> may be used after +a succesful match to refer to a named buffer. See C for more +details on the C<%+> hash. + +If multiple distinct capture buffers have the same name then the +$+{NAME} will refer to the leftmost defined buffer in the match. + +The forms C<(?'NAME'pattern)> and C<(?pattern)> are equivalent. + +B While the notation of this construct is the same as the similar +function in .NET regexes, the behavior is not, in Perl the buffers are +numbered sequentially regardless of being named or not. Thus in the +pattern + + /(x)(?y)(z)/ + +$+{foo} will be the same as $2, and $3 will contain 'z' instead of +the opposite which is what a .NET regex hacker might expect. + +Currently NAME is restricted to word chars only. In other words, it +must match C. + +=item C<< \k >> + +=item C<< \k'name' >> + +Named backreference. Similar to numeric backreferences, except that +the group is designated by name and not number. If multiple groups +have the same name then it refers to the leftmost defined group in +the current match. + +It is an error to refer to a name not defined by a C<(?)> +earlier in the pattern. + +Both forms are equivalent. + =item C<(?{ code })> +X<(?{})> X X X B: This extended regular expression feature is considered -highly experimental, and may be changed or deleted without notice. +experimental, and may be changed without notice. Code executed that +has side effects may not perform identically from version to version +due to the effect of future optimisations in the regex engine. -This zero-width assertion evaluate any embedded Perl code. It +This zero-width assertion evaluates any embedded Perl code. It always succeeds, and its C is not interpolated. Currently, the rules to determine where the C ends are somewhat convoluted. +This feature can be used together with the special variable C<$^N> to +capture the results of submatches in variables without having to keep +track of the number of nested parentheses. For example: + + $_ = "The brown fox jumps over the lazy dog"; + /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i; + print "color = $color, animal = $animal\n"; + +Inside the C<(?{...})> block, C<$_> 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 $_ = 'a' x 8; - m< + m< (?{ $cnt = 0 }) # Initialize $cnt. ( - a + a (?{ local $cnt = $cnt + 1; # Update $cnt, backtracking-safe. }) - )* + )* aaaa (?{ $res = $cnt }) # On success copy to non-localized # location. @@ -549,11 +786,18 @@ The assignment to C<$^R> above is properly localized, so the old value of C<$^R> is restored if the assertion is backtracked; compare L<"Backtracking">. +Due to an unfortunate implementation issue, the Perl code contained in these +blocks is treated as a compile time closure that can have seemingly bizarre +consequences when used with lexically scoped variables inside of subroutines +or loops. There are various workarounds for this, including simply using +global variables instead. If you are using this construct and strange results +occur then check for the use of lexically scoped variables. + 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 -L). +L). This restriction is because of the wide-spread and remarkably convenient custom of using run-time determined strings as patterns. For example: @@ -568,19 +812,33 @@ 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 -module. See L for details about both these mechanisms. +compartment. See L for details about both these mechanisms. + +Because perl's regex engine is not currently re-entrant, interpolated +code may not invoke the regex engine either directly with C or C), +or indirectly with functions such as C. =item C<(??{ code })> +X<(??{})> +X X X B: This extended regular expression feature is considered -highly experimental, and may be changed or deleted without notice. -A simplified version of the syntax may be introduced for commonly -used idioms. +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. +if it were inserted instead of this construct. Note that this means +that the contents of capture buffers 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 buffer defined outside. +Thus, + + ('a' x 100)=~/(??{'(.)' x 100})/ + +B match, it will B set $1. The C is not interpolated. As before, the rules to determine where the C ends are currently somewhat convoluted. @@ -597,10 +855,196 @@ The following pattern matches a parenthesized group: \) }x; -=item C<< (?>pattern) >> +See also C<(?PARNO)> for a different, more efficient way to accomplish +the same task. + +Because perl's regex engine is not currently re-entrant, delayed +code may not 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. + +=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 buffer as an independent +pattern that must match at the current position. Capture buffers +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 buffer to recurse to. C<(?R)> recurses to +the beginning of the whole pattern. C<(?0)> is an alternate syntax for +C<(?R)>. If PARNO is preceded by a plus or minus sign then it is assumed +to be relative, with negative numbers indicating preceding capture buffers +and positive ones following. Thus C<(?-1)> refers to the most recently +declared buffer, and C<(?+1)> indicates the next buffer to be declared. + +The following pattern matches a function foo() which may contain +balanced parentheses as the argument. + + $re = qr{ ( # paren group 1 (full function) + foo + ( # paren group 2 (parens) + \( + ( # paren group 3 (contents of parens) + (?: + (?> [^()]+ ) # Non-parens without backtracking + | + (?2) # Recurse to start of paren group 2 + )* + ) + \) + ) + ) + }x; + +If the pattern was used as follows + + 'foo(bar(baz)+baz(bop))'=~/$re/ + and print "\$1 = $1\n", + "\$2 = $2\n", + "\$3 = $3\n"; + +the output produced should be the following: + + $1 = foo(bar(baz)+baz(bop)) + $2 = (bar(baz)+baz(bop)) + $3 = bar(baz)+baz(bop) + +If there is no corresponding capture buffer defined, then it is a +fatal error. Recursing deeper than 50 times without consuming any input +string will also result in a fatal error. The maximum depth is compiled +into perl, so changing it requires a custom build. + +The following shows how using negative indexing can make it +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) { + # do something here... + } -B: This extended regular expression feature is considered -highly experimental, and may be changed or deleted without notice. +B that this pattern does not behave the same way as the equivalent +PCRE or Python construct of the same form. In perl you can backtrack into +a recursed group, in PCRE and Python the recursed into group is treated +as atomic. Also, modifiers are resolved at compile time, so constructs +like (?i:(?1)) or (?:(?i)(?1)) do not affect how the sub-pattern will +be processed. + +=item C<(?&NAME)> +X<(?&NAME)> + +Recurse to a named subpattern. Identical to (?PARNO) except that the +parenthesis to recurse to is determined by name. If multiple parens have +the same name, then it recurses to the leftmost. + +It is an error to refer to a name that is not declared somewhere in the +pattern. + +=item C<(?(condition)yes-pattern|no-pattern)> +X<(?()> + +=item C<(?(condition)yes-pattern)> + +Conditional expression. C<(condition)> should be either an integer in +parentheses (which is valid if the corresponding pair of parentheses +matched), a look-ahead/look-behind/evaluate zero-width assertion, a +name in angle brackets or single quotes (which is valid if a buffer +with the given name matched), or 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 +be true only when evaluated during recursion in the named group. + +Here's a summary of the possible predicates: + +=over 4 + +=item (1) (2) ... + +Checks if the numbered capturing buffer has matched something. + +=item () ('NAME') + +Checks if a buffer with the given name has matched something. + +=item (?{ CODE }) + +Treats the code block as the condition. + +=item (R) + +Checks if the expression has been evaluated inside of recursion. + +=item (R1) (R2) ... + +Checks if the expression has been evaluated while executing directly +inside of the n-th capture group. This check is the regex equivalent of + + if ((caller(0))[3] eq 'subname') { ... } + +In other words, it does not check the full recursion stack. + +=item (R&NAME) + +Similar to C<(R1)>, this predicate checks to see if we're executing +directly inside of the leftmost group with a given name (this is the same +logic used by C<(?&NAME)> to disambiguate). It does not check the full +stack, but only the name of the innermost active recursion. + +=item (DEFINE) + +In this case, the yes-pattern is never directly executed, and no +no-pattern is allowed. Similar in spirit to C<(?{0})> but more efficient. +See below for details. + +=back + +For example: + + m{ ( \( )? + [^()]+ + (?(1) \) ) + }x + +matches a chunk of non-parentheses, possibly included in parentheses +themselves. + +A special form is the C<(DEFINE)> 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. +This way, you can define a set of regular expression rules that can be +bundled into any pattern you choose. + +It is recommended that for this usage you put the DEFINE block at the +end of the pattern, and that you name any subpatterns defined within it. + +Also, it's worth noting that patterns defined this way probably will +not be as efficient, as the optimiser is not very clever about +handling them. + +An example of how this might be used is as follows: + + /(?(&NAME_PAT))(?(&ADDRESS_PAT)) + (?(DEFINE) + (....) + (....) + )/x + +Note that capture buffers matched inside of recursion are not accessible +after the recursion returns, so the extra layer of capturing buffers are +necessary. Thus C<$+{NAME_PAT}> would not be defined even though +C<$+{NAME}> would be. + +=item C<< (?>pattern) >> +X X X X An "independent" subexpression, one which matches the substring that a I C would match if anchored at the given @@ -630,12 +1074,12 @@ in the rest of a regular expression.) Consider this pattern: m{ \( - ( - [^()]+ # x+ - | + ( + [^()]+ # x+ + | \( [^()]* \) )+ - \) + \) }x That will efficiently match a nonempty group with matching parentheses @@ -649,13 +1093,13 @@ seconds, but that each extra letter doubles this time. This exponential performance will make it appear that your program has hung. However, a tiny change to this pattern - m{ \( - ( - (?> [^()]+ ) # change x+ above to (?> x+ ) - | + m{ \( + ( + (?> [^()]+ ) # change x+ above to (?> x+ ) + | \( [^()]* \) )+ - \) + \) }x which uses C<< (?>...) >> matches exactly when the one above does (verifying @@ -663,7 +1107,7 @@ 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 the C pragma or B<-w> switch saying it -C<"matches the null string many times">): +C<"matches null string many times in regex">. On simple groups, such as the pattern C<< (?> [^()]+ ) >>, a comparable effect may be achieved by negative look-ahead, as in C<[^()]+ (?! [^()] )>. @@ -690,30 +1134,277 @@ one of these: Which one you pick depends on which of these expressions better reflects the above specification of comments. -=item C<(?(condition)yes-pattern|no-pattern)> +In some literature this construct is called "atomic matching" or +"possessive matching". -=item C<(?(condition)yes-pattern)> +Possessive quantifiers are equivalent to putting the item they are applied +to inside of one of these constructs. The following equivalences apply: -B: This extended regular expression feature is considered -highly experimental, and may be changed or deleted without notice. + Quantifier Form Bracketing Form + --------------- --------------- + PAT*+ (?>PAT*) + PAT++ (?>PAT+) + PAT?+ (?>PAT?) + PAT{min,max}+ (?>PAT{min,max}) -Conditional expression. C<(condition)> should be either an integer in -parentheses (which is valid if the corresponding pair of parentheses -matched), or look-ahead/look-behind/evaluate zero-width assertion. +=back -For example: +=head2 Special Backtracking Control Verbs - m{ ( \( )? - [^()]+ - (?(1) \) ) - }x +B These patterns are experimental and subject to change or +removal in a future version of perl. Their usage in production code should +be noted to avoid problems during upgrades. -matches a chunk of non-parentheses, possibly included in parentheses -themselves. +These special patterns are generally of the form C<(*VERB:ARG)>. Unless +otherwise stated the ARG argument is optional; in some cases, it is +forbidden. + +Any pattern containing a special backtracking verb that allows an argument +has the special behaviour that when executed it sets the current packages' +C<$REGERROR> and C<$REGMARK> variables. When doing so the following +rules apply: + +On failure, the C<$REGERROR> variable will be set to the ARG value of the +verb pattern, if the verb was involved in the failure of the match. If the +ARG part of the pattern was omitted, then C<$REGERROR> will be set to the +name of the last C<(*MARK:NAME)> pattern executed, or to TRUE if there was +none. Also, the C<$REGMARK> variable will be set to FALSE. + +On a successful match, the C<$REGERROR> variable will be set to FALSE, and +the C<$REGMARK> variable will be set to the name of the last +C<(*MARK:NAME)> pattern executed. See the explanation for the +C<(*MARK:NAME)> verb below for more details. + +B C<$REGERROR> and C<$REGMARK> are not magic variables like C<$1> +and most other regex related variables. They are not local to a scope, nor +readonly, but instead are volatile package variables similar to C<$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 + +=item Verbs that take an argument + +=over 4 + +=item C<(*PRUNE)> C<(*PRUNE:NAME)> +X<(*PRUNE)> X<(*PRUNE:NAME)> + +This zero-width pattern prunes the backtracking tree at the current point +when backtracked into on failure. Consider the pattern C, +where A and B are complex patterns. Until the C<(*PRUNE)> verb is reached, +A may backtrack as necessary to match. Once it is reached, matching +continues in B, which may also backtrack as necessary; however, should B +not match, then no further backtracking will take place, and the pattern +will fail outright at the current starting position. + +The following example counts all the possible matching strings in a +pattern (without actually matching any of them). + + 'aaab' =~ /a+b?(?{print "$&\n"; $count++})(*FAIL)/; + print "Count=$count\n"; + +which produces: + + aaab + aaa + aa + a + aab + aa + a + ab + a + Count=9 + +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 +at each matching startpoint like so: + + aaab + aab + ab + Count=3 + +Any number of C<(*PRUNE)> assertions may be used in a pattern. + +See also C<< (?>pattern) >> and possessive quantifiers for other ways to +control backtracking. In some cases, the use of C<(*PRUNE)> can be +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)> + +This zero-width pattern is similar to C<(*PRUNE)>, except that on +failure it also signifies that whatever text that was matched leading up +to the C<(*SKIP)> pattern being executed cannot be part of I match +of this pattern. This effectively means that the regex engine "skips" forward +to this position on failure and tries to match again, (assuming that +there is sufficient room to match). + +The name of the C<(*SKIP:NAME)> pattern has special significance. If a +C<(*MARK:NAME)> was encountered while matching, then it is that position +which is used as the "skip point". If no C<(*MARK)> of that name was +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 +is twice as long: + + 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/; + print "Count=$count\n"; + +outputs + + aaab + aaab + Count=2 + +Once the 'aaab' at the start of the string has matched, and the C<(*SKIP)> +executed, the next startpoint will be where the cursor was when the +C<(*SKIP)> was executed. + +As a shortcut C<(*MARK:NAME)> can be written C<(*:NAME)>. + +=item C<(*MARK:NAME)> C<(*:NAME)> +X<(*MARK)> C<(*MARK:NAME)> C<(*: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 +mark may be given a name. A later C<(*SKIP)> pattern will then skip +forward to that point if backtracked into on failure. Any number of +C<(*MARK)> patterns are allowed, and the NAME portion is optional and may +be duplicated. + +In addition to interacting with the C<(*SKIP)> pattern, C<(*MARK:NAME)> +can be used to "label" a pattern branch, so that after matching, the +program can determine which branches of the pattern were involved in the +match. + +When a match is successful, the C<$REGMARK> variable will be set to the +name of the most recently executed C<(*MARK:NAME)> that was involved +in the match. + +This can be used to determine which branch of a pattern was matched +without using a seperate capture buffer for each branch, which in turn +can result in a performance improvement, as perl cannot optimize +C as efficiently as something like +C. + +When a match has failed, and unless another verb has been involved in +failing the match and has provided its own name to use, the C<$REGERROR> +variable will be set to the name of the most recently executed +C<(*MARK:NAME)>. + +See C<(*SKIP)> for more details. + +=item C<(*THEN)> C<(*THEN:NAME)> + +This is similar to the "cut group" operator C<::> from Perl6. 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). + +Its name comes from the observation that this operation combined with the +alternation operator (C<|>) can be used to create what is essentially a +pattern-based if/then/else block: + + ( COND (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) + +Note that if this operator is used and NOT inside of an alternation then +it acts exactly like the C<(*PRUNE)> operator. + + / A (*PRUNE) B / + +is the same as + + / A (*THEN) B / + +but + + / ( A (*THEN) B | C (*THEN) D ) / + +is not the same as + + / ( A (*PRUNE) B | C (*PRUNE) D ) / + +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. + +=item C<(*COMMIT)> +X<(*COMMIT)> + +This is the Perl6 "commit pattern" C<< >> or C<:::>. It's a +zero-width pattern similar to C<(*SKIP)>, except that when backtracked +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"; + +outputs + + aaab + Count=1 + +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)> + +This pattern matches nothing and always fails. It can be used to force the +engine to backtrack. It is equivalent to C<(?!)>, but easier to read. In +fact, C<(?!)> gets optimised into C<(*FAIL)> internally. + +It is probably useful only when combined with C<(?{})> or C<(??{})>. + +=item C<(*ACCEPT)> +X<(*ACCEPT)> + +B This feature is highly experimental. It is not recommended +for production code. + +This pattern matches nothing and causes the end of successful matching at +the point at which the C<(*ACCEPT)> pattern was encountered, regardless of +whether there is actually more to match in the string. When inside of a +nested pattern, such as recursion or a dynamically generated subbpattern +via C<(??{})>, only the innermost pattern is ended immediately. + +If the C<(*ACCEPT)> is inside of capturing buffers then the buffers are +marked as ended at the point at which the C<(*ACCEPT)> was encountered. +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 parens were matched, such as in the +string 'ACDE', then the C and C would have to be matched as well. + +=back =back =head2 Backtracking +X X NOTE: This section presents an abstract approximation of regular expression behavior. For a more rigorous (and complicated) view of @@ -771,7 +1462,7 @@ and the first "bar" thereafter. got Here's another example: let's say you'd like to match a number at the end -of a string, and you also want to keep the preceding part the match. +of a string, and you also want to keep the preceding part of the match. So you write this: $_ = "I have 2 numbers: 53147"; @@ -827,7 +1518,7 @@ multiple ways it might succeed, you need to understand backtracking to know which variety of success you will achieve. When using look-ahead assertions and negations, this can all get even -tricker. Imagine you'd like to find a sequence of non-digits not +trickier. Imagine you'd like to find a sequence of non-digits not followed by "123". You might try to write that as $_ = "ABC123"; @@ -837,16 +1528,16 @@ followed by "123". You might try to write that as But that isn't going to match; at least, not the way you're hoping. It claims that there is no 123 in the string. Here's a clearer picture of -why it that pattern matches, contrary to popular expectations: +why that pattern matches, contrary to popular expectations: - $x = 'ABC123' ; - $y = 'ABC445' ; + $x = 'ABC123'; + $y = 'ABC445'; - print "1: got $1\n" if $x =~ /^(ABC)(?!123)/ ; - print "2: got $1\n" if $y =~ /^(ABC)(?!123)/ ; + print "1: got $1\n" if $x =~ /^(ABC)(?!123)/; + print "2: got $1\n" if $y =~ /^(ABC)(?!123)/; - print "3: got $1\n" if $x =~ /^(\D*)(?!123)/ ; - print "4: got $1\n" if $y =~ /^(\D*)(?!123)/ ; + print "3: got $1\n" if $x =~ /^(\D*)(?!123)/; + print "4: got $1\n" if $y =~ /^(\D*)(?!123)/; This prints @@ -881,8 +1572,8 @@ are zero-width expressions--they only look, but don't consume any of the string in their match. So rewriting this way produces what you'd expect; that is, case 5 will fail, but case 6 succeeds: - print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/ ; - print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/ ; + print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/; + print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/; 6: got ABC @@ -901,21 +1592,26 @@ ways they can use backtracking to try match. For example, without internal optimizations done by the regular expression engine, this will take a painfully long time to run: - 'aaaaaaaaaaaa' =~ /((a{0,5}){0,5}){0,5}[c]/ + 'aaaaaaaaaaaa' =~ /((a{0,5}){0,5})*[c]/ -And if you used C<*>'s instead of limiting it to 0 through 5 matches, -then it would take forever--or until you ran out of stack space. +And if you used C<*>'s in the internal groups instead of limiting them +to 0 through 5 matches, then it would take forever--or until you ran +out of stack space. Moreover, these internal optimizations are not +always applicable. For example, if you put C<{0,5}> instead of C<*> +on the external group, no current optimization is applicable, and the +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 zero-length look-ahead/look-behind assertions will not backtrack to make -the tail match, since they are in "logical" context: only +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) >>>. =head2 Version 8 Regular Expressions +X X X In case you're not familiar with the "regular" Version 8 regex routines, here are the pattern-matching rules not described above. @@ -929,7 +1625,7 @@ series of characters 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 -in C<[]>, which will match any one character from the list. If the +in C<[]>, which will match any character from the list. If the first character after the "[" is "^", the class matches any character not in the list. Within a list, the "-" character specifies a range, so that C represents all characters between "a" and "z", @@ -939,10 +1635,10 @@ escape it with a backslash. "-" is also taken literally when it is at the end of the list, just before the closing "]". (The following all specify the same class of three characters: C<[-az]>, C<[az-]>, and C<[a\-z]>. All are different from C<[a-z]>, which -specifies a class containing twenty-six characters.) -Also, if you try to use the character classes C<\w>, C<\W>, C<\s>, -C<\S>, C<\d>, or C<\D> as endpoints of a range, that's not a range, -the "-" is understood literally. +specifies a class containing twenty-six characters, even on EBCDIC-based +character sets.) Also, if you try to use the character +classes C<\w>, C<\W>, C<\s>, C<\S>, C<\d>, or C<\D> as endpoints of +a range, the "-" is understood literally. Note also that the whole range idea is rather unportable between character sets--and even within character sets they may cause results @@ -954,11 +1650,11 @@ spell out the character sets in full. Characters may be specified using a metacharacter syntax much like that used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return, "\f" a form feed, etc. More generally, \I, where I is a string -of octal digits, matches the character whose ASCII value is I. -Similarly, \xI, where I are hexadecimal digits, matches the -character whose ASCII value is I. The expression \cI matches the -ASCII character control-I. Finally, the "." metacharacter matches any -character except "\n" (unless you use C). +of octal digits, matches the character whose coded character set value +is I. Similarly, \xI, where I are hexadecimal digits, +matches the character whose numeric value is I. The expression \cI +matches the character control-I. Finally, the "." metacharacter +matches any character except "\n" (unless you use C). You can specify a series of alternatives for a pattern using "|" to separate them, so that C will match any of "fee", "fie", @@ -1000,7 +1696,7 @@ Some people get too used to writing things like: This is grandfathered for the RHS of a substitute to avoid shocking the B addicts, but it's a dirty habit to get into. That's because in -PerlThink, the righthand side of a C is a double-quoted string. C<\1> in +PerlThink, the righthand side of an C is a double-quoted string. C<\1> in the usual double-quoted string means a control-A. The customary Unix meaning of C<\1> is kludged in for C. However, if you get into the habit of doing that, you get yourself into trouble if you then add an C @@ -1061,17 +1757,17 @@ zero-length substring. Thus m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x; -is made equivalent to +is made equivalent to - m{ (?: NON_ZERO_LENGTH )* - | - (?: ZERO_LENGTH )? + m{ (?: NON_ZERO_LENGTH )* + | + (?: ZERO_LENGTH )? }x; The higher level-loops preserve an additional state between iterations: -whether the last match was zero-length. To break the loop, the following +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">), +This prohibition interacts with backtracking (see L<"Backtracking">), and so the I match is chosen if the I match is of zero length. @@ -1080,12 +1776,12 @@ For example: $_ = 'bar'; s/\w??/<$&>/g; -results in C<"<><><><>">. At each position of the string the best -match given by non-greedy C is the zero-length match, and the I<><><> >>. At each position of the string the best +match given by non-greedy C is the zero-length match, and the I match is what is matched by C<\w>. Thus zero-length matches alternate with one-character-long matches. -Similarly, for repeated C the second-best match is the match at the +Similarly, for repeated C the second-best match is the match at the position one notch further in the string. The additional state of being I is associated with @@ -1120,13 +1816,13 @@ one match at a given position is possible. This section describes the notion of better/worse for combining operators. In the description below C and C are regular subexpressions. -=over +=over 4 =item C 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. +which can be matched by C. If C is better match for C than C, C is a better match than C. @@ -1176,10 +1872,10 @@ else in the whole regular expression.) For this grouping operator there is no need to describe the ordering, since only whether or not C can match is important. -=item C<(??{ EXPR })> +=item C<(??{ EXPR })>, C<(?PARNO)> The ordering is the same as for the regular expression which is -the result of EXPR. +the result of EXPR, or the pattern contained by capture buffer PARNO. =item C<(?(condition)yes-pattern|no-pattern)> @@ -1200,7 +1896,7 @@ Overloaded constants (see L) provide a simple way to extend the functionality of the RE engine. Suppose that we want to enable a new RE escape-sequence C<\Y|> which -matches at boundary between white-space characters and non-whitespace +matches at boundary between whitespace characters and non-whitespace characters. Note that C<(?=\S)(? matches exactly at these positions, so we want to have each C<\Y|> in the place of the more complicated version. We can create a module C to do @@ -1217,14 +1913,16 @@ this: sub invalid { die "/$_[0]/: invalid escape '\\$_[1]'"} - my %rules = ( '\\' => '\\', + # We must also take care of not escaping the legitimate \\Y| + # sequence, hence the presence of '\\' in the conversion rules. + my %rules = ( '\\' => '\\\\', 'Y|' => qr/(?=\S)(?. + +L. + L. L. @@ -1262,5 +1964,7 @@ L. L. +L. + I by Jeffrey Friedl, published by O'Reilly and Associates.