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 =item p X

X X @@ -74,14 +114,24 @@ rather than the regex itself. See L for further explanation of the g and c modifiers. +=item a, d, l and u +X X X X + +These modifiers, all new in 5.14, affect which character-set semantics +(Unicode, etc.) are used, as described below in +L. + =back -These are usually written as "the C modifier", even though the delimiter -in question might not really be a slash. Any of these -modifiers may also be embedded within the regular expression itself using -the C<(?...)> construct. See below. +Regular expression modifiers are usually written in documentation +as e.g., "the C modifier", even though the delimiter +in question might not really be a slash. The modifiers C +may also be embedded within the regular expression itself using +the C<(?...)> construct, see L below. + +=head3 /x -The C modifier itself needs a little more explanation. It tells +C tells the regular expression parser to ignore most whitespace that is neither backslashed nor within a character class. You can use this to break up your regular expression into (slightly) more readable parts. The C<#> @@ -96,7 +146,7 @@ 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. And note that C doesn't affect -whether space interpretation within a single multi-character construct. For +space interpretation within a single multi-character construct. For example in C<\x{...}>, regardless of the C modifier, there can be no spaces. Same for a L such as C<{3}> or C<{5,}>. Similarly, C<(?:...)> can't have a space between the C and C<:>, @@ -104,10 +154,309 @@ but can between the C<(> and C. Within any delimiters for such a construct, allowed spaces are not affected by C, and depend on the construct. For example, C<\x{...}> can't have spaces because hexadecimal numbers don't have spaces in them. But, Unicode properties can have spaces, so -in C<\p{...}> there can be spaces that follow the Unicode rules, for which see +in C<\p{...}> there can be spaces that follow the Unicode rules, for which see L. X +=head3 Character set modifiers + +C, C, C, and C, available starting in 5.14, are called +the character set modifiers; they affect the character set semantics +used for the regular expression. + +The C, C, and C modifiers are not likely to be of much use +to you, and so you need not worry about them very much. They exist for +Perl's internal use, so that complex regular expression data structures +can be automatically serialized and later exactly reconstituted, +including all their nuances. But, since Perl can't keep a secret, and +there may be rare instances where they are useful, they are documented +here. + +The C modifier, on the other hand, may be useful. Its purpose is to +allow code that is to work mostly on ASCII data to not have to concern +itself with Unicode. + +Briefly, C sets the character set to that of whatever Bocale is in +effect at the time of the execution of the pattern match. + +C sets the character set to Bnicode. + +C also sets the character set to Unicode, BUT adds several +restrictions for BSCII-safe matching. + +C is the old, problematic, pre-5.14 Befault character set +behavior. Its only use is to force that old behavior. + +At any given time, exactly one of these modifiers is in effect. Their +existence allows Perl to keep the originally compiled behavior of a +regular expression, regardless of what rules are in effect when it is +actually executed. And if it is interpolated into a larger regex, the +original's rules continue to apply to it, and only it. + +The C and C modifiers are automatically selected for +regular expressions compiled within the scope of various pragmas, +and we recommend that in general, you use those pragmas instead of +specifying these modifiers explicitly. For one thing, the modifiers +affect only pattern matching, and do not extend to even any replacement +done, whereas using the pragmas give consistent results for all +appropriate operations within their scopes. For example, + + s/foo/\Ubar/il + +will match "foo" using the locale's rules for case-insensitive matching, +but the C does not affect how the C<\U> operates. Most likely you +want both of them to use locale rules. To do this, instead compile the +regular expression within the scope of C. This both +implicitly adds the C and applies locale rules to the C<\U>. The +lesson is to C and not C explicitly. + +Similarly, it would be better to use C +instead of, + + s/foo/\Lbar/iu + +to get Unicode rules, as the C<\L> in the former (but not necessarily +the latter) would also use Unicode rules. + +More detail on each of the modifiers follows. Most likely you don't +need to know this detail for C, C, and C, and can skip ahead +to La|/Ea (and Eaa)>. + +=head4 /l + +means to use the current locale's rules (see L) when pattern +matching. For example, C<\w> will match the "word" characters of that +locale, and C<"/i"> case-insensitive matching will match according to +the locale's case folding rules. The locale used will be the one in +effect at the time of execution of the pattern match. This may not be +the same as the compilation-time locale, and can differ from one match +to another if there is an intervening call of the +L. + +Perl only supports single-byte locales. This means that code points +above 255 are treated as Unicode no matter what locale is in effect. +Under Unicode rules, there are a few case-insensitive matches that cross +the 255/256 boundary. These are disallowed under C. For example, +0xFF (on ASCII platforms) does not caselessly match the character at +0x178, C, because 0xFF may not be +C in the current locale, and Perl +has no way of knowing if that character even exists in the locale, much +less what code point it is. + +This modifier may be specified to be the default by C, but +see L. +X + +=head4 /u + +means to use Unicode rules when pattern matching. On ASCII platforms, +this means that the code points between 128 and 255 take on their +Latin-1 (ISO-8859-1) meanings (which are the same as Unicode's). +(Otherwise Perl considers their meanings to be undefined.) Thus, +under this modifier, the ASCII platform effectively becomes a Unicode +platform; and hence, for example, C<\w> will match any of the more than +100_000 word characters in Unicode. + +Unlike most locales, which are specific to a language and country pair, +Unicode classifies all the characters that are letters I in +the world as +C<\w>. For example, your locale might not think that C is a letter (unless you happen to speak Icelandic), but +Unicode does. Similarly, all the characters that are decimal digits +somewhere in the world will match C<\d>; this is hundreds, not 10, +possible matches. And some of those digits look like some of the 10 +ASCII digits, but mean a different number, so a human could easily think +a number is a different quantity than it really is. For example, +C (U+09EA) looks very much like an +C (U+0038). And, C<\d+>, may match strings of digits +that are a mixture from different writing systems, creating a security +issue. L can be used to sort +this out. Or the C modifier can be used to force C<\d> to match +just the ASCII 0 through 9. + +Also, under this modifier, case-insensitive matching works on the full +set of Unicode +characters. The C, for example matches the letters "k" and +"K"; and C matches the sequence "ff", which, +if you're not prepared, might make it look like a hexadecimal constant, +presenting another potential security issue. See +L for a detailed discussion of Unicode +security issues. + +On the EBCDIC platforms that Perl handles, the native character set is +equivalent to Latin-1. Thus this modifier changes behavior only when +the C<"/i"> modifier is also specified, and it turns out it affects only +two characters, giving them full Unicode semantics: the C +will match the Greek capital and small letters C, otherwise not; and +the C will match any of C, C, +C, and C, otherwise not. + +This modifier may be specified to be the default by C, C, or +C> (or higher), +but see L. +X + +=head4 /d + +This modifier means to use the "Default" native rules of the platform +except when there is cause to use Unicode rules instead, as follows: + +=over 4 + +=item 1 + +the target string is encoded in UTF-8; or + +=item 2 + +the pattern is encoded in UTF-8; or + +=item 3 + +the pattern explicitly mentions a code point that is above 255 (say by +C<\x{100}>); or + +=item 4 + +the pattern uses a Unicode name (C<\N{...}>); or + +=item 5 + +the pattern uses a Unicode property (C<\p{...}>) + +=back + +Another mnemonic for this modifier is "Depends", as the rules actually +used depend on various things, and as a result you can get unexpected +results. See L. The Unicode Bug has +become rather infamous, leading to yet another (printable) name for this +modifier, "Dodgy". + +On ASCII platforms, the native rules are ASCII, and on EBCDIC platforms +(at least the ones that Perl handles), they are Latin-1. + +Here are some examples of how that works on an ASCII platform: + + $str = "\xDF"; # $str is not in UTF-8 format. + $str =~ /^\w/; # No match, as $str isn't in UTF-8 format. + $str .= "\x{0e0b}"; # Now $str is in UTF-8 format. + $str =~ /^\w/; # Match! $str is now in UTF-8 format. + chop $str; + $str =~ /^\w/; # Still a match! $str remains in UTF-8 format. + +This modifier is automatically selected by default when none of the +others are, so yet another name for it is "Default". + +Because of the unexpected behaviors associated with this modifier, you +probably should only use it to maintain weird backward compatibilities. + +=head4 /a (and /aa) + +This modifier stands for ASCII-restrict (or ASCII-safe). This modifier, +unlike the others, may be doubled-up to increase its effect. + +When it appears singly, it causes the sequences C<\d>, C<\s>, C<\w>, and +the Posix character classes to match only in the ASCII range. They thus +revert to their pre-5.6, pre-Unicode meanings. Under C, C<\d> +always means precisely the digits C<"0"> to C<"9">; C<\s> means the five +characters C<[ \f\n\r\t]>; C<\w> means the 63 characters +C<[A-Za-z0-9_]>; and likewise, all the Posix classes such as +C<[[:print:]]> match only the appropriate ASCII-range characters. + +This modifier is useful for people who only incidentally use Unicode, +and who do not wish to be burdened with its complexities and security +concerns. + +With C, one can write C<\d> with confidence that it will only match +ASCII characters, and should the need arise to match beyond ASCII, you +can instead use C<\p{Digit}> (or C<\p{Word}> for C<\w>). There are +similar C<\p{...}> constructs that can match beyond ASCII both white +space (see L), and Posix classes (see +L). Thus, this modifier +doesn't mean you can't use Unicode, it means that to get Unicode +matching you must explicitly use a construct (C<\p{}>, C<\P{}>) that +signals Unicode. + +As you would expect, this modifier causes, for example, C<\D> to mean +the same thing as C<[^0-9]>; in fact, all non-ASCII characters match +C<\D>, C<\S>, and C<\W>. C<\b> still means to match at the boundary +between C<\w> and C<\W>, using the C definitions of them (similarly +for C<\B>). + +Otherwise, C behaves like the C modifier, in that +case-insensitive matching uses Unicode semantics; for example, "k" will +match the Unicode C<\N{KELVIN SIGN}> under C matching, and code +points in the Latin1 range, above ASCII will have Unicode rules when it +comes to case-insensitive matching. + +To forbid ASCII/non-ASCII matches (like "k" with C<\N{KELVIN SIGN}>), +specify the "a" twice, for example C or C. (The first +occurrence of "a" restricts the C<\d>, etc., and the second occurrence +adds the C restrictions.) But, note that code points outside the +ASCII range will use Unicode rules for C matching, so the modifier +doesn't really restrict things to just ASCII; it just forbids the +intermixing of ASCII and non-ASCII. + +To summarize, this modifier provides protection for applications that +don't wish to be exposed to all of Unicode. Specifying it twice +gives added protection. + +This modifier may be specified to be the default by C +or C. If you do so, you may actually have occasion to use +the C modifier explictly if there are a few regular expressions +where you do want full Unicode rules (but even here, it's best if +everything were under feature C<"unicode_strings">, along with the +C). Also see L. +X +X + +=head4 Which character set modifier is in effect? + +Which of these modifiers is in effect at any given point in a regular +expression depends on a fairly complex set of interactions. These have +been designed so that in general you don't have to worry about it, but +this section gives the gory details. As +explained below in L it is possible to explicitly +specify modifiers that apply only to portions of a regular expression. +The innermost always has priority over any outer ones, and one applying +to the whole expression has priority over any of the default settings that are +described in the remainder of this section. + +The Cfoo'|re/"'/flags' mode">> pragma can be used to set +default modifiers (including these) for regular expressions compiled +within its scope. This pragma has precedence over the other pragmas +listed below that also change the defaults. + +Otherwise, C> sets the default modifier to C; +and C>, or +C> (or higher) set the default to +C when not in the same scope as either C> +or C>. +(C> also +sets the default to C, overriding any plain C.) +Unlike the mechanisms mentioned above, these +affect operations besides regular expressions pattern matching, and so +give more consistent results with other operators, including using +C<\U>, C<\l>, etc. in substitution replacements. + +If none of the above apply, for backwards compatibility reasons, the +C modifier is the one in effect by default. As this can lead to +unexpected results, it is best to specify which other rule set should be +used. + +=head4 Character set modifier behavior prior to Perl 5.14 + +Prior to 5.14, there were no explicit modifiers, but C was implied +for regexes compiled within the scope of C, and C was +implied otherwise. However, interpolating a regex into a larger regex +would ignore the original compilation in favor of whatever was in effect +at the time of the second compilation. There were a number of +inconsistencies (bugs) with the C modifier, where Unicode rules +would be used when inappropriate, and vice versa. C<\p{}> did not imply +Unicode rules, and neither did all occurrences of C<\N{}>, until 5.12. + =head2 Regular Expressions =head3 Metacharacters @@ -142,7 +491,7 @@ 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<$*>, -but this practice has been removed in perl 5.9.) +but this option was removed in perl 5.10.) X<^> X<$> X To simplify multi-line substitutions, the "." character never matches a @@ -162,9 +511,25 @@ X X X<*> X<+> X X<{n}> X<{n,}> X<{n,m}> {n,} Match at least n times {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. In particular, the lower bound -is not optional.) The "*" quantifier is equivalent to C<{0,}>, the "+" +(If a curly bracket occurs in any other context and does not form part of +a backslashed sequence like C<\x{...}>, it is treated +as a regular character. In particular, the lower quantifier bound +is not optional. However, in Perl v5.18, it is planned to issue a +deprecation warning for all such occurrences, and in Perl v5.20 to +require literal uses of a curly bracket to be escaped, say by preceding +them with a backslash or enclosing them within square brackets, (C<"\{"> +or C<"[{]">). This change will allow for future syntax extensions (like +making the lower bound of a quantifier optional), and better error +checking of quantifiers. Now, a typo in a quantifier silently causes +it to be treated as the literal characters. For example, + + /o{4,3}/ + +looks like a quantifier that matches 0 times, since 4 is greater than 3, +but it really means to match the sequence of six characters +S>.) + +The "*" quantifier is equivalent to C<{0,}>, the "+" quantifier to C<{1,}>, and the "?" quantifier to C<{0,1}>. n and m are limited to non-negative 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 @@ -183,7 +548,7 @@ X X<*?> X<+?> X X<{n}?> X<{n,}?> X<{n,m}?> *? Match 0 or more times, not greedily +? Match 1 or more times, not greedily ?? Match 0 or 1 time, not greedily - {n}? Match exactly n times, not greedily + {n}? Match exactly n times, not greedily (redundant) {n,}? Match at least n times, not greedily {n,m}? Match at least n but not more than m times, not greedily @@ -212,7 +577,8 @@ string" problem can be most efficiently performed when written as: /"(?:[^"\\]++|\\.)*+"/ as we know that if the final quote does not match, backtracking will not -help. See the independent subexpression C<< (?>...) >> for more details; +help. See the independent subexpression +Lpattern) >>> for more details; possessive quantifiers are just syntactic sugar for that construct. For instance the above example could also be written as follows: @@ -220,7 +586,7 @@ instance the above example could also be written as follows: =head3 Escape sequences -Because patterns are processed as double quoted strings, the following +Because patterns are processed as double-quoted strings, the following also work: \t tab (HT, TAB) @@ -258,7 +624,7 @@ X<\g> X<\k> X<\K> X uppercase character. \w [3] Match a "word" character (alphanumeric plus "_", plus other connector punctuation chars plus Unicode - marks + marks) \W [3] Match a non-"word" character \s [3] Match a whitespace character \S [3] Match a non-whitespace character @@ -357,10 +723,11 @@ The C<\G> assertion can be used to chain global matches (using C), as described in L. 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 +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. Note that the rule for zero-length -matches is modified somewhat, in that contents to the left of C<\G> is +matches (see L) +is modified somewhat, in that contents to the left of C<\G> are not counted when determining the length of the match. Thus the following will not match forever: X<\G> @@ -445,7 +812,8 @@ is probably not what you intended. The C<\g> and C<\k> notations were introduced in Perl 5.10.0. Prior to that there were no named nor relative numbered capture groups. Absolute numbered -groups were referred to using C<\1>, C<\2>, etc, and this notation is still +groups were referred to using C<\1>, +C<\2>, etc., and this notation is still accepted (and likely always will be). But it leads to some ambiguities if there are more than 9 capture groups, as C<\10> could mean either the tenth capture group, or the character whose ordinal in octal is 010 (a backspace in @@ -460,7 +828,7 @@ digits padded with leading zeros, since a leading zero implies an octal constant. The C<\I> notation also works in certain circumstances outside -the pattern. See L below for details.) +the pattern. See L below for details. Examples: @@ -528,8 +896,10 @@ use C<$&>, C<$`> or C<$'>, then patterns I capturing parentheses will not be penalized. So avoid C<$&>, C<$'>, and C<$`> 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<$&> is not so costly as the -other two. +already paid the price. As of 5.17.4, the presence of each of the three +variables in a program is recorded separately, and depending on +circumstances, perl may be able be more efficient knowing that only C<$&> +rather than all three have been seen, for example. X<$&> X<$`> X<$'> As a workaround for this problem, Perl 5.10.0 introduces C<${^PREMATCH}>, @@ -567,10 +937,13 @@ backslash interpolation may lead to confusing results. If you I to use literal backslashes within C<\Q...\E>, consult L. +C and C<\Q> are fully described in L. + =head2 Extended Patterns Perl also defines a consistent extension syntax for features not -found in standard tools like B and B. The syntax is a +found in standard tools like B and +B. The syntax for most of these is a pair of parentheses with a question mark as the first thing within the parentheses. The character after the question mark indicates the extension. @@ -584,9 +957,9 @@ status. A question mark was chosen for this and for the minimal-matching construct because 1) question marks are rare in older regular expressions, and 2) whenever you see one, you should stop and -"question" exactly what is going on. That's psychology... +"question" exactly what is going on. That's psychology.... -=over 10 +=over 4 =item C<(?#text)> X<(?#)> @@ -596,9 +969,9 @@ whitespace formatting, a simple C<#> will suffice. Note that Perl closes the comment as soon as it sees a C<)>, so there is no way to put a literal C<)> in the comment. -=item C<(?dlupimsx-imsx)> +=item C<(?adlupimsx-imsx)> -=item C<(?^lupimsx)> +=item C<(?^alupimsx)> X<(?)> X<(?^)> One or more embedded pattern-match modifiers, to be turned on (or @@ -607,8 +980,8 @@ 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, taken from an argument, or specified in a table -somewhere. Consider the case where some patterns want to be case -sensitive and some do not: The case insensitive ones merely need to +somewhere. Consider the case where some patterns want to be +case-sensitive and some do not: The case-insensitive ones merely need to include C<(?i)> at the front of the pattern. For example: $pattern = "foobar"; @@ -628,60 +1001,25 @@ repetition of the previous word, assuming the C modifier, and no C modifier outside this group. These modifiers do not carry over into named subpatterns called in the -enclosing group. In other words, a pattern such as C<((?i)(&NAME))> does not +enclosing group. In other words, a pattern such as C<((?i)(?&NAME))> does not change the case-sensitivity of the "NAME" pattern. +Any of these modifiers can be set to apply globally to all regular +expressions compiled within the scope of a C. See +L. + Starting in Perl 5.14, a C<"^"> (caret or circumflex accent) immediately after the C<"?"> is a shorthand equivalent to C. Flags (except C<"d">) may follow the caret to override it. But a minus sign is not legal with it. -Also, starting in Perl 5.14, are modifiers C<"d">, C<"l">, and C<"u">, -which for 5.14 may not be used as suffix modifiers. - -C<"l"> means to use a locale (see L) when pattern matching. -The locale used will be the one in effect at the time of execution of -the pattern match. This may not be the same as the compilation-time -locale, and can differ from one match to another if there is an -intervening call of the -L. -This modifier is automatically set if the regular expression is compiled -within the scope of a C<"use locale"> pragma. - -C<"u"> means to use Unicode semantics when pattern matching. It is -automatically set if the regular expression is compiled within the scope -of a L|feature> pragma (and isn't -also in the scope of L|locale> nor -L|bytes> pragmas. It is not fully implemented at the -time of this writing, but work is being done to complete the job. On -EBCDIC platforms this currently has no effect, but on ASCII platforms, -it effectively turns them into Latin-1 platforms. That is, the ASCII -characters remain as ASCII characters (since ASCII is a subset of -Latin-1), but the non-ASCII code points are treated as Latin-1 -characters. Right now, this only applies to the C<"\b">, C<"\s">, and -C<"\w"> pattern matching operators, plus their complements. For -example, when this option is not on, C<"\w"> matches precisely -C<[A-Za-z0-9_]> (on a non-utf8 string). When the option is on, it -matches not just those, but all the Latin-1 word characters (such as an -"n" with a tilde). It thus matches exactly the same set of code points -from 0 to 255 as it would if the string were encoded in utf8. - -C<"d"> means to use the traditional Perl pattern matching behavior. -This is dualistic (hence the name C<"d">, which also could stand for -"default"). When this is in effect, Perl matches utf8-encoded strings -using Unicode rules, and matches non-utf8-encoded strings using the -platform's native character set rules. -See L. It is automatically selected by -default if the regular expression is compiled neither within the scope -of a C<"use locale"> pragma nor a -pragma. - -Note that the C, C, C

, 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 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 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. -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<$REGERROR> variable will be set to the name of the most recently executed C<(*MARK:NAME)>. -See C<(*SKIP)> for more details. +See L for more details. As a shortcut C<(*MARK:NAME)> can be written C<(*:NAME)>. =item C<(*THEN)> C<(*THEN:NAME)> -This is similar to the "cut group" operator C<::> from Perl 6. Like +This is similar to the "cut group" operator C<::> from Perl 6. Like C<(*PRUNE)>, this verb always matches, and when backtracked into on failure, it causes the regex engine to try the next alternation in the -innermost enclosing group (capturing or otherwise). +innermost enclosing group (capturing or otherwise) that has alternations. +The two branches of a C<(?(condition)yes-pattern|no-pattern)> do not +count as an alternation, as far as C<(*THEN)> is concerned. Its name comes from the observation that this operation combined with the alternation operator (C<|>) can be used to create what is essentially a @@ -1528,15 +1925,21 @@ is the same as but - / ( A (*THEN) B | C (*THEN) D ) / + / ( A (*THEN) B | C ) / is not the same as - / ( A (*PRUNE) B | C (*PRUNE) D ) / + / ( A (*PRUNE) B | C ) / as after matching the A but failing on the B the C<(*THEN)> verb will backtrack and try C; but the C<(*PRUNE)> verb will simply fail. +=back + +=item Verbs without an argument + +=over 4 + =item C<(*COMMIT)> X<(*COMMIT)> @@ -1546,8 +1949,8 @@ into on failure it causes the match to fail outright. No further attempts to find a valid match by advancing the start pointer will occur again. For example, - 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\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.