1PERLRE(1) Perl Programmers Reference Guide PERLRE(1)
2
6 perlre - Perl regular expressions
7
9 This page describes the syntax of regular expressions in Perl.
10 If you haven't used regular expressions before, a tutorial introduction
12 is available in perlretut. If you know just a little about them, a
13 quick-start introduction is available in perlrequick.
14 Except for "The Basics" section, this page assumes you are familiar
16 with regular expression basics, like what is a "pattern", what does it
17 look like, and how it is basically used. For a reference on how they
18 are used, plus various examples of the same, see discussions of "m//",
19 "s///", "qr//" and "??" in "Regexp Quote-Like Operators" in perlop.
20 New in v5.22, "use re 'strict'" applies stricter rules than otherwise
22 when compiling regular expression patterns. It can find things that,
23 while legal, may not be what you intended.
24 The Basics
26 Regular expressions are strings with the very particular syntax and
27 meaning described in this document and auxiliary documents referred to
28 by this one. The strings are called "patterns". Patterns are used to
29 determine if some other string, called the "target", has (or doesn't
30 have) the characteristics specified by the pattern. We call this
31 "matching" the target string against the pattern. Usually the match is
32 done by having the target be the first operand, and the pattern be the
33 second operand, of one of the two binary operators "=~" and "!~",
34 listed in "Binding Operators" in perlop; and the pattern will have been
35 converted from an ordinary string by one of the operators in "Regexp
36 Quote-Like Operators" in perlop, like so:
37 $foo =~ m/abc/
39 This evaluates to true if and only if the string in the variable $foo
41 contains somewhere in it, the sequence of characters "a", "b", then
42 "c". (The "=~ m", or match operator, is described in
43 "m/PATTERN/msixpodualngc" in perlop.)
44 Patterns that aren't already stored in some variable must be delimited,
46 at both ends, by delimiter characters. These are often, as in the
47 example above, forward slashes, and the typical way a pattern is
48 written in documentation is with those slashes. In most cases, the
49 delimiter is the same character, fore and aft, but there are a few
50 cases where a character looks like it has a mirror-image mate, where
51 the opening version is the beginning delimiter, and the closing one is
52 the ending delimiter, like
53 $foo =~ m<abc>
55 Most times, the pattern is evaluated in double-quotish context, but it
57 is possible to choose delimiters to force single-quotish, like
58 $foo =~ m'abc'
60 If the pattern contains its delimiter within it, that delimiter must be
62 escaped. Prefixing it with a backslash (e.g., "/foo\/bar/") serves
63 this purpose.
64 Any single character in a pattern matches that same character in the
66 target string, unless the character is a metacharacter with a special
67 meaning described in this document. A sequence of non-metacharacters
68 matches the same sequence in the target string, as we saw above with
69 "m/abc/".
70 Only a few characters (all of them being ASCII punctuation characters)
72 are metacharacters. The most commonly used one is a dot ".", which
73 normally matches almost any character (including a dot itself).
74 You can cause characters that normally function as metacharacters to be
76 interpreted literally by prefixing them with a "\", just like the
77 pattern's delimiter must be escaped if it also occurs within the
78 pattern. Thus, "\." matches just a literal dot, "." instead of its
79 normal meaning. This means that the backslash is also a metacharacter,
80 so "\\" matches a single "\". And a sequence that contains an escaped
81 metacharacter matches the same sequence (but without the escape) in the
82 target string. So, the pattern "/blur\\fl/" would match any target
83 string that contains the sequence "blur\fl".
84 The metacharacter "|" is used to match one thing or another. Thus
86 $foo =~ m/this|that/
88 is TRUE if and only if $foo contains either the sequence "this" or the
90 sequence "that". Like all metacharacters, prefixing the "|" with a
91 backslash makes it match the plain punctuation character; in its case,
92 the VERTICAL LINE.
93 $foo =~ m/this\|that/
95 is TRUE if and only if $foo contains the sequence "this|that".
97 You aren't limited to just a single "|".
99 $foo =~ m/fee|fie|foe|fum/
101 is TRUE if and only if $foo contains any of those 4 sequences from the
103 children's story "Jack and the Beanstalk".
104 As you can see, the "|" binds less tightly than a sequence of ordinary
106 characters. We can override this by using the grouping metacharacters,
107 the parentheses "(" and ")".
108 $foo =~ m/th(is|at) thing/
110 is TRUE if and only if $foo contains either the sequence "this thing"
112 or the sequence "that thing". The portions of the string that match
113 the portions of the pattern enclosed in parentheses are normally made
114 available separately for use later in the pattern, substitution, or
115 program. This is called "capturing", and it can get complicated. See
116 "Capture groups".
117 The first alternative includes everything from the last pattern
119 delimiter ("(", "(?:" (described later), etc. or the beginning of the
120 pattern) up to the first "|", and the last alternative contains
121 everything from the last "|" to the next closing pattern delimiter.
122 That's why it's common practice to include alternatives in parentheses:
123 to minimize confusion about where they start and end.
124 Alternatives are tried from left to right, so the first alternative
126 found for which the entire expression matches, is the one that is
127 chosen. This means that alternatives are not necessarily greedy. For
128 example: when matching "foo|foot" against "barefoot", only the "foo"
129 part will match, as that is the first alternative tried, and it
130 successfully matches the target string. (This might not seem important,
131 but it is important when you are capturing matched text using
132 parentheses.)
133 Besides taking away the special meaning of a metacharacter, a prefixed
135 backslash changes some letter and digit characters away from matching
136 just themselves to instead have special meaning. These are called
137 "escape sequences", and all such are described in perlrebackslash. A
138 backslash sequence (of a letter or digit) that doesn't currently have
139 special meaning to Perl will raise a warning if warnings are enabled,
140 as those are reserved for potential future use.
141 One such sequence is "\b", which matches a boundary of some sort.
143 "\b{wb}" and a few others give specialized types of boundaries. (They
144 are all described in detail starting at "\b{}, \b, \B{}, \B" in
145 perlrebackslash.) Note that these don't match characters, but the
146 zero-width spaces between characters. They are an example of a zero-
147 width assertion. Consider again,
148 $foo =~ m/fee|fie|foe|fum/
150 It evaluates to TRUE if, besides those 4 words, any of the sequences
152 "feed", "field", "Defoe", "fume", and many others are in $foo. By
153 judicious use of "\b" (or better (because it is designed to handle
154 natural language) "\b{wb}"), we can make sure that only the Giant's
155 words are matched:
156 $foo =~ m/\b(fee|fie|foe|fum)\b/
158 $foo =~ m/\b{wb}(fee|fie|foe|fum)\b{wb}/
159 The final example shows that the characters "{" and "}" are
161 metacharacters.
162 Another use for escape sequences is to specify characters that cannot
164 (or which you prefer not to) be written literally. These are described
165 in detail in "Character Escapes" in perlrebackslash, but the next three
166 paragraphs briefly describe some of them.
167 Various control characters can be written in C language style: "\n"
169 matches a newline, "\t" a tab, "\r" a carriage return, "\f" a form
170 feed, etc.
171 More generally, "\nnn", where nnn is a string of three octal digits,
173 matches the character whose native code point is nnn. You can easily
174 run into trouble if you don't have exactly three digits. So always use
175 three, or since Perl 5.14, you can use "\o{...}" to specify any number
176 of octal digits.
177 Similarly, "\xnn", where nn are hexadecimal digits, matches the
179 character whose native ordinal is nn. Again, not using exactly two
180 digits is a recipe for disaster, but you can use "\x{...}" to specify
181 any number of hex digits.
182 Besides being a metacharacter, the "." is an example of a "character
184 class", something that can match any single character of a given set of
185 them. In its case, the set is just about all possible characters.
186 Perl predefines several character classes besides the "."; there is a
187 separate reference page about just these, perlrecharclass.
188 You can define your own custom character classes, by putting into your
190 pattern in the appropriate place(s), a list of all the characters you
191 want in the set. You do this by enclosing the list within "[]" bracket
192 characters. These are called "bracketed character classes" when we are
193 being precise, but often the word "bracketed" is dropped. (Dropping it
194 usually doesn't cause confusion.) This means that the "[" character is
195 another metacharacter. It doesn't match anything just by itself; it is
196 used only to tell Perl that what follows it is a bracketed character
197 class. If you want to match a literal left square bracket, you must
198 escape it, like "\[". The matching "]" is also a metacharacter; again
199 it doesn't match anything by itself, but just marks the end of your
200 custom class to Perl. It is an example of a "sometimes metacharacter".
201 It isn't a metacharacter if there is no corresponding "[", and matches
202 its literal self:
203 print "]" =~ /]/; # prints 1
205 The list of characters within the character class gives the set of
207 characters matched by the class. "[abc]" matches a single "a" or "b"
208 or "c". But if the first character after the "[" is "^", the class
209 instead matches any character not in the list. Within a list, the "-"
210 character specifies a range of characters, so that "a-z" represents all
211 characters between "a" and "z", inclusive. If you want either "-" or
212 "]" itself to be a member of a class, put it at the start of the list
213 (possibly after a "^"), or escape it with a backslash. "-" is also
214 taken literally when it is at the end of the list, just before the
215 closing "]". (The following all specify the same class of three
216 characters: "[-az]", "[az-]", and "[a\-z]". All are different from
217 "[a-z]", which specifies a class containing twenty-six characters, even
218 on EBCDIC-based character sets.)
219 There is lots more to bracketed character classes; full details are in
221 "Bracketed Character Classes" in perlrecharclass.
222 Metacharacters
224 "The Basics" introduced some of the metacharacters. This section gives
226 them all. Most of them have the same meaning as in the egrep command.
227 Only the "\" is always a metacharacter. The others are metacharacters
229 just sometimes. The following tables lists all of them, summarizes
230 their use, and gives the contexts where they are metacharacters.
231 Outside those contexts or if prefixed by a "\", they match their
232 corresponding punctuation character. In some cases, their meaning
233 varies depending on various pattern modifiers that alter the default
234 behaviors. See "Modifiers".
235 PURPOSE WHERE
237 \ Escape the next character Always, except when
238 escaped by another \
239 ^ Match the beginning of the string Not in []
240 (or line, if /m is used)
241 ^ Complement the [] class At the beginning of []
242 . Match any single character except newline Not in []
243 (under /s, includes newline)
244 $ Match the end of the string Not in [], but can
245 (or before newline at the end of the mean interpolate a
246 string; or before any newline if /m is scalar
247 used)
248 | Alternation Not in []
249 () Grouping Not in []
250 [ Start Bracketed Character class Not in []
251 ] End Bracketed Character class Only in [], and
252 not first
253 * Matches the preceding element 0 or more Not in []
254 times
255 + Matches the preceding element 1 or more Not in []
256 times
257 ? Matches the preceding element 0 or 1 Not in []
258 times
259 { Starts a sequence that gives number(s) Not in []
260 of times the preceding element can be
261 matched
262 { when following certain escape sequences
263 starts a modifier to the meaning of the
264 sequence
265 } End sequence started by {
266 - Indicates a range Only in [] interior
267 # Beginning of comment, extends to line end Only with /x modifier
268 Notice that most of the metacharacters lose their special meaning when
270 they occur in a bracketed character class, except "^" has a different
271 meaning when it is at the beginning of such a class. And "-" and "]"
272 are metacharacters only at restricted positions within bracketed
273 character classes; while "}" is a metacharacter only when closing a
274 special construct started by "{".
275 In double-quotish context, as is usually the case, you need to be
277 careful about "$" and the non-metacharacter "@". Those could
278 interpolate variables, which may or may not be what you intended.
279 These rules were designed for compactness of expression, rather than
281 legibility and maintainability. The "/x and /xx" pattern modifiers
282 allow you to insert white space to improve readability. And use of
283 "re 'strict'" adds extra checking to catch some typos that might
284 silently compile into something unintended.
285 By default, the "^" character is guaranteed to match only the beginning
287 of the string, the "$" character only the end (or before the newline at
288 the end), and Perl does certain optimizations with the assumption that
289 the string contains only one line. Embedded newlines will not be
290 matched by "^" or "$". You may, however, wish to treat a string as a
291 multi-line buffer, such that the "^" will match after any newline
292 within the string (except if the newline is the last character in the
293 string), and "$" will match before any newline. At the cost of a
294 little more overhead, you can do this by using the "/m" modifier on the
295 pattern match operator. (Older programs did this by setting $*, but
296 this option was removed in perl 5.10.)
297 To simplify multi-line substitutions, the "." character never matches a
299 newline unless you use the "/s" modifier, which in effect tells Perl to
300 pretend the string is a single line--even if it isn't.
301 Modifiers
303 Overview
304 The default behavior for matching can be changed, using various
306 modifiers. Modifiers that relate to the interpretation of the pattern
307 are listed just below. Modifiers that alter the way a pattern is used
308 by Perl are detailed in "Regexp Quote-Like Operators" in perlop and
309 "Gory details of parsing quoted constructs" in perlop. Modifiers can
310 be added dynamically; see "Extended Patterns" below.
311 "m" Treat the string being matched against as multiple lines. That is,
313 change "^" and "$" from matching the start of the string's first
314 line and the end of its last line to matching the start and end of
315 each line within the string.
316 "s" Treat the string as single line. That is, change "." to match any
318 character whatsoever, even a newline, which normally it would not
319 match.
320 Used together, as "/ms", they let the "." match any character
322 whatsoever, while still allowing "^" and "$" to match,
323 respectively, just after and just before newlines within the
324 string.
325 "i" Do case-insensitive pattern matching. For example, "A" will match
327 "a" under "/i".
328 If locale matching rules are in effect, the case map is taken from
330 the current locale for code points less than 255, and from Unicode
331 rules for larger code points. However, matches that would cross
332 the Unicode rules/non-Unicode rules boundary (ords 255/256) will
333 not succeed, unless the locale is a UTF-8 one. See perllocale.
334 There are a number of Unicode characters that match a sequence of
336 multiple characters under "/i". For example, "LATIN SMALL LIGATURE
337 FI" should match the sequence "fi". Perl is not currently able to
338 do this when the multiple characters are in the pattern and are
339 split between groupings, or when one or more are quantified. Thus
340 "\N{LATIN SMALL LIGATURE FI}" =~ /fi/i; # Matches
342 "\N{LATIN SMALL LIGATURE FI}" =~ /[fi][fi]/i; # Doesn't match!
343 "\N{LATIN SMALL LIGATURE FI}" =~ /fi*/i; # Doesn't match!
344 # The below doesn't match, and it isn't clear what $1 and $2 would
346 # be even if it did!!
347 "\N{LATIN SMALL LIGATURE FI}" =~ /(f)(i)/i; # Doesn't match!
348 Perl doesn't match multiple characters in a bracketed character
350 class unless the character that maps to them is explicitly
351 mentioned, and it doesn't match them at all if the character class
352 is inverted, which otherwise could be highly confusing. See
353 "Bracketed Character Classes" in perlrecharclass, and "Negation" in
354 perlrecharclass.
355 "x" and "xx"
357 Extend your pattern's legibility by permitting whitespace and
358 comments. Details in "/x and /xx"
359 "p" Preserve the string matched such that "${^PREMATCH}", "${^MATCH}",
361 and "${^POSTMATCH}" are available for use after matching.
362 In Perl 5.20 and higher this is ignored. Due to a new copy-on-write
364 mechanism, "${^PREMATCH}", "${^MATCH}", and "${^POSTMATCH}" will be
365 available after the match regardless of the modifier.
366 "a", "d", "l", and "u"
368 These modifiers, all new in 5.14, affect which character-set rules
369 (Unicode, etc.) are used, as described below in "Character set
370 modifiers".
371 "n" Prevent the grouping metacharacters "()" from capturing. This
373 modifier, new in 5.22, will stop $1, $2, etc... from being filled
374 in.
375 "hello" =~ /(hi|hello)/; # $1 is "hello"
377 "hello" =~ /(hi|hello)/n; # $1 is undef
378 This is equivalent to putting "?:" at the beginning of every
380 capturing group:
381 "hello" =~ /(?:hi|hello)/; # $1 is undef
383 "/n" can be negated on a per-group basis. Alternatively, named
385 captures may still be used.
386 "hello" =~ /(?-n:(hi|hello))/n; # $1 is "hello"
388 "hello" =~ /(?<greet>hi|hello)/n; # $1 is "hello", $+{greet} is
389 # "hello"
390 Other Modifiers
392 There are a number of flags that can be found at the end of regular
393 expression constructs that are not generic regular expression
394 flags, but apply to the operation being performed, like matching or
395 substitution ("m//" or "s///" respectively).
396 Flags described further in "Using regular expressions in Perl" in
398 perlretut are:
399 c - keep the current position during repeated matching
401 g - globally match the pattern repeatedly in the string
402 Substitution-specific modifiers described in
404 "s/PATTERN/REPLACEMENT/msixpodualngcer" in perlop are:
405 e - evaluate the right-hand side as an expression
407 ee - evaluate the right side as a string then eval the result
408 o - pretend to optimize your code, but actually introduce bugs
409 r - perform non-destructive substitution and return the new value
410 Regular expression modifiers are usually written in documentation as
412 e.g., "the "/x" modifier", even though the delimiter in question might
413 not really be a slash. The modifiers "/imnsxadlup" may also be
414 embedded within the regular expression itself using the "(?...)"
415 construct, see "Extended Patterns" below.
416 Details on some modifiers
418 Some of the modifiers require more explanation than given in the
420 "Overview" above.
421 "/x" and "/xx"
423 A single "/x" tells the regular expression parser to ignore most
425 whitespace that is neither backslashed nor within a bracketed character
426 class. You can use this to break up your regular expression into more
427 readable parts. Also, the "#" character is treated as a metacharacter
428 introducing a comment that runs up to the pattern's closing delimiter,
429 or to the end of the current line if the pattern extends onto the next
430 line. Hence, this is very much like an ordinary Perl code comment.
431 (You can include the closing delimiter within the comment only if you
432 precede it with a backslash, so be careful!)
433 Use of "/x" means that if you want real whitespace or "#" characters in
435 the pattern (outside a bracketed character class, which is unaffected
436 by "/x"), then you'll either have to escape them (using backslashes or
437 "\Q...\E") or encode them using octal, hex, or "\N{}" or "\p{name=...}"
438 escapes. It is ineffective to try to continue a comment onto the next
439 line by escaping the "\n" with a backslash or "\Q".
440 You can use "(?#text)" to create a comment that ends earlier than the
442 end of the current line, but "text" also can't contain the closing
443 delimiter unless escaped with a backslash.
444 A common pitfall is to forget that "#" characters (outside a bracketed
446 character class) begin a comment under "/x" and are not matched
447 literally. Just keep that in mind when trying to puzzle out why a
448 particular "/x" pattern isn't working as expected. Inside a bracketed
449 character class, "#" retains its non-special, literal meaning.
450 Starting in Perl v5.26, if the modifier has a second "x" within it, the
452 effect of a single "/x" is increased. The only difference is that
453 inside bracketed character classes, non-escaped (by a backslash) SPACE
454 and TAB characters are not added to the class, and hence can be
455 inserted to make the classes more readable:
456 / [d-e g-i 3-7]/xx
458 /[ ! @ " # $ % ^ & * () = ? <> ' ]/xx
459 may be easier to grasp than the squashed equivalents
461 /[d-eg-i3-7]/
463 /[!@"#$%^&*()=?<>']/
464 Note that this unfortunately doesn't mean that your bracketed classes
466 can contain comments or extend over multiple lines. A "#" inside a
467 character class is still just a literal "#", and doesn't introduce a
468 comment. And, unless the closing bracket is on the same line as the
469 opening one, the newline character (and everything on the next line(s)
470 until terminated by a "]" will be part of the class, just as if you'd
471 written "\n".
472 Taken together, these features go a long way towards making Perl's
474 regular expressions more readable. Here's an example:
475 # Delete (most) C comments.
477 $program =~ s {
478 /\* # Match the opening delimiter.
479 .*? # Match a minimal number of characters.
480 \*/ # Match the closing delimiter.
481 } []gsx;
482 Note that anything inside a "\Q...\E" stays unaffected by "/x". And
484 note that "/x" doesn't affect space interpretation within a single
485 multi-character construct. For example "(?:...)" can't have a space
486 between the "(", "?", and ":". Within any delimiters for such a
487 construct, allowed spaces are not affected by "/x", and depend on the
488 construct. For example, all constructs using curly braces as
489 delimiters, such as "\x{...}" can have blanks within but adjacent to
490 the braces, but not elsewhere, and no non-blank space characters. An
491 exception are Unicode properties which follow Unicode rules, for which
492 see "Properties accessible through \p{} and \P{}" in perluniprops.
493 The set of characters that are deemed whitespace are those that Unicode
495 calls "Pattern White Space", namely:
496 U+0009 CHARACTER TABULATION
498 U+000A LINE FEED
499 U+000B LINE TABULATION
500 U+000C FORM FEED
501 U+000D CARRIAGE RETURN
502 U+0020 SPACE
503 U+0085 NEXT LINE
504 U+200E LEFT-TO-RIGHT MARK
505 U+200F RIGHT-TO-LEFT MARK
506 U+2028 LINE SEPARATOR
507 U+2029 PARAGRAPH SEPARATOR
508 Character set modifiers
510 "/d", "/u", "/a", and "/l", available starting in 5.14, are called the
512 character set modifiers; they affect the character set rules used for
513 the regular expression.
514 The "/d", "/u", and "/l" modifiers are not likely to be of much use to
516 you, and so you need not worry about them very much. They exist for
517 Perl's internal use, so that complex regular expression data structures
518 can be automatically serialized and later exactly reconstituted,
519 including all their nuances. But, since Perl can't keep a secret, and
520 there may be rare instances where they are useful, they are documented
521 here.
522 The "/a" modifier, on the other hand, may be useful. Its purpose is to
524 allow code that is to work mostly on ASCII data to not have to concern
525 itself with Unicode.
526 Briefly, "/l" sets the character set to that of whatever Locale is in
528 effect at the time of the execution of the pattern match.
529 "/u" sets the character set to Unicode.
531 "/a" also sets the character set to Unicode, BUT adds several
533 restrictions for ASCII-safe matching.
534 "/d" is the old, problematic, pre-5.14 Default character set behavior.
536 Its only use is to force that old behavior.
537 At any given time, exactly one of these modifiers is in effect. Their
539 existence allows Perl to keep the originally compiled behavior of a
540 regular expression, regardless of what rules are in effect when it is
541 actually executed. And if it is interpolated into a larger regex, the
542 original's rules continue to apply to it, and don't affect the other
543 parts.
544 The "/l" and "/u" modifiers are automatically selected for regular
546 expressions compiled within the scope of various pragmas, and we
547 recommend that in general, you use those pragmas instead of specifying
548 these modifiers explicitly. For one thing, the modifiers affect only
549 pattern matching, and do not extend to even any replacement done,
550 whereas using the pragmas gives consistent results for all appropriate
551 operations within their scopes. For example,
552 s/foo/\Ubar/il
554 will match "foo" using the locale's rules for case-insensitive
556 matching, but the "/l" does not affect how the "\U" operates. Most
557 likely you want both of them to use locale rules. To do this, instead
558 compile the regular expression within the scope of "use locale". This
559 both implicitly adds the "/l", and applies locale rules to the "\U".
560 The lesson is to "use locale", and not "/l" explicitly.
561 Similarly, it would be better to use "use feature 'unicode_strings'"
563 instead of,
564 s/foo/\Lbar/iu
566 to get Unicode rules, as the "\L" in the former (but not necessarily
568 the latter) would also use Unicode rules.
569 More detail on each of the modifiers follows. Most likely you don't
571 need to know this detail for "/l", "/u", and "/d", and can skip ahead
572 to /a.
573 /l
575 means to use the current locale's rules (see perllocale) when pattern
577 matching. For example, "\w" will match the "word" characters of that
578 locale, and "/i" case-insensitive matching will match according to the
579 locale's case folding rules. The locale used will be the one in effect
580 at the time of execution of the pattern match. This may not be the
581 same as the compilation-time locale, and can differ from one match to
582 another if there is an intervening call of the setlocale() function.
583 Prior to v5.20, Perl did not support multi-byte locales. Starting
585 then, UTF-8 locales are supported. No other multi byte locales are
586 ever likely to be supported. However, in all locales, one can have
587 code points above 255 and these will always be treated as Unicode no
588 matter what locale is in effect.
589 Under Unicode rules, there are a few case-insensitive matches that
591 cross the 255/256 boundary. Except for UTF-8 locales in Perls v5.20
592 and later, these are disallowed under "/l". For example, 0xFF (on
593 ASCII platforms) does not caselessly match the character at 0x178,
594 "LATIN CAPITAL LETTER Y WITH DIAERESIS", because 0xFF may not be "LATIN
595 SMALL LETTER Y WITH DIAERESIS" in the current locale, and Perl has no
596 way of knowing if that character even exists in the locale, much less
597 what code point it is.
598 In a UTF-8 locale in v5.20 and later, the only visible difference
600 between locale and non-locale in regular expressions should be
601 tainting, if your perl supports taint checking (see perlsec).
602 This modifier may be specified to be the default by "use locale", but
604 see "Which character set modifier is in effect?".
605 /u
607 means to use Unicode rules when pattern matching. On ASCII platforms,
609 this means that the code points between 128 and 255 take on their
610 Latin-1 (ISO-8859-1) meanings (which are the same as Unicode's).
611 (Otherwise Perl considers their meanings to be undefined.) Thus, under
612 this modifier, the ASCII platform effectively becomes a Unicode
613 platform; and hence, for example, "\w" will match any of the more than
614 100_000 word characters in Unicode.
615 Unlike most locales, which are specific to a language and country pair,
617 Unicode classifies all the characters that are letters somewhere in the
618 world as "\w". For example, your locale might not think that "LATIN
619 SMALL LETTER ETH" is a letter (unless you happen to speak Icelandic),
620 but Unicode does. Similarly, all the characters that are decimal
621 digits somewhere in the world will match "\d"; this is hundreds, not
622 10, possible matches. And some of those digits look like some of the
623 10 ASCII digits, but mean a different number, so a human could easily
624 think a number is a different quantity than it really is. For example,
625 "BENGALI DIGIT FOUR" (U+09EA) looks very much like an "ASCII DIGIT
626 EIGHT" (U+0038), and "LEPCHA DIGIT SIX" (U+1C46) looks very much like
627 an "ASCII DIGIT FIVE" (U+0035). And, "\d+", may match strings of
628 digits that are a mixture from different writing systems, creating a
629 security issue. A fraudulent website, for example, could display the
630 price of something using U+1C46, and it would appear to the user that
631 something cost 500 units, but it really costs 600. A browser that
632 enforced script runs ("Script Runs") would prevent that fraudulent
633 display. "num()" in Unicode::UCD can also be used to sort this out.
634 Or the "/a" modifier can be used to force "\d" to match just the ASCII
635 0 through 9.
636 Also, under this modifier, case-insensitive matching works on the full
638 set of Unicode characters. The "KELVIN SIGN", for example matches the
639 letters "k" and "K"; and "LATIN SMALL LIGATURE FF" matches the sequence
640 "ff", which, if you're not prepared, might make it look like a
641 hexadecimal constant, presenting another potential security issue. See
642 <https://unicode.org/reports/tr36> for a detailed discussion of Unicode
643 security issues.
644 This modifier may be specified to be the default by "use feature
646 'unicode_strings", "use locale ':not_characters'", or "use v5.12" (or
647 higher), but see "Which character set modifier is in effect?".
648 /d
650 IMPORTANT: Because of the unpredictable behaviors this modifier causes,
652 only use it to maintain weird backward compatibilities. Use the
653 "unicode_strings" feature in new code to avoid inadvertently enabling
654 this modifier by default.
655 What does this modifier do? It "Depends"!
657 This modifier means to use platform-native matching rules except when
659 there is cause to use Unicode rules instead, as follows:
660 1. the target string's UTF8 flag (see below) is set; or
662 2. the pattern's UTF8 flag (see below) is set; or
664 3. the pattern explicitly mentions a code point that is above 255 (say
666 by "\x{100}"); or
667 4. the pattern uses a Unicode name ("\N{...}"); or
669 5. the pattern uses a Unicode property ("\p{...}" or "\P{...}"); or
671 6. the pattern uses a Unicode break ("\b{...}" or "\B{...}"); or
673 7. the pattern uses "(?[ ])"
675 8. the pattern uses "(*script_run: ...)"
677 Regarding the "UTF8 flag" references above: normally Perl applications
679 shouldn't think about that flag. It's part of Perl's internals, so it
680 can change whenever Perl wants. "/d" may thus cause unpredictable
681 results. See "The "Unicode Bug"" in perlunicode. This bug has become
682 rather infamous, leading to yet other (without swearing) names for this
683 modifier like "Dicey" and "Dodgy".
684 Here are some examples of how that works on an ASCII platform:
686 $str = "\xDF"; #
688 utf8::downgrade($str); # $str is not UTF8-flagged.
689 $str =~ /^\w/; # No match, since no UTF8 flag.
690 $str .= "\x{0e0b}"; # Now $str is UTF8-flagged.
692 $str =~ /^\w/; # Match! $str is now UTF8-flagged.
693 chop $str;
694 $str =~ /^\w/; # Still a match! $str retains its UTF8 flag.
695 Under Perl's default configuration this modifier is automatically
697 selected by default when none of the others are, so yet another name
698 for it (unfortunately) is "Default".
699 Whenever you can, use the "unicode_strings" to cause to be the default
701 instead.
702 /a (and /aa)
704 This modifier stands for ASCII-restrict (or ASCII-safe). This modifier
706 may be doubled-up to increase its effect.
707 When it appears singly, it causes the sequences "\d", "\s", "\w", and
709 the Posix character classes to match only in the ASCII range. They
710 thus revert to their pre-5.6, pre-Unicode meanings. Under "/a", "\d"
711 always means precisely the digits "0" to "9"; "\s" means the five
712 characters "[ \f\n\r\t]", and starting in Perl v5.18, the vertical tab;
713 "\w" means the 63 characters "[A-Za-z0-9_]"; and likewise, all the
714 Posix classes such as "[[:print:]]" match only the appropriate ASCII-
715 range characters.
716 This modifier is useful for people who only incidentally use Unicode,
718 and who do not wish to be burdened with its complexities and security
719 concerns.
720 With "/a", one can write "\d" with confidence that it will only match
722 ASCII characters, and should the need arise to match beyond ASCII, you
723 can instead use "\p{Digit}" (or "\p{Word}" for "\w"). There are
724 similar "\p{...}" constructs that can match beyond ASCII both white
725 space (see "Whitespace" in perlrecharclass), and Posix classes (see
726 "POSIX Character Classes" in perlrecharclass). Thus, this modifier
727 doesn't mean you can't use Unicode, it means that to get Unicode
728 matching you must explicitly use a construct ("\p{}", "\P{}") that
729 signals Unicode.
730 As you would expect, this modifier causes, for example, "\D" to mean
732 the same thing as "[^0-9]"; in fact, all non-ASCII characters match
733 "\D", "\S", and "\W". "\b" still means to match at the boundary
734 between "\w" and "\W", using the "/a" definitions of them (similarly
735 for "\B").
736 Otherwise, "/a" behaves like the "/u" modifier, in that case-
738 insensitive matching uses Unicode rules; for example, "k" will match
739 the Unicode "\N{KELVIN SIGN}" under "/i" matching, and code points in
740 the Latin1 range, above ASCII will have Unicode rules when it comes to
741 case-insensitive matching.
742 To forbid ASCII/non-ASCII matches (like "k" with "\N{KELVIN SIGN}"),
744 specify the "a" twice, for example "/aai" or "/aia". (The first
745 occurrence of "a" restricts the "\d", etc., and the second occurrence
746 adds the "/i" restrictions.) But, note that code points outside the
747 ASCII range will use Unicode rules for "/i" matching, so the modifier
748 doesn't really restrict things to just ASCII; it just forbids the
749 intermixing of ASCII and non-ASCII.
750 To summarize, this modifier provides protection for applications that
752 don't wish to be exposed to all of Unicode. Specifying it twice gives
753 added protection.
754 This modifier may be specified to be the default by "use re '/a'" or
756 "use re '/aa'". If you do so, you may actually have occasion to use
757 the "/u" modifier explicitly if there are a few regular expressions
758 where you do want full Unicode rules (but even here, it's best if
759 everything were under feature "unicode_strings", along with the "use re
760 '/aa'"). Also see "Which character set modifier is in effect?".
761 Which character set modifier is in effect?
763 Which of these modifiers is in effect at any given point in a regular
765 expression depends on a fairly complex set of interactions. These have
766 been designed so that in general you don't have to worry about it, but
767 this section gives the gory details. As explained below in "Extended
768 Patterns" it is possible to explicitly specify modifiers that apply
769 only to portions of a regular expression. The innermost always has
770 priority over any outer ones, and one applying to the whole expression
771 has priority over any of the default settings that are described in the
772 remainder of this section.
773 The "use re '/foo'" pragma can be used to set default modifiers
775 (including these) for regular expressions compiled within its scope.
776 This pragma has precedence over the other pragmas listed below that
777 also change the defaults.
778 Otherwise, "use locale" sets the default modifier to "/l"; and "use
780 feature 'unicode_strings", or "use v5.12" (or higher) set the default
781 to "/u" when not in the same scope as either "use locale" or "use
782 bytes". ("use locale ':not_characters'" also sets the default to "/u",
783 overriding any plain "use locale".) Unlike the mechanisms mentioned
784 above, these affect operations besides regular expressions pattern
785 matching, and so give more consistent results with other operators,
786 including using "\U", "\l", etc. in substitution replacements.
787 If none of the above apply, for backwards compatibility reasons, the
789 "/d" modifier is the one in effect by default. As this can lead to
790 unexpected results, it is best to specify which other rule set should
791 be used.
792 Character set modifier behavior prior to Perl 5.14
794 Prior to 5.14, there were no explicit modifiers, but "/l" was implied
796 for regexes compiled within the scope of "use locale", and "/d" was
797 implied otherwise. However, interpolating a regex into a larger regex
798 would ignore the original compilation in favor of whatever was in
799 effect at the time of the second compilation. There were a number of
800 inconsistencies (bugs) with the "/d" modifier, where Unicode rules
801 would be used when inappropriate, and vice versa. "\p{}" did not imply
802 Unicode rules, and neither did all occurrences of "\N{}", until 5.12.
803 Regular Expressions
805 Quantifiers
806 Quantifiers are used when a particular portion of a pattern needs to
808 match a certain number (or numbers) of times. If there isn't a
809 quantifier the number of times to match is exactly one. The following
810 standard quantifiers are recognized:
811 * Match 0 or more times
813 + Match 1 or more times
814 ? Match 1 or 0 times
815 {n} Match exactly n times
816 {n,} Match at least n times
817 {,n} Match at most n times
818 {n,m} Match at least n but not more than m times
819 (If a non-escaped curly bracket occurs in a context other than one of
821 the quantifiers listed above, where it does not form part of a
822 backslashed sequence like "\x{...}", it is either a fatal syntax error,
823 or treated as a regular character, generally with a deprecation warning
824 raised. To escape it, you can precede it with a backslash ("\{") or
825 enclose it within square brackets ("[{]"). This change will allow for
826 future syntax extensions (like making the lower bound of a quantifier
827 optional), and better error checking of quantifiers).
828 The "*" quantifier is equivalent to "{0,}", the "+" quantifier to
830 "{1,}", and the "?" quantifier to "{0,1}". n and m are limited to non-
831 negative integral values less than a preset limit defined when perl is
832 built. This is usually 65534 on the most common platforms. The actual
833 limit can be seen in the error message generated by code such as this:
834 $_ **= $_ , / {$_} / for 2 .. 42;
836 By default, a quantified subpattern is "greedy", that is, it will match
838 as many times as possible (given a particular starting location) while
839 still allowing the rest of the pattern to match. If you want it to
840 match the minimum number of times possible, follow the quantifier with
841 a "?". Note that the meanings don't change, just the "greediness":
842 *? Match 0 or more times, not greedily
844 +? Match 1 or more times, not greedily
845 ?? Match 0 or 1 time, not greedily
846 {n}? Match exactly n times, not greedily (redundant)
847 {n,}? Match at least n times, not greedily
848 {,n}? Match at most n times, not greedily
849 {n,m}? Match at least n but not more than m times, not greedily
850 Normally when a quantified subpattern does not allow the rest of the
852 overall pattern to match, Perl will backtrack. However, this behaviour
853 is sometimes undesirable. Thus Perl provides the "possessive"
854 quantifier form as well.
855 *+ Match 0 or more times and give nothing back
857 ++ Match 1 or more times and give nothing back
858 ?+ Match 0 or 1 time and give nothing back
859 {n}+ Match exactly n times and give nothing back (redundant)
860 {n,}+ Match at least n times and give nothing back
861 {,n}+ Match at most n times and give nothing back
862 {n,m}+ Match at least n but not more than m times and give nothing back
863 For instance,
865 'aaaa' =~ /a++a/
867 will never match, as the "a++" will gobble up all the "a"'s in the
869 string and won't leave any for the remaining part of the pattern. This
870 feature can be extremely useful to give perl hints about where it
871 shouldn't backtrack. For instance, the typical "match a double-quoted
872 string" problem can be most efficiently performed when written as:
873 /"(?:[^"\\]++|\\.)*+"/
875 as we know that if the final quote does not match, backtracking will
877 not help. See the independent subexpression "(?>pattern)" for more
878 details; possessive quantifiers are just syntactic sugar for that
879 construct. For instance the above example could also be written as
880 follows:
881 /"(?>(?:(?>[^"\\]+)|\\.)*)"/
883 Note that the possessive quantifier modifier can not be combined with
885 the non-greedy modifier. This is because it would make no sense.
886 Consider the follow equivalency table:
887 Illegal Legal
889 ------------ ------
890 X??+ X{0}
891 X+?+ X{1}
892 X{min,max}?+ X{min}
893 Escape sequences
895 Because patterns are processed as double-quoted strings, the following
897 also work:
898 \t tab (HT, TAB)
900 \n newline (LF, NL)
901 \r return (CR)
902 \f form feed (FF)
903 \a alarm (bell) (BEL)
904 \e escape (think troff) (ESC)
905 \cK control char (example: VT)
906 \x{}, \x00 character whose ordinal is the given hexadecimal number
907 \N{name} named Unicode character or character sequence
908 \N{U+263D} Unicode character (example: FIRST QUARTER MOON)
909 \o{}, \000 character whose ordinal is the given octal number
910 \l lowercase next char (think vi)
911 \u uppercase next char (think vi)
912 \L lowercase until \E (think vi)
913 \U uppercase until \E (think vi)
914 \Q quote (disable) pattern metacharacters until \E
915 \E end either case modification or quoted section, think vi
916 Details are in "Quote and Quote-like Operators" in perlop.
918 Character Classes and other Special Escapes
920 In addition, Perl defines the following:
922 Sequence Note Description
924 [...] [1] Match a character according to the rules of the
925 bracketed character class defined by the "...".
926 Example: [a-z] matches "a" or "b" or "c" ... or "z"
927 [[:...:]] [2] Match a character according to the rules of the POSIX
928 character class "..." within the outer bracketed
929 character class. Example: [[:upper:]] matches any
930 uppercase character.
931 (?[...]) [8] Extended bracketed character class
932 \w [3] Match a "word" character (alphanumeric plus "_", plus
933 other connector punctuation chars plus Unicode
934 marks)
935 \W [3] Match a non-"word" character
936 \s [3] Match a whitespace character
937 \S [3] Match a non-whitespace character
938 \d [3] Match a decimal digit character
939 \D [3] Match a non-digit character
940 \pP [3] Match P, named property. Use \p{Prop} for longer names
941 \PP [3] Match non-P
942 \X [4] Match Unicode "eXtended grapheme cluster"
943 \1 [5] Backreference to a specific capture group or buffer.
944 '1' may actually be any positive integer.
945 \g1 [5] Backreference to a specific or previous group,
946 \g{-1} [5] The number may be negative indicating a relative
947 previous group and may optionally be wrapped in
948 curly brackets for safer parsing.
949 \g{name} [5] Named backreference
950 \k<name> [5] Named backreference
951 \k'name' [5] Named backreference
952 \k{name} [5] Named backreference
953 \K [6] Keep the stuff left of the \K, don't include it in $&
954 \N [7] Any character but \n. Not affected by /s modifier
955 \v [3] Vertical whitespace
956 \V [3] Not vertical whitespace
957 \h [3] Horizontal whitespace
958 \H [3] Not horizontal whitespace
959 \R [4] Linebreak
960 [1] See "Bracketed Character Classes" in perlrecharclass for details.
962 [2] See "POSIX Character Classes" in perlrecharclass for details.
964 [3] See "Unicode Character Properties" in perlunicode for details
966 [4] See "Misc" in perlrebackslash for details.
968 [5] See "Capture groups" below for details.
970 [6] See "Extended Patterns" below for details.
972 [7] Note that "\N" has two meanings. When of the form "\N{NAME}", it
974 matches the character or character sequence whose name is NAME; and
975 similarly when of the form "\N{U+hex}", it matches the character
976 whose Unicode code point is hex. Otherwise it matches any
977 character but "\n".
978 [8] See "Extended Bracketed Character Classes" in perlrecharclass for
980 details.
981 Assertions
983 Besides "^" and "$", Perl defines the following zero-width assertions:
985 \b{} Match at Unicode boundary of specified type
987 \B{} Match where corresponding \b{} doesn't match
988 \b Match a \w\W or \W\w boundary
989 \B Match except at a \w\W or \W\w boundary
990 \A Match only at beginning of string
991 \Z Match only at end of string, or before newline at the end
992 \z Match only at end of string
993 \G Match only at pos() (e.g. at the end-of-match position
994 of prior m//g)
995 A Unicode boundary ("\b{}"), available starting in v5.22, is a spot
997 between two characters, or before the first character in the string, or
998 after the final character in the string where certain criteria defined
999 by Unicode are met. See "\b{}, \b, \B{}, \B" in perlrebackslash for
1000 details.
1001 A word boundary ("\b") is a spot between two characters that has a "\w"
1003 on one side of it and a "\W" on the other side of it (in either order),
1004 counting the imaginary characters off the beginning and end of the
1005 string as matching a "\W". (Within character classes "\b" represents
1006 backspace rather than a word boundary, just as it normally does in any
1007 double-quoted string.) The "\A" and "\Z" are just like "^" and "$",
1008 except that they won't match multiple times when the "/m" modifier is
1009 used, while "^" and "$" will match at every internal line boundary. To
1010 match the actual end of the string and not ignore an optional trailing
1011 newline, use "\z".
1012 The "\G" assertion can be used to chain global matches (using "m//g"),
1014 as described in "Regexp Quote-Like Operators" in perlop. It is also
1015 useful when writing "lex"-like scanners, when you have several patterns
1016 that you want to match against consequent substrings of your string;
1017 see the previous reference. The actual location where "\G" will match
1018 can also be influenced by using "pos()" as an lvalue: see "pos" in
1019 perlfunc. Note that the rule for zero-length matches (see "Repeated
1020 Patterns Matching a Zero-length Substring") is modified somewhat, in
1021 that contents to the left of "\G" are not counted when determining the
1022 length of the match. Thus the following will not match forever:
1023 my $string = 'ABC';
1025 pos($string) = 1;
1026 while ($string =~ /(.\G)/g) {
1027 print $1;
1028 }
1029 It will print 'A' and then terminate, as it considers the match to be
1031 zero-width, and thus will not match at the same position twice in a
1032 row.
1033 It is worth noting that "\G" improperly used can result in an infinite
1035 loop. Take care when using patterns that include "\G" in an
1036 alternation.
1037 Note also that "s///" will refuse to overwrite part of a substitution
1039 that has already been replaced; so for example this will stop after the
1040 first iteration, rather than iterating its way backwards through the
1041 string:
1042 $_ = "123456789";
1044 pos = 6;
1045 s/.(?=.\G)/X/g;
1046 print; # prints 1234X6789, not XXXXX6789
1047 Capture groups
1049 The grouping construct "( ... )" creates capture groups (also referred
1051 to as capture buffers). To refer to the current contents of a group
1052 later on, within the same pattern, use "\g1" (or "\g{1}") for the
1053 first, "\g2" (or "\g{2}") for the second, and so on. This is called a
1054 backreference.
1055 There is no limit to the number of captured substrings that you may
1064 use. Groups are numbered with the leftmost open parenthesis being
1065 number 1, etc. If a group did not match, the associated backreference
1066 won't match either. (This can happen if the group is optional, or in a
1067 different branch of an alternation.) You can omit the "g", and write
1068 "\1", etc, but there are some issues with this form, described below.
1069 You can also refer to capture groups relatively, by using a negative
1071 number, so that "\g-1" and "\g{-1}" both refer to the immediately
1072 preceding capture group, and "\g-2" and "\g{-2}" both refer to the
1073 group before it. For example:
1074 /
1076 (Y) # group 1
1077 ( # group 2
1078 (X) # group 3
1079 \g{-1} # backref to group 3
1080 \g{-3} # backref to group 1
1081 )
1082 /x
1083 would match the same as "/(Y) ( (X) \g3 \g1 )/x". This allows you to
1085 interpolate regexes into larger regexes and not have to worry about the
1086 capture groups being renumbered.
1087 You can dispense with numbers altogether and create named capture
1089 groups. The notation is "(?<name>...)" to declare and "\g{name}" to
1090 reference. (To be compatible with .Net regular expressions, "\g{name}"
1091 may also be written as "\k{name}", "\k<name>" or "\k'name'".) name
1092 must not begin with a number, nor contain hyphens. When different
1093 groups within the same pattern have the same name, any reference to
1094 that name assumes the leftmost defined group. Named groups count in
1095 absolute and relative numbering, and so can also be referred to by
1096 those numbers. (It's possible to do things with named capture groups
1097 that would otherwise require "(??{})".)
1098 Capture group contents are dynamically scoped and available to you
1100 outside the pattern until the end of the enclosing block or until the
1101 next successful match, whichever comes first. (See "Compound
1102 Statements" in perlsyn.) You can refer to them by absolute number
1103 (using "$1" instead of "\g1", etc); or by name via the "%+" hash, using
1104 "$+{name}".
1105 Braces are required in referring to named capture groups, but are
1107 optional for absolute or relative numbered ones. Braces are safer when
1108 creating a regex by concatenating smaller strings. For example if you
1109 have "qr/$a$b/", and $a contained "\g1", and $b contained "37", you
1110 would get "/\g137/" which is probably not what you intended.
1111 If you use braces, you may also optionally add any number of blank
1113 (space or tab) characters within but adjacent to the braces, like
1114 "\g{ -1 }", or "\k{ name }".
1115 The "\g" and "\k" notations were introduced in Perl 5.10.0. Prior to
1117 that there were no named nor relative numbered capture groups.
1118 Absolute numbered groups were referred to using "\1", "\2", etc., and
1119 this notation is still accepted (and likely always will be). But it
1120 leads to some ambiguities if there are more than 9 capture groups, as
1121 "\10" could mean either the tenth capture group, or the character whose
1122 ordinal in octal is 010 (a backspace in ASCII). Perl resolves this
1123 ambiguity by interpreting "\10" as a backreference only if at least 10
1124 left parentheses have opened before it. Likewise "\11" is a
1125 backreference only if at least 11 left parentheses have opened before
1126 it. And so on. "\1" through "\9" are always interpreted as
1127 backreferences. There are several examples below that illustrate these
1128 perils. You can avoid the ambiguity by always using "\g{}" or "\g" if
1129 you mean capturing groups; and for octal constants always using "\o{}",
1130 or for "\077" and below, using 3 digits padded with leading zeros,
1131 since a leading zero implies an octal constant.
1132 The "\digit" notation also works in certain circumstances outside the
1134 pattern. See "Warning on \1 Instead of $1" below for details.
1135 Examples:
1137 s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words
1139 /(.)\g1/ # find first doubled char
1141 and print "'$1' is the first doubled character\n";
1142 /(?<char>.)\k<char>/ # ... a different way
1144 and print "'$+{char}' is the first doubled character\n";
1145 /(?'char'.)\g1/ # ... mix and match
1147 and print "'$1' is the first doubled character\n";
1148 if (/Time: (..):(..):(..)/) { # parse out values
1150 $hours = $1;
1151 $minutes = $2;
1152 $seconds = $3;
1153 }
1154 /(.)(.)(.)(.)(.)(.)(.)(.)(.)\g10/ # \g10 is a backreference
1156 /(.)(.)(.)(.)(.)(.)(.)(.)(.)\10/ # \10 is octal
1157 /((.)(.)(.)(.)(.)(.)(.)(.)(.))\10/ # \10 is a backreference
1158 /((.)(.)(.)(.)(.)(.)(.)(.)(.))\010/ # \010 is octal
1159 $a = '(.)\1'; # Creates problems when concatenated.
1161 $b = '(.)\g{1}'; # Avoids the problems.
1162 "aa" =~ /${a}/; # True
1163 "aa" =~ /${b}/; # True
1164 "aa0" =~ /${a}0/; # False!
1165 "aa0" =~ /${b}0/; # True
1166 "aa\x08" =~ /${a}0/; # True!
1167 "aa\x08" =~ /${b}0/; # False
1168 Several special variables also refer back to portions of the previous
1170 match. $+ returns whatever the last bracket match matched. $& returns
1171 the entire matched string. (At one point $0 did also, but now it
1172 returns the name of the program.) "$`" returns everything before the
1173 matched string. "$'" returns everything after the matched string. And
1174 $^N contains whatever was matched by the most-recently closed group
1175 (submatch). $^N can be used in extended patterns (see below), for
1176 example to assign a submatch to a variable.
1177 These special variables, like the "%+" hash and the numbered match
1179 variables ($1, $2, $3, etc.) are dynamically scoped until the end of
1180 the enclosing block or until the next successful match, whichever comes
1181 first. (See "Compound Statements" in perlsyn.)
1182 The "@{^CAPTURE}" array may be used to access ALL of the capture
1184 buffers as an array without needing to know how many there are. For
1185 instance
1186 $string=~/$pattern/ and @captured = @{^CAPTURE};
1188 will place a copy of each capture variable, $1, $2 etc, into the
1190 @captured array.
1191 Be aware that when interpolating a subscript of the "@{^CAPTURE}" array
1193 you must use demarcated curly brace notation:
1194 print "@{^CAPTURE[0]}";
1196 See "Demarcated variable names using braces" in perldata for more on
1198 this notation.
1199 NOTE: Failed matches in Perl do not reset the match variables, which
1201 makes it easier to write code that tests for a series of more specific
1202 cases and remembers the best match.
1203 WARNING: If your code is to run on Perl 5.16 or earlier, beware that
1205 once Perl sees that you need one of $&, "$`", or "$'" anywhere in the
1206 program, it has to provide them for every pattern match. This may
1207 substantially slow your program.
1208 Perl uses the same mechanism to produce $1, $2, etc, so you also pay a
1210 price for each pattern that contains capturing parentheses. (To avoid
1211 this cost while retaining the grouping behaviour, use the extended
1212 regular expression "(?: ... )" instead.) But if you never use $&, "$`"
1213 or "$'", then patterns without capturing parentheses will not be
1214 penalized. So avoid $&, "$'", and "$`" if you can, but if you can't
1215 (and some algorithms really appreciate them), once you've used them
1216 once, use them at will, because you've already paid the price.
1217 Perl 5.16 introduced a slightly more efficient mechanism that notes
1219 separately whether each of "$`", $&, and "$'" have been seen, and thus
1220 may only need to copy part of the string. Perl 5.20 introduced a much
1221 more efficient copy-on-write mechanism which eliminates any slowdown.
1222 As another workaround for this problem, Perl 5.10.0 introduced
1224 "${^PREMATCH}", "${^MATCH}" and "${^POSTMATCH}", which are equivalent
1225 to "$`", $& and "$'", except that they are only guaranteed to be
1226 defined after a successful match that was executed with the "/p"
1227 (preserve) modifier. The use of these variables incurs no global
1228 performance penalty, unlike their punctuation character equivalents,
1229 however at the trade-off that you have to tell perl when you want to
1230 use them. As of Perl 5.20, these three variables are equivalent to
1231 "$`", $& and "$'", and "/p" is ignored.
1232 Quoting metacharacters
1234 Backslashed metacharacters in Perl are alphanumeric, such as "\b",
1235 "\w", "\n". Unlike some other regular expression languages, there are
1236 no backslashed symbols that aren't alphanumeric. So anything that
1237 looks like "\\", "\(", "\)", "\[", "\]", "\{", or "\}" is always
1238 interpreted as a literal character, not a metacharacter. This was once
1239 used in a common idiom to disable or quote the special meanings of
1240 regular expression metacharacters in a string that you want to use for
1241 a pattern. Simply quote all non-"word" characters:
1242 $pattern =~ s/(\W)/\\$1/g;
1244 (If "use locale" is set, then this depends on the current locale.)
1246 Today it is more common to use the "quotemeta()" function or the "\Q"
1247 metaquoting escape sequence to disable all metacharacters' special
1248 meanings like this:
1249 /$unquoted\Q$quoted\E$unquoted/
1251 Beware that if you put literal backslashes (those not inside
1253 interpolated variables) between "\Q" and "\E", double-quotish backslash
1254 interpolation may lead to confusing results. If you need to use
1255 literal backslashes within "\Q...\E", consult "Gory details of parsing
1256 quoted constructs" in perlop.
1257 "quotemeta()" and "\Q" are fully described in "quotemeta" in perlfunc.
1259 Extended Patterns
1261 Perl also defines a consistent extension syntax for features not found
1262 in standard tools like awk and lex. The syntax for most of these is a
1263 pair of parentheses with a question mark as the first thing within the
1264 parentheses. The character after the question mark indicates the
1265 extension.
1266 A question mark was chosen for this and for the minimal-matching
1268 construct because 1) question marks are rare in older regular
1269 expressions, and 2) whenever you see one, you should stop and
1270 "question" exactly what is going on. That's psychology....
1271 "(?#text)"
1273 A comment. The text is ignored. Note that Perl closes the comment
1274 as soon as it sees a ")", so there is no way to put a literal ")"
1275 in the comment. The pattern's closing delimiter must be escaped by
1276 a backslash if it appears in the comment.
1277 See "/x" for another way to have comments in patterns.
1279 Note that a comment can go just about anywhere, except in the
1281 middle of an escape sequence. Examples:
1282 qr/foo(?#comment)bar/' # Matches 'foobar'
1284 # The pattern below matches 'abcd', 'abccd', or 'abcccd'
1286 qr/abc(?#comment between literal and its quantifier){1,3}d/
1287 # The pattern below generates a syntax error, because the '\p' must
1289 # be followed immediately by a '{'.
1290 qr/\p(?#comment between \p and its property name){Any}/
1291 # The pattern below generates a syntax error, because the initial
1293 # '\(' is a literal opening parenthesis, and so there is nothing
1294 # for the closing ')' to match
1295 qr/\(?#the backslash means this isn't a comment)p{Any}/
1296 # Comments can be used to fold long patterns into multiple lines
1298 qr/First part of a long regex(?#
1299 )remaining part/
1300 "(?adlupimnsx-imnsx)"
1302 "(?^alupimnsx)"
1303 Zero or more embedded pattern-match modifiers, to be turned on (or
1304 turned off if preceded by "-") for the remainder of the pattern or
1305 the remainder of the enclosing pattern group (if any).
1306 This is particularly useful for dynamically-generated patterns,
1308 such as those read in from a configuration file, taken from an
1309 argument, or specified in a table somewhere. Consider the case
1310 where some patterns want to be case-sensitive and some do not: The
1311 case-insensitive ones merely need to include "(?i)" at the front of
1312 the pattern. For example:
1313 $pattern = "foobar";
1315 if ( /$pattern/i ) { }
1316 # more flexible:
1318 $pattern = "(?i)foobar";
1320 if ( /$pattern/ ) { }
1321 These modifiers are restored at the end of the enclosing group. For
1323 example,
1324 ( (?i) blah ) \s+ \g1
1326 will match "blah" in any case, some spaces, and an exact (including
1328 the case!) repetition of the previous word, assuming the "/x"
1329 modifier, and no "/i" modifier outside this group.
1330 These modifiers do not carry over into named subpatterns called in
1332 the enclosing group. In other words, a pattern such as
1333 "((?i)(?&NAME))" does not change the case-sensitivity of the NAME
1334 pattern.
1335 A modifier is overridden by later occurrences of this construct in
1337 the same scope containing the same modifier, so that
1338 /((?im)foo(?-m)bar)/
1340 matches all of "foobar" case insensitively, but uses "/m" rules for
1342 only the "foo" portion. The "a" flag overrides "aa" as well;
1343 likewise "aa" overrides "a". The same goes for "x" and "xx".
1344 Hence, in
1345 /(?-x)foo/xx
1347 both "/x" and "/xx" are turned off during matching "foo". And in
1349 /(?x)foo/x
1351 "/x" but NOT "/xx" is turned on for matching "foo". (One might
1353 mistakenly think that since the inner "(?x)" is already in the
1354 scope of "/x", that the result would effectively be the sum of
1355 them, yielding "/xx". It doesn't work that way.) Similarly, doing
1356 something like "(?xx-x)foo" turns off all "x" behavior for matching
1357 "foo", it is not that you subtract 1 "x" from 2 to get 1 "x"
1358 remaining.
1359 Any of these modifiers can be set to apply globally to all regular
1361 expressions compiled within the scope of a "use re". See "'/flags'
1362 mode" in re.
1363 Starting in Perl 5.14, a "^" (caret or circumflex accent)
1365 immediately after the "?" is a shorthand equivalent to "d-imnsx".
1366 Flags (except "d") may follow the caret to override it. But a
1367 minus sign is not legal with it.
1368 Note that the "a", "d", "l", "p", and "u" modifiers are special in
1370 that they can only be enabled, not disabled, and the "a", "d", "l",
1371 and "u" modifiers are mutually exclusive: specifying one de-
1372 specifies the others, and a maximum of one (or two "a"'s) may
1373 appear in the construct. Thus, for example, "(?-p)" will warn when
1374 compiled under "use warnings"; "(?-d:...)" and "(?dl:...)" are
1375 fatal errors.
1376 Note also that the "p" modifier is special in that its presence
1378 anywhere in a pattern has a global effect.
1379 Having zero modifiers makes this a no-op (so why did you specify
1381 it, unless it's generated code), and starting in v5.30, warns under
1382 "use re 'strict'".
1383 "(?:pattern)"
1385 "(?adluimnsx-imnsx:pattern)"
1386 "(?^aluimnsx:pattern)"
1387 This is for clustering, not capturing; it groups subexpressions
1388 like "()", but doesn't make backreferences as "()" does. So
1389 @fields = split(/\b(?:a|b|c)\b/)
1391 matches the same field delimiters as
1393 @fields = split(/\b(a|b|c)\b/)
1395 but doesn't spit out the delimiters themselves as extra fields
1397 (even though that's the behaviour of "split" in perlfunc when its
1398 pattern contains capturing groups). It's also cheaper not to
1399 capture characters if you don't need to.
1400 Any letters between "?" and ":" act as flags modifiers as with
1402 "(?adluimnsx-imnsx)". For example,
1403 /(?s-i:more.*than).*million/i
1405 is equivalent to the more verbose
1407 /(?:(?s-i)more.*than).*million/i
1409 Note that any "()" constructs enclosed within this one will still
1411 capture unless the "/n" modifier is in effect.
1412 Like the "(?adlupimnsx-imnsx)" construct, "aa" and "a" override
1414 each other, as do "xx" and "x". They are not additive. So, doing
1415 something like "(?xx-x:foo)" turns off all "x" behavior for
1416 matching "foo".
1417 Starting in Perl 5.14, a "^" (caret or circumflex accent)
1419 immediately after the "?" is a shorthand equivalent to "d-imnsx".
1420 Any positive flags (except "d") may follow the caret, so
1421 (?^x:foo)
1423 is equivalent to
1425 (?x-imns:foo)
1427 The caret tells Perl that this cluster doesn't inherit the flags of
1429 any surrounding pattern, but uses the system defaults ("d-imnsx"),
1430 modified by any flags specified.
1431 The caret allows for simpler stringification of compiled regular
1433 expressions. These look like
1434 (?^:pattern)
1436 with any non-default flags appearing between the caret and the
1438 colon. A test that looks at such stringification thus doesn't need
1439 to have the system default flags hard-coded in it, just the caret.
1440 If new flags are added to Perl, the meaning of the caret's
1441 expansion will change to include the default for those flags, so
1442 the test will still work, unchanged.
1443 Specifying a negative flag after the caret is an error, as the flag
1445 is redundant.
1446 Mnemonic for "(?^...)": A fresh beginning since the usual use of a
1448 caret is to match at the beginning.
1449 "(?|pattern)"
1451 This is the "branch reset" pattern, which has the special property
1452 that the capture groups are numbered from the same starting point
1453 in each alternation branch. It is available starting from perl
1454 5.10.0.
1455 Capture groups are numbered from left to right, but inside this
1457 construct the numbering is restarted for each branch.
1458 The numbering within each branch will be as normal, and any groups
1460 following this construct will be numbered as though the construct
1461 contained only one branch, that being the one with the most capture
1462 groups in it.
1463 This construct is useful when you want to capture one of a number
1465 of alternative matches.
1466 Consider the following pattern. The numbers underneath show in
1468 which group the captured content will be stored.
1469 # before ---------------branch-reset----------- after
1471 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1472 # 1 2 2 3 2 3 4
1473 Be careful when using the branch reset pattern in combination with
1475 named captures. Named captures are implemented as being aliases to
1476 numbered groups holding the captures, and that interferes with the
1477 implementation of the branch reset pattern. If you are using named
1478 captures in a branch reset pattern, it's best to use the same
1479 names, in the same order, in each of the alternations:
1480 /(?| (?<a> x ) (?<b> y )
1482 | (?<a> z ) (?<b> w )) /x
1483 Not doing so may lead to surprises:
1485 "12" =~ /(?| (?<a> \d+ ) | (?<b> \D+))/x;
1487 say $+{a}; # Prints '12'
1488 say $+{b}; # *Also* prints '12'.
1489 The problem here is that both the group named "a" and the group
1491 named "b" are aliases for the group belonging to $1.
1492 Lookaround Assertions
1494 Lookaround assertions are zero-width patterns which match a
1495 specific pattern without including it in $&. Positive assertions
1496 match when their subpattern matches, negative assertions match when
1497 their subpattern fails. Lookbehind matches text up to the current
1498 match position, lookahead matches text following the current match
1499 position.
1500 "(?=pattern)"
1502 "(*pla:pattern)"
1503 "(*positive_lookahead:pattern)"
1504 A zero-width positive lookahead assertion. For example,
1505 "/\w+(?=\t)/" matches a word followed by a tab, without
1506 including the tab in $&.
1507 "(?!pattern)"
1509 "(*nla:pattern)"
1510 "(*negative_lookahead:pattern)"
1511 A zero-width negative lookahead assertion. For example
1512 "/foo(?!bar)/" matches any occurrence of "foo" that isn't
1513 followed by "bar". Note however that lookahead and lookbehind
1514 are NOT the same thing. You cannot use this for lookbehind.
1515 If you are looking for a "bar" that isn't preceded by a "foo",
1517 "/(?!foo)bar/" will not do what you want. That's because the
1518 "(?!foo)" is just saying that the next thing cannot be
1519 "foo"--and it's not, it's a "bar", so "foobar" will match. Use
1520 lookbehind instead (see below).
1521 "(?<=pattern)"
1523 "\K"
1524 "(*plb:pattern)"
1525 "(*positive_lookbehind:pattern)"
1526 A zero-width positive lookbehind assertion. For example,
1527 "/(?<=\t)\w+/" matches a word that follows a tab, without
1528 including the tab in $&.
1529 Prior to Perl 5.30, it worked only for fixed-width lookbehind,
1531 but starting in that release, it can handle variable lengths
1532 from 1 to 255 characters as an experimental feature. The
1533 feature is enabled automatically if you use a variable length
1534 positive lookbehind assertion.
1535 In Perl 5.35.10 the scope of the experimental nature of this
1537 construct has been reduced, and experimental warnings will only
1538 be produced when the construct contains capturing parenthesis.
1539 The warnings will be raised at pattern compilation time, unless
1540 turned off, in the "experimental::vlb" category. This is to
1541 warn you that the exact contents of capturing buffers in a
1542 variable length positive lookbehind is not well defined and is
1543 subject to change in a future release of perl.
1544 Currently if you use capture buffers inside of a positive
1546 variable length lookbehind the result will be the longest and
1547 thus leftmost match possible. This means that
1548 "aax" =~ /(?=x)(?<=(a|aa))/
1550 "aax" =~ /(?=x)(?<=(aa|a))/
1551 "aax" =~ /(?=x)(?<=(a{1,2}?)/
1552 "aax" =~ /(?=x)(?<=(a{1,2})/
1553 will all result in $1 containing "aa". It is possible in a
1555 future release of perl we will change this behavior.
1556 There is a special form of this construct, called "\K"
1558 (available since Perl 5.10.0), which causes the regex engine to
1559 "keep" everything it had matched prior to the "\K" and not
1560 include it in $&. This effectively provides non-experimental
1561 variable-length lookbehind of any length.
1562 And, there is a technique that can be used to handle variable
1564 length lookbehinds on earlier releases, and longer than 255
1565 characters. It is described in
1566 <http://www.drregex.com/2019/02/variable-length-lookbehinds-actually.html>.
1567 Note that under "/i", a few single characters match two or
1569 three other characters. This makes them variable length, and
1570 the 255 length applies to the maximum number of characters in
1571 the match. For example "qr/\N{LATIN SMALL LETTER SHARP S}/i"
1572 matches the sequence "ss". Your lookbehind assertion could
1573 contain 127 Sharp S characters under "/i", but adding a 128th
1574 would generate a compilation error, as that could match 256 "s"
1575 characters in a row.
1576 The use of "\K" inside of another lookaround assertion is
1578 allowed, but the behaviour is currently not well defined.
1579 For various reasons "\K" may be significantly more efficient
1581 than the equivalent "(?<=...)" construct, and it is especially
1582 useful in situations where you want to efficiently remove
1583 something following something else in a string. For instance
1584 s/(foo)bar/$1/g;
1586 can be rewritten as the much more efficient
1588 s/foo\Kbar//g;
1590 Use of the non-greedy modifier "?" may not give you the
1592 expected results if it is within a capturing group within the
1593 construct.
1594 "(?<!pattern)"
1596 "(*nlb:pattern)"
1597 "(*negative_lookbehind:pattern)"
1598 A zero-width negative lookbehind assertion. For example
1599 "/(?<!bar)foo/" matches any occurrence of "foo" that does not
1600 follow "bar".
1601 Prior to Perl 5.30, it worked only for fixed-width lookbehind,
1603 but starting in that release, it can handle variable lengths
1604 from 1 to 255 characters as an experimental feature. The
1605 feature is enabled automatically if you use a variable length
1606 negative lookbehind assertion.
1607 In Perl 5.35.10 the scope of the experimental nature of this
1609 construct has been reduced, and experimental warnings will only
1610 be produced when the construct contains capturing parentheses.
1611 The warnings will be raised at pattern compilation time, unless
1612 turned off, in the "experimental::vlb" category. This is to
1613 warn you that the exact contents of capturing buffers in a
1614 variable length negative lookbehind is not well defined and is
1615 subject to change in a future release of perl.
1616 Currently if you use capture buffers inside of a negative
1618 variable length lookbehind the result may not be what you
1619 expect, for instance:
1620 say "axfoo"=~/(?=foo)(?<!(a|ax)(?{ say $1 }))/ ? "y" : "n";
1622 will output the following:
1624 a
1626 no
1627 which does not make sense as this should print out "ax" as the
1629 "a" does not line up at the correct place. Another example
1630 would be:
1631 say "yes: '$1-$2'" if "aayfoo"=~/(?=foo)(?<!(a|aa)(a|aa)x)/;
1633 will output the following:
1635 yes: 'aa-a'
1637 It is possible in a future release of perl we will change this
1639 behavior so both of these examples produced more reasonable
1640 output.
1641 Note that we are confident that the construct will match and
1643 reject patterns appropriately, the undefined behavior strictly
1644 relates to the value of the capture buffer during or after
1645 matching.
1646 There is a technique that can be used to handle variable length
1648 lookbehind on earlier releases, and longer than 255 characters.
1649 It is described in
1650 <http://www.drregex.com/2019/02/variable-length-lookbehinds-actually.html>.
1651 Note that under "/i", a few single characters match two or
1653 three other characters. This makes them variable length, and
1654 the 255 length applies to the maximum number of characters in
1655 the match. For example "qr/\N{LATIN SMALL LETTER SHARP S}/i"
1656 matches the sequence "ss". Your lookbehind assertion could
1657 contain 127 Sharp S characters under "/i", but adding a 128th
1658 would generate a compilation error, as that could match 256 "s"
1659 characters in a row.
1660 Use of the non-greedy modifier "?" may not give you the
1662 expected results if it is within a capturing group within the
1663 construct.
1664 "(?<NAME>pattern)"
1666 "(?'NAME'pattern)"
1667 A named capture group. Identical in every respect to normal
1668 capturing parentheses "()" but for the additional fact that the
1669 group can be referred to by name in various regular expression
1670 constructs (like "\g{NAME}") and can be accessed by name after a
1671 successful match via "%+" or "%-". See perlvar for more details on
1672 the "%+" and "%-" hashes.
1673 If multiple distinct capture groups have the same name, then
1675 $+{NAME} will refer to the leftmost defined group in the match.
1676 The forms "(?'NAME'pattern)" and "(?<NAME>pattern)" are equivalent.
1678 NOTE: While the notation of this construct is the same as the
1680 similar function in .NET regexes, the behavior is not. In Perl the
1681 groups are numbered sequentially regardless of being named or not.
1682 Thus in the pattern
1683 /(x)(?<foo>y)(z)/
1685 $+{foo} will be the same as $2, and $3 will contain 'z' instead of
1687 the opposite which is what a .NET regex hacker might expect.
1688 Currently NAME is restricted to simple identifiers only. In other
1690 words, it must match "/^[_A-Za-z][_A-Za-z0-9]*\z/" or its Unicode
1691 extension (see utf8), though it isn't extended by the locale (see
1692 perllocale).
1693 NOTE: In order to make things easier for programmers with
1695 experience with the Python or PCRE regex engines, the pattern
1696 "(?P<NAME>pattern)" may be used instead of "(?<NAME>pattern)";
1697 however this form does not support the use of single quotes as a
1698 delimiter for the name.
1699 "\k<NAME>"
1701 "\k'NAME'"
1702 "\k{NAME}"
1703 Named backreference. Similar to numeric backreferences, except that
1704 the group is designated by name and not number. If multiple groups
1705 have the same name then it refers to the leftmost defined group in
1706 the current match.
1707 It is an error to refer to a name not defined by a "(?<NAME>)"
1709 earlier in the pattern.
1710 All three forms are equivalent, although with "\k{ NAME }", you may
1712 optionally have blanks within but adjacent to the braces, as shown.
1713 NOTE: In order to make things easier for programmers with
1715 experience with the Python or PCRE regex engines, the pattern
1716 "(?P=NAME)" may be used instead of "\k<NAME>".
1717 "(?{ code })"
1719 WARNING: Using this feature safely requires that you understand its
1720 limitations. Code executed that has side effects may not perform
1721 identically from version to version due to the effect of future
1722 optimisations in the regex engine. For more information on this,
1723 see "Embedded Code Execution Frequency".
1724 This zero-width assertion executes any embedded Perl code. It
1726 always succeeds, and its return value is set as $^R.
1727 In literal patterns, the code is parsed at the same time as the
1729 surrounding code. While within the pattern, control is passed
1730 temporarily back to the perl parser, until the logically-balancing
1731 closing brace is encountered. This is similar to the way that an
1732 array index expression in a literal string is handled, for example
1733 "abc$array[ 1 + f('[') + g()]def"
1735 In particular, braces do not need to be balanced:
1737 s/abc(?{ f('{'); })/def/
1739 Even in a pattern that is interpolated and compiled at run-time,
1741 literal code blocks will be compiled once, at perl compile time;
1742 the following prints "ABCD":
1743 print "D";
1745 my $qr = qr/(?{ BEGIN { print "A" } })/;
1746 my $foo = "foo";
1747 /$foo$qr(?{ BEGIN { print "B" } })/;
1748 BEGIN { print "C" }
1749 In patterns where the text of the code is derived from run-time
1751 information rather than appearing literally in a source code
1752 /pattern/, the code is compiled at the same time that the pattern
1753 is compiled, and for reasons of security, "use re 'eval'" must be
1754 in scope. This is to stop user-supplied patterns containing code
1755 snippets from being executable.
1756 In situations where you need to enable this with "use re 'eval'",
1758 you should also have taint checking enabled, if your perl supports
1759 it. Better yet, use the carefully constrained evaluation within a
1760 Safe compartment. See perlsec for details about both these
1761 mechanisms.
1762 From the viewpoint of parsing, lexical variable scope and closures,
1764 /AAA(?{ BBB })CCC/
1766 behaves approximately like
1768 /AAA/ && do { BBB } && /CCC/
1770 Similarly,
1772 qr/AAA(?{ BBB })CCC/
1774 behaves approximately like
1776 sub { /AAA/ && do { BBB } && /CCC/ }
1778 In particular:
1780 { my $i = 1; $r = qr/(?{ print $i })/ }
1782 my $i = 2;
1783 /$r/; # prints "1"
1784 Inside a "(?{...})" block, $_ refers to the string the regular
1786 expression is matching against. You can also use "pos()" to know
1787 what is the current position of matching within this string.
1788 The code block introduces a new scope from the perspective of
1790 lexical variable declarations, but not from the perspective of
1791 "local" and similar localizing behaviours. So later code blocks
1792 within the same pattern will still see the values which were
1793 localized in earlier blocks. These accumulated localizations are
1794 undone either at the end of a successful match, or if the assertion
1795 is backtracked (compare "Backtracking"). For example,
1796 $_ = 'a' x 8;
1798 m<
1799 (?{ $cnt = 0 }) # Initialize $cnt.
1800 (
1801 a
1802 (?{
1803 local $cnt = $cnt + 1; # Update $cnt,
1804 # backtracking-safe.
1805 })
1806 )*
1807 aaaa
1808 (?{ $res = $cnt }) # On success copy to
1809 # non-localized location.
1810 >x;
1811 will initially increment $cnt up to 8; then during backtracking,
1813 its value will be unwound back to 4, which is the value assigned to
1814 $res. At the end of the regex execution, $cnt will be wound back
1815 to its initial value of 0.
1816 This assertion may be used as the condition in a
1818 (?(condition)yes-pattern|no-pattern)
1820 switch. If not used in this way, the result of evaluation of code
1822 is put into the special variable $^R. This happens immediately, so
1823 $^R can be used from other "(?{ code })" assertions inside the same
1824 regular expression.
1825 The assignment to $^R above is properly localized, so the old value
1827 of $^R is restored if the assertion is backtracked; compare
1828 "Backtracking".
1829 Note that the special variable $^N is particularly useful with
1831 code blocks to capture the results of submatches in variables
1832 without having to keep track of the number of nested parentheses.
1833 For example:
1834 $_ = "The brown fox jumps over the lazy dog";
1836 /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i;
1837 print "color = $color, animal = $animal\n";
1838 "(??{ code })"
1840 WARNING: Using this feature safely requires that you understand its
1841 limitations. Code executed that has side effects may not perform
1842 identically from version to version due to the effect of future
1843 optimisations in the regex engine. For more information on this,
1844 see "Embedded Code Execution Frequency".
1845 This is a "postponed" regular subexpression. It behaves in exactly
1847 the same way as a "(?{ code })" code block as described above,
1848 except that its return value, rather than being assigned to $^R, is
1849 treated as a pattern, compiled if it's a string (or used as-is if
1850 its a qr// object), then matched as if it were inserted instead of
1851 this construct.
1852 During the matching of this sub-pattern, it has its own set of
1854 captures which are valid during the sub-match, but are discarded
1855 once control returns to the main pattern. For example, the
1856 following matches, with the inner pattern capturing "B" and
1857 matching "BB", while the outer pattern captures "A";
1858 my $inner = '(.)\1';
1860 "ABBA" =~ /^(.)(??{ $inner })\1/;
1861 print $1; # prints "A";
1862 Note that this means that there is no way for the inner pattern to
1864 refer to a capture group defined outside. (The code block itself
1865 can use $1, etc., to refer to the enclosing pattern's capture
1866 groups.) Thus, although
1867 ('a' x 100)=~/(??{'(.)' x 100})/
1869 will match, it will not set $1 on exit.
1871 The following pattern matches a parenthesized group:
1873 $re = qr{
1875 \(
1876 (?:
1877 (?> [^()]+ ) # Non-parens without backtracking
1878 |
1879 (??{ $re }) # Group with matching parens
1880 )*
1881 \)
1882 }x;
1883 See also "(?PARNO)" for a different, more efficient way to
1885 accomplish the same task.
1886 Executing a postponed regular expression too many times without
1888 consuming any input string will also result in a fatal error. The
1889 depth at which that happens is compiled into perl, so it can be
1890 changed with a custom build.
1891 "(?PARNO)" "(?-PARNO)" "(?+PARNO)" "(?R)" "(?0)"
1893 Recursive subpattern. Treat the contents of a given capture buffer
1894 in the current pattern as an independent subpattern and attempt to
1895 match it at the current position in the string. Information about
1896 capture state from the caller for things like backreferences is
1897 available to the subpattern, but capture buffers set by the
1898 subpattern are not visible to the caller.
1899 Similar to "(??{ code })" except that it does not involve executing
1901 any code or potentially compiling a returned pattern string;
1902 instead it treats the part of the current pattern contained within
1903 a specified capture group as an independent pattern that must match
1904 at the current position. Also different is the treatment of capture
1905 buffers, unlike "(??{ code })" recursive patterns have access to
1906 their caller's match state, so one can use backreferences safely.
1907 PARNO is a sequence of digits (not starting with 0) whose value
1909 reflects the paren-number of the capture group to recurse to.
1910 "(?R)" recurses to the beginning of the whole pattern. "(?0)" is an
1911 alternate syntax for "(?R)". If PARNO is preceded by a plus or
1912 minus sign then it is assumed to be relative, with negative numbers
1913 indicating preceding capture groups and positive ones following.
1914 Thus "(?-1)" refers to the most recently declared group, and
1915 "(?+1)" indicates the next group to be declared. Note that the
1916 counting for relative recursion differs from that of relative
1917 backreferences, in that with recursion unclosed groups are
1918 included.
1919 The following pattern matches a function "foo()" which may contain
1921 balanced parentheses as the argument.
1922 $re = qr{ ( # paren group 1 (full function)
1924 foo
1925 ( # paren group 2 (parens)
1926 \(
1927 ( # paren group 3 (contents of parens)
1928 (?:
1929 (?> [^()]+ ) # Non-parens without backtracking
1930 |
1931 (?2) # Recurse to start of paren group 2
1932 )*
1933 )
1934 \)
1935 )
1936 )
1937 }x;
1938 If the pattern was used as follows
1940 'foo(bar(baz)+baz(bop))'=~/$re/
1942 and print "\$1 = $1\n",
1943 "\$2 = $2\n",
1944 "\$3 = $3\n";
1945 the output produced should be the following:
1947 $1 = foo(bar(baz)+baz(bop))
1949 $2 = (bar(baz)+baz(bop))
1950 $3 = bar(baz)+baz(bop)
1951 If there is no corresponding capture group defined, then it is a
1953 fatal error. Recursing deeply without consuming any input string
1954 will also result in a fatal error. The depth at which that happens
1955 is compiled into perl, so it can be changed with a custom build.
1956 The following shows how using negative indexing can make it easier
1958 to embed recursive patterns inside of a "qr//" construct for later
1959 use:
1960 my $parens = qr/(\((?:[^()]++|(?-1))*+\))/;
1962 if (/foo $parens \s+ \+ \s+ bar $parens/x) {
1963 # do something here...
1964 }
1965 Note that this pattern does not behave the same way as the
1967 equivalent PCRE or Python construct of the same form. In Perl you
1968 can backtrack into a recursed group, in PCRE and Python the
1969 recursed into group is treated as atomic. Also, modifiers are
1970 resolved at compile time, so constructs like "(?i:(?1))" or
1971 "(?:(?i)(?1))" do not affect how the sub-pattern will be processed.
1972 "(?&NAME)"
1974 Recurse to a named subpattern. Identical to "(?PARNO)" except that
1975 the parenthesis to recurse to is determined by name. If multiple
1976 parentheses have the same name, then it recurses to the leftmost.
1977 It is an error to refer to a name that is not declared somewhere in
1979 the pattern.
1980 NOTE: In order to make things easier for programmers with
1982 experience with the Python or PCRE regex engines the pattern
1983 "(?P>NAME)" may be used instead of "(?&NAME)".
1984 "(?(condition)yes-pattern|no-pattern)"
1986 "(?(condition)yes-pattern)"
1987 Conditional expression. Matches yes-pattern if condition yields a
1988 true value, matches no-pattern otherwise. A missing pattern always
1989 matches.
1990 "(condition)" should be one of:
1992 an integer in parentheses
1994 (which is valid if the corresponding pair of parentheses
1995 matched);
1996 a lookahead/lookbehind/evaluate zero-width assertion;
1998 a name in angle brackets or single quotes
1999 (which is valid if a group with the given name matched);
2000 the special symbol "(R)"
2002 (true when evaluated inside of recursion or eval).
2003 Additionally the "R" may be followed by a number, (which will
2004 be true when evaluated when recursing inside of the appropriate
2005 group), or by "&NAME", in which case it will be true only when
2006 evaluated during recursion in the named group.
2007 Here's a summary of the possible predicates:
2009 "(1)" "(2)" ...
2011 Checks if the numbered capturing group has matched something.
2012 Full syntax: "(?(1)then|else)"
2013 "(<NAME>)" "('NAME')"
2015 Checks if a group with the given name has matched something.
2016 Full syntax: "(?(<name>)then|else)"
2017 "(?=...)" "(?!...)" "(?<=...)" "(?<!...)"
2019 Checks whether the pattern matches (or does not match, for the
2020 "!" variants). Full syntax: "(?(?=lookahead)then|else)"
2021 "(?{ CODE })"
2023 Treats the return value of the code block as the condition.
2024 Full syntax: "(?(?{ code })then|else)"
2025 "(R)"
2027 Checks if the expression has been evaluated inside of
2028 recursion. Full syntax: "(?(R)then|else)"
2029 "(R1)" "(R2)" ...
2031 Checks if the expression has been evaluated while executing
2032 directly inside of the n-th capture group. This check is the
2033 regex equivalent of
2034 if ((caller(0))[3] eq 'subname') { ... }
2036 In other words, it does not check the full recursion stack.
2038 Full syntax: "(?(R1)then|else)"
2040 "(R&NAME)"
2042 Similar to "(R1)", this predicate checks to see if we're
2043 executing directly inside of the leftmost group with a given
2044 name (this is the same logic used by "(?&NAME)" to
2045 disambiguate). It does not check the full stack, but only the
2046 name of the innermost active recursion. Full syntax:
2047 "(?(R&name)then|else)"
2048 "(DEFINE)"
2050 In this case, the yes-pattern is never directly executed, and
2051 no no-pattern is allowed. Similar in spirit to "(?{0})" but
2052 more efficient. See below for details. Full syntax:
2053 "(?(DEFINE)definitions...)"
2054 For example:
2056 m{ ( \( )?
2058 [^()]+
2059 (?(1) \) )
2060 }x
2061 matches a chunk of non-parentheses, possibly included in
2063 parentheses themselves.
2064 A special form is the "(DEFINE)" predicate, which never executes
2066 its yes-pattern directly, and does not allow a no-pattern. This
2067 allows one to define subpatterns which will be executed only by the
2068 recursion mechanism. This way, you can define a set of regular
2069 expression rules that can be bundled into any pattern you choose.
2070 It is recommended that for this usage you put the DEFINE block at
2072 the end of the pattern, and that you name any subpatterns defined
2073 within it.
2074 Also, it's worth noting that patterns defined this way probably
2076 will not be as efficient, as the optimizer is not very clever about
2077 handling them.
2078 An example of how this might be used is as follows:
2080 /(?<NAME>(?&NAME_PAT))(?<ADDR>(?&ADDRESS_PAT))
2082 (?(DEFINE)
2083 (?<NAME_PAT>....)
2084 (?<ADDRESS_PAT>....)
2085 )/x
2086 Note that capture groups matched inside of recursion are not
2088 accessible after the recursion returns, so the extra layer of
2089 capturing groups is necessary. Thus $+{NAME_PAT} would not be
2090 defined even though $+{NAME} would be.
2091 Finally, keep in mind that subpatterns created inside a DEFINE
2093 block count towards the absolute and relative number of captures,
2094 so this:
2095 my @captures = "a" =~ /(.) # First capture
2097 (?(DEFINE)
2098 (?<EXAMPLE> 1 ) # Second capture
2099 )/x;
2100 say scalar @captures;
2101 Will output 2, not 1. This is particularly important if you intend
2103 to compile the definitions with the "qr//" operator, and later
2104 interpolate them in another pattern.
2105 "(?>pattern)"
2107 "(*atomic:pattern)"
2108 An "independent" subexpression, one which matches the substring
2109 that a standalone pattern would match if anchored at the given
2110 position, and it matches nothing other than this substring. This
2111 construct is useful for optimizations of what would otherwise be
2112 "eternal" matches, because it will not backtrack (see
2113 "Backtracking"). It may also be useful in places where the "grab
2114 all you can, and do not give anything back" semantic is desirable.
2115 For example: "^(?>a*)ab" will never match, since "(?>a*)" (anchored
2117 at the beginning of string, as above) will match all characters "a"
2118 at the beginning of string, leaving no "a" for "ab" to match. In
2119 contrast, "a*ab" will match the same as "a+b", since the match of
2120 the subgroup "a*" is influenced by the following group "ab" (see
2121 "Backtracking"). In particular, "a*" inside "a*ab" will match
2122 fewer characters than a standalone "a*", since this makes the tail
2123 match.
2124 "(?>pattern)" does not disable backtracking altogether once it has
2126 matched. It is still possible to backtrack past the construct, but
2127 not into it. So "((?>a*)|(?>b*))ar" will still match "bar".
2128 An effect similar to "(?>pattern)" may be achieved by writing
2130 "(?=(pattern))\g{-1}". This matches the same substring as a
2131 standalone "a+", and the following "\g{-1}" eats the matched
2132 string; it therefore makes a zero-length assertion into an analogue
2133 of "(?>...)". (The difference between these two constructs is that
2134 the second one uses a capturing group, thus shifting ordinals of
2135 backreferences in the rest of a regular expression.)
2136 Consider this pattern:
2138 m{ \(
2140 (
2141 [^()]+ # x+
2142 |
2143 \( [^()]* \)
2144 )+
2145 \)
2146 }x
2147 That will efficiently match a nonempty group with matching
2149 parentheses two levels deep or less. However, if there is no such
2150 group, it will take virtually forever on a long string. That's
2151 because there are so many different ways to split a long string
2152 into several substrings. This is what "(.+)+" is doing, and
2153 "(.+)+" is similar to a subpattern of the above pattern. Consider
2154 how the pattern above detects no-match on "((()aaaaaaaaaaaaaaaaaa"
2155 in several seconds, but that each extra letter doubles this time.
2156 This exponential performance will make it appear that your program
2157 has hung. However, a tiny change to this pattern
2158 m{ \(
2160 (
2161 (?> [^()]+ ) # change x+ above to (?> x+ )
2162 |
2163 \( [^()]* \)
2164 )+
2165 \)
2166 }x
2167 which uses "(?>...)" matches exactly when the one above does
2169 (verifying this yourself would be a productive exercise), but
2170 finishes in a fourth the time when used on a similar string with
2171 1000000 "a"s. Be aware, however, that, when this construct is
2172 followed by a quantifier, it currently triggers a warning message
2173 under the "use warnings" pragma or -w switch saying it "matches
2174 null string many times in regex".
2175 On simple groups, such as the pattern "(?> [^()]+ )", a comparable
2177 effect may be achieved by negative lookahead, as in "[^()]+ (?!
2178 [^()] )". This was only 4 times slower on a string with 1000000
2179 "a"s.
2180 The "grab all you can, and do not give anything back" semantic is
2182 desirable in many situations where on the first sight a simple
2183 "()*" looks like the correct solution. Suppose we parse text with
2184 comments being delimited by "#" followed by some optional
2185 (horizontal) whitespace. Contrary to its appearance, "#[ \t]*" is
2186 not the correct subexpression to match the comment delimiter,
2187 because it may "give up" some whitespace if the remainder of the
2188 pattern can be made to match that way. The correct answer is
2189 either one of these:
2190 (?>#[ \t]*)
2192 #[ \t]*(?![ \t])
2193 For example, to grab non-empty comments into $1, one should use
2195 either one of these:
2196 / (?> \# [ \t]* ) ( .+ ) /x;
2198 / \# [ \t]* ( [^ \t] .* ) /x;
2199 Which one you pick depends on which of these expressions better
2201 reflects the above specification of comments.
2202 In some literature this construct is called "atomic matching" or
2204 "possessive matching".
2205 Possessive quantifiers are equivalent to putting the item they are
2207 applied to inside of one of these constructs. The following
2208 equivalences apply:
2209 Quantifier Form Bracketing Form
2211 --------------- ---------------
2212 PAT*+ (?>PAT*)
2213 PAT++ (?>PAT+)
2214 PAT?+ (?>PAT?)
2215 PAT{min,max}+ (?>PAT{min,max})
2216 Nested "(?>...)" constructs are not no-ops, even if at first glance
2218 they might seem to be. This is because the nested "(?>...)" can
2219 restrict internal backtracking that otherwise might occur. For
2220 example,
2221 "abc" =~ /(?>a[bc]*c)/
2223 matches, but
2225 "abc" =~ /(?>a(?>[bc]*)c)/
2227 does not.
2229 "(?[ ])"
2231 See "Extended Bracketed Character Classes" in perlrecharclass.
2232 Backtracking
2234 NOTE: This section presents an abstract approximation of regular
2235 expression behavior. For a more rigorous (and complicated) view of the
2236 rules involved in selecting a match among possible alternatives, see
2237 "Combining RE Pieces".
2238 A fundamental feature of regular expression matching involves the
2240 notion called backtracking, which is currently used (when needed) by
2241 all regular non-possessive expression quantifiers, namely "*", "*?",
2242 "+", "+?", "{n,m}", and "{n,m}?". Backtracking is often optimized
2243 internally, but the general principle outlined here is valid.
2244 For a regular expression to match, the entire regular expression must
2246 match, not just part of it. So if the beginning of a pattern
2247 containing a quantifier succeeds in a way that causes later parts in
2248 the pattern to fail, the matching engine backs up and recalculates the
2249 beginning part--that's why it's called backtracking.
2250 Here is an example of backtracking: Let's say you want to find the
2252 word following "foo" in the string "Food is on the foo table.":
2253 $_ = "Food is on the foo table.";
2255 if ( /\b(foo)\s+(\w+)/i ) {
2256 print "$2 follows $1.\n";
2257 }
2258 When the match runs, the first part of the regular expression
2260 ("\b(foo)") finds a possible match right at the beginning of the
2261 string, and loads up $1 with "Foo". However, as soon as the matching
2262 engine sees that there's no whitespace following the "Foo" that it had
2263 saved in $1, it realizes its mistake and starts over again one
2264 character after where it had the tentative match. This time it goes
2265 all the way until the next occurrence of "foo". The complete regular
2266 expression matches this time, and you get the expected output of "table
2267 follows foo."
2268 Sometimes minimal matching can help a lot. Imagine you'd like to match
2270 everything between "foo" and "bar". Initially, you write something
2271 like this:
2272 $_ = "The food is under the bar in the barn.";
2274 if ( /foo(.*)bar/ ) {
2275 print "got <$1>\n";
2276 }
2277 Which perhaps unexpectedly yields:
2279 got <d is under the bar in the >
2281 That's because ".*" was greedy, so you get everything between the first
2283 "foo" and the last "bar". Here it's more effective to use minimal
2284 matching to make sure you get the text between a "foo" and the first
2285 "bar" thereafter.
2286 if ( /foo(.*?)bar/ ) { print "got <$1>\n" }
2288 got <d is under the >
2289 Here's another example. Let's say you'd like to match a number at the
2291 end of a string, and you also want to keep the preceding part of the
2292 match. So you write this:
2293 $_ = "I have 2 numbers: 53147";
2295 if ( /(.*)(\d*)/ ) { # Wrong!
2296 print "Beginning is <$1>, number is <$2>.\n";
2297 }
2298 That won't work at all, because ".*" was greedy and gobbled up the
2300 whole string. As "\d*" can match on an empty string the complete
2301 regular expression matched successfully.
2302 Beginning is <I have 2 numbers: 53147>, number is <>.
2304 Here are some variants, most of which don't work:
2306 $_ = "I have 2 numbers: 53147";
2308 @pats = qw{
2309 (.*)(\d*)
2310 (.*)(\d+)
2311 (.*?)(\d*)
2312 (.*?)(\d+)
2313 (.*)(\d+)$
2314 (.*?)(\d+)$
2315 (.*)\b(\d+)$
2316 (.*\D)(\d+)$
2317 };
2318 for $pat (@pats) {
2320 printf "%-12s ", $pat;
2321 if ( /$pat/ ) {
2322 print "<$1> <$2>\n";
2323 } else {
2324 print "FAIL\n";
2325 }
2326 }
2327 That will print out:
2329 (.*)(\d*) <I have 2 numbers: 53147> <>
2331 (.*)(\d+) <I have 2 numbers: 5314> <7>
2332 (.*?)(\d*) <> <>
2333 (.*?)(\d+) <I have > <2>
2334 (.*)(\d+)$ <I have 2 numbers: 5314> <7>
2335 (.*?)(\d+)$ <I have 2 numbers: > <53147>
2336 (.*)\b(\d+)$ <I have 2 numbers: > <53147>
2337 (.*\D)(\d+)$ <I have 2 numbers: > <53147>
2338 As you see, this can be a bit tricky. It's important to realize that a
2340 regular expression is merely a set of assertions that gives a
2341 definition of success. There may be 0, 1, or several different ways
2342 that the definition might succeed against a particular string. And if
2343 there are multiple ways it might succeed, you need to understand
2344 backtracking to know which variety of success you will achieve.
2345 When using lookahead assertions and negations, this can all get even
2347 trickier. Imagine you'd like to find a sequence of non-digits not
2348 followed by "123". You might try to write that as
2349 $_ = "ABC123";
2351 if ( /^\D*(?!123)/ ) { # Wrong!
2352 print "Yup, no 123 in $_\n";
2353 }
2354 But that isn't going to match; at least, not the way you're hoping. It
2356 claims that there is no 123 in the string. Here's a clearer picture of
2357 why that pattern matches, contrary to popular expectations:
2358 $x = 'ABC123';
2360 $y = 'ABC445';
2361 print "1: got $1\n" if $x =~ /^(ABC)(?!123)/;
2363 print "2: got $1\n" if $y =~ /^(ABC)(?!123)/;
2364 print "3: got $1\n" if $x =~ /^(\D*)(?!123)/;
2366 print "4: got $1\n" if $y =~ /^(\D*)(?!123)/;
2367 This prints
2369 2: got ABC
2371 3: got AB
2372 4: got ABC
2373 You might have expected test 3 to fail because it seems to a more
2375 general purpose version of test 1. The important difference between
2376 them is that test 3 contains a quantifier ("\D*") and so can use
2377 backtracking, whereas test 1 will not. What's happening is that you've
2378 asked "Is it true that at the start of $x, following 0 or more non-
2379 digits, you have something that's not 123?" If the pattern matcher had
2380 let "\D*" expand to "ABC", this would have caused the whole pattern to
2381 fail.
2382 The search engine will initially match "\D*" with "ABC". Then it will
2384 try to match "(?!123)" with "123", which fails. But because a
2385 quantifier ("\D*") has been used in the regular expression, the search
2386 engine can backtrack and retry the match differently in the hope of
2387 matching the complete regular expression.
2388 The pattern really, really wants to succeed, so it uses the standard
2390 pattern back-off-and-retry and lets "\D*" expand to just "AB" this
2391 time. Now there's indeed something following "AB" that is not "123".
2392 It's "C123", which suffices.
2393 We can deal with this by using both an assertion and a negation. We'll
2395 say that the first part in $1 must be followed both by a digit and by
2396 something that's not "123". Remember that the lookaheads are zero-
2397 width expressions--they only look, but don't consume any of the string
2398 in their match. So rewriting this way produces what you'd expect; that
2399 is, case 5 will fail, but case 6 succeeds:
2400 print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/;
2402 print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/;
2403 6: got ABC
2405 In other words, the two zero-width assertions next to each other work
2407 as though they're ANDed together, just as you'd use any built-in
2408 assertions: "/^$/" matches only if you're at the beginning of the line
2409 AND the end of the line simultaneously. The deeper underlying truth is
2410 that juxtaposition in regular expressions always means AND, except when
2411 you write an explicit OR using the vertical bar. "/ab/" means match
2412 "a" AND (then) match "b", although the attempted matches are made at
2413 different positions because "a" is not a zero-width assertion, but a
2414 one-width assertion.
2415 WARNING: Particularly complicated regular expressions can take
2417 exponential time to solve because of the immense number of possible
2418 ways they can use backtracking to try for a match. For example,
2419 without internal optimizations done by the regular expression engine,
2420 this will take a painfully long time to run:
2421 'aaaaaaaaaaaa' =~ /((a{0,5}){0,5})*[c]/
2423 And if you used "*"'s in the internal groups instead of limiting them
2425 to 0 through 5 matches, then it would take forever--or until you ran
2426 out of stack space. Moreover, these internal optimizations are not
2427 always applicable. For example, if you put "{0,5}" instead of "*" on
2428 the external group, no current optimization is applicable, and the
2429 match takes a long time to finish.
2430 A powerful tool for optimizing such beasts is what is known as an
2432 "independent group", which does not backtrack (see "(?>pattern)").
2433 Note also that zero-length lookahead/lookbehind assertions will not
2434 backtrack to make the tail match, since they are in "logical" context:
2435 only whether they match is considered relevant. For an example where
2436 side-effects of lookahead might have influenced the following match,
2437 see "(?>pattern)".
2438 Script Runs
2440 A script run is basically a sequence of characters, all from the same
2441 Unicode script (see "Scripts" in perlunicode), such as Latin or Greek.
2442 In most places a single word would never be written in multiple
2443 scripts, unless it is a spoofing attack. An infamous example, is
2444 paypal.com
2446 Those letters could all be Latin (as in the example just above), or
2448 they could be all Cyrillic (except for the dot), or they could be a
2449 mixture of the two. In the case of an internet address the ".com"
2450 would be in Latin, And any Cyrillic ones would cause it to be a
2451 mixture, not a script run. Someone clicking on such a link would not
2452 be directed to the real Paypal website, but an attacker would craft a
2453 look-alike one to attempt to gather sensitive information from the
2454 person.
2455 Starting in Perl 5.28, it is now easy to detect strings that aren't
2457 script runs. Simply enclose just about any pattern like either of
2458 these:
2459 (*script_run:pattern)
2461 (*sr:pattern)
2462 What happens is that after pattern succeeds in matching, it is
2464 subjected to the additional criterion that every character in it must
2465 be from the same script (see exceptions below). If this isn't true,
2466 backtracking occurs until something all in the same script is found
2467 that matches, or all possibilities are exhausted. This can cause a lot
2468 of backtracking, but generally, only malicious input will result in
2469 this, though the slow down could cause a denial of service attack. If
2470 your needs permit, it is best to make the pattern atomic to cut down on
2471 the amount of backtracking. This is so likely to be what you want,
2472 that instead of writing this:
2473 (*script_run:(?>pattern))
2475 you can write either of these:
2477 (*atomic_script_run:pattern)
2479 (*asr:pattern)
2480 (See "(?>pattern)".)
2482 In Taiwan, Japan, and Korea, it is common for text to have a mixture of
2484 characters from their native scripts and base Chinese. Perl follows
2485 Unicode's UTS 39 (<https://unicode.org/reports/tr39/>) Unicode Security
2486 Mechanisms in allowing such mixtures. For example, the Japanese
2487 scripts Katakana and Hiragana are commonly mixed together in practice,
2488 along with some Chinese characters, and hence are treated as being in a
2489 single script run by Perl.
2490 The rules used for matching decimal digits are slightly stricter. Many
2492 scripts have their own sets of digits equivalent to the Western 0
2493 through 9 ones. A few, such as Arabic, have more than one set. For a
2494 string to be considered a script run, all digits in it must come from
2495 the same set of ten, as determined by the first digit encountered. As
2496 an example,
2497 qr/(*script_run: \d+ \b )/x
2499 guarantees that the digits matched will all be from the same set of 10.
2501 You won't get a look-alike digit from a different script that has a
2502 different value than what it appears to be.
2503 Unicode has three pseudo scripts that are handled specially.
2505 "Unknown" is applied to code points whose meaning has yet to be
2507 determined. Perl currently will match as a script run, any single
2508 character string consisting of one of these code points. But any
2509 string longer than one code point containing one of these will not be
2510 considered a script run.
2511 "Inherited" is applied to characters that modify another, such as an
2513 accent of some type. These are considered to be in the script of the
2514 master character, and so never cause a script run to not match.
2515 The other one is "Common". This consists of mostly punctuation, emoji,
2517 and characters used in mathematics and music, the ASCII digits 0
2518 through 9, and full-width forms of these digits. These characters can
2519 appear intermixed in text in many of the world's scripts. These also
2520 don't cause a script run to not match. But like other scripts, all
2521 digits in a run must come from the same set of 10.
2522 This construct is non-capturing. You can add parentheses to pattern to
2524 capture, if desired. You will have to do this if you plan to use
2525 "(*ACCEPT) (*ACCEPT:arg)" and not have it bypass the script run
2526 checking.
2527 The "Script_Extensions" property as modified by UTS 39
2529 (<https://unicode.org/reports/tr39/>) is used as the basis for this
2530 feature.
2531 To summarize,
2533 • All length 0 or length 1 sequences are script runs.
2535 • A longer sequence is a script run if and only if all of the
2537 following conditions are met:
2538 1. No code point in the sequence has the "Script_Extension"
2542 property of "Unknown".
2543 This currently means that all code points in the sequence have
2545 been assigned by Unicode to be characters that aren't private
2546 use nor surrogate code points.
2547 2. All characters in the sequence come from the Common script
2549 and/or the Inherited script and/or a single other script.
2550 The script of a character is determined by the
2552 "Script_Extensions" property as modified by UTS 39
2553 (<https://unicode.org/reports/tr39/>), as described above.
2554 3. All decimal digits in the sequence come from the same block of
2556 10 consecutive digits.
2557 Special Backtracking Control Verbs
2559 These special patterns are generally of the form "(*VERB:arg)". Unless
2560 otherwise stated the arg argument is optional; in some cases, it is
2561 mandatory.
2562 Any pattern containing a special backtracking verb that allows an
2564 argument has the special behaviour that when executed it sets the
2565 current package's $REGERROR and $REGMARK variables. When doing so the
2566 following rules apply:
2567 On failure, the $REGERROR variable will be set to the arg value of the
2569 verb pattern, if the verb was involved in the failure of the match. If
2570 the arg part of the pattern was omitted, then $REGERROR will be set to
2571 the name of the last "(*MARK:NAME)" pattern executed, or to TRUE if
2572 there was none. Also, the $REGMARK variable will be set to FALSE.
2573 On a successful match, the $REGERROR variable will be set to FALSE, and
2575 the $REGMARK variable will be set to the name of the last
2576 "(*MARK:NAME)" pattern executed. See the explanation for the
2577 "(*MARK:NAME)" verb below for more details.
2578 NOTE: $REGERROR and $REGMARK are not magic variables like $1 and most
2580 other regex-related variables. They are not local to a scope, nor
2581 readonly, but instead are volatile package variables similar to
2582 $AUTOLOAD. They are set in the package containing the code that
2583 executed the regex (rather than the one that compiled it, where those
2584 differ). If necessary, you can use "local" to localize changes to
2585 these variables to a specific scope before executing a regex.
2586 If a pattern does not contain a special backtracking verb that allows
2588 an argument, then $REGERROR and $REGMARK are not touched at all.
2589 Verbs
2591 "(*PRUNE)" "(*PRUNE:NAME)"
2592 This zero-width pattern prunes the backtracking tree at the
2593 current point when backtracked into on failure. Consider the
2594 pattern "/A (*PRUNE) B/", where A and B are complex patterns.
2595 Until the "(*PRUNE)" verb is reached, A may backtrack as
2596 necessary to match. Once it is reached, matching continues in B,
2597 which may also backtrack as necessary; however, should B not
2598 match, then no further backtracking will take place, and the
2599 pattern will fail outright at the current starting position.
2600 The following example counts all the possible matching strings
2602 in a pattern (without actually matching any of them).
2603 'aaab' =~ /a+b?(?{print "$&\n"; $count++})(*FAIL)/;
2605 print "Count=$count\n";
2606 which produces:
2608 aaab
2610 aaa
2611 aa
2612 a
2613 aab
2614 aa
2615 a
2616 ab
2617 a
2618 Count=9
2619 If we add a "(*PRUNE)" before the count like the following
2621 'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/;
2623 print "Count=$count\n";
2624 we prevent backtracking and find the count of the longest
2626 matching string at each matching starting point like so:
2627 aaab
2629 aab
2630 ab
2631 Count=3
2632 Any number of "(*PRUNE)" assertions may be used in a pattern.
2634 See also "(?>pattern)" and possessive quantifiers for other ways
2636 to control backtracking. In some cases, the use of "(*PRUNE)"
2637 can be replaced with a "(?>pattern)" with no functional
2638 difference; however, "(*PRUNE)" can be used to handle cases that
2639 cannot be expressed using a "(?>pattern)" alone.
2640 "(*SKIP)" "(*SKIP:NAME)"
2642 This zero-width pattern is similar to "(*PRUNE)", except that on
2643 failure it also signifies that whatever text that was matched
2644 leading up to the "(*SKIP)" pattern being executed cannot be
2645 part of any match of this pattern. This effectively means that
2646 the regex engine "skips" forward to this position on failure and
2647 tries to match again, (assuming that there is sufficient room to
2648 match).
2649 The name of the "(*SKIP:NAME)" pattern has special significance.
2651 If a "(*MARK:NAME)" was encountered while matching, then it is
2652 that position which is used as the "skip point". If no "(*MARK)"
2653 of that name was encountered, then the "(*SKIP)" operator has no
2654 effect. When used without a name the "skip point" is where the
2655 match point was when executing the "(*SKIP)" pattern.
2656 Compare the following to the examples in "(*PRUNE)"; note the
2658 string is twice as long:
2659 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/;
2661 print "Count=$count\n";
2662 outputs
2664 aaab
2666 aaab
2667 Count=2
2668 Once the 'aaab' at the start of the string has matched, and the
2670 "(*SKIP)" executed, the next starting point will be where the
2671 cursor was when the "(*SKIP)" was executed.
2672 "(*MARK:NAME)" "(*:NAME)"
2674 This zero-width pattern can be used to mark the point reached in
2675 a string when a certain part of the pattern has been
2676 successfully matched. This mark may be given a name. A later
2677 "(*SKIP)" pattern will then skip forward to that point if
2678 backtracked into on failure. Any number of "(*MARK)" patterns
2679 are allowed, and the NAME portion may be duplicated.
2680 In addition to interacting with the "(*SKIP)" pattern,
2682 "(*MARK:NAME)" can be used to "label" a pattern branch, so that
2683 after matching, the program can determine which branches of the
2684 pattern were involved in the match.
2685 When a match is successful, the $REGMARK variable will be set to
2687 the name of the most recently executed "(*MARK:NAME)" that was
2688 involved in the match.
2689 This can be used to determine which branch of a pattern was
2691 matched without using a separate capture group for each branch,
2692 which in turn can result in a performance improvement, as perl
2693 cannot optimize "/(?:(x)|(y)|(z))/" as efficiently as something
2694 like "/(?:x(*MARK:x)|y(*MARK:y)|z(*MARK:z))/".
2695 When a match has failed, and unless another verb has been
2697 involved in failing the match and has provided its own name to
2698 use, the $REGERROR variable will be set to the name of the most
2699 recently executed "(*MARK:NAME)".
2700 See "(*SKIP)" for more details.
2702 As a shortcut "(*MARK:NAME)" can be written "(*:NAME)".
2704 "(*THEN)" "(*THEN:NAME)"
2706 This is similar to the "cut group" operator "::" from Raku.
2707 Like "(*PRUNE)", this verb always matches, and when backtracked
2708 into on failure, it causes the regex engine to try the next
2709 alternation in the innermost enclosing group (capturing or
2710 otherwise) that has alternations. The two branches of a
2711 "(?(condition)yes-pattern|no-pattern)" do not count as an
2712 alternation, as far as "(*THEN)" is concerned.
2713 Its name comes from the observation that this operation combined
2715 with the alternation operator ("|") can be used to create what
2716 is essentially a pattern-based if/then/else block:
2717 ( COND (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ )
2719 Note that if this operator is used and NOT inside of an
2721 alternation then it acts exactly like the "(*PRUNE)" operator.
2722 / A (*PRUNE) B /
2724 is the same as
2726 / A (*THEN) B /
2728 but
2730 / ( A (*THEN) B | C ) /
2732 is not the same as
2734 / ( A (*PRUNE) B | C ) /
2736 as after matching the A but failing on the B the "(*THEN)" verb
2738 will backtrack and try C; but the "(*PRUNE)" verb will simply
2739 fail.
2740 "(*COMMIT)" "(*COMMIT:arg)"
2742 This is the Raku "commit pattern" "<commit>" or ":::". It's a
2743 zero-width pattern similar to "(*SKIP)", except that when
2744 backtracked into on failure it causes the match to fail
2745 outright. No further attempts to find a valid match by advancing
2746 the start pointer will occur again. For example,
2747 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/;
2749 print "Count=$count\n";
2750 outputs
2752 aaab
2754 Count=1
2755 In other words, once the "(*COMMIT)" has been entered, and if
2757 the pattern does not match, the regex engine will not try any
2758 further matching on the rest of the string.
2759 "(*FAIL)" "(*F)" "(*FAIL:arg)"
2761 This pattern matches nothing and always fails. It can be used to
2762 force the engine to backtrack. It is equivalent to "(?!)", but
2763 easier to read. In fact, "(?!)" gets optimised into "(*FAIL)"
2764 internally. You can provide an argument so that if the match
2765 fails because of this "FAIL" directive the argument can be
2766 obtained from $REGERROR.
2767 It is probably useful only when combined with "(?{})" or
2769 "(??{})".
2770 "(*ACCEPT)" "(*ACCEPT:arg)"
2772 This pattern matches nothing and causes the end of successful
2773 matching at the point at which the "(*ACCEPT)" pattern was
2774 encountered, regardless of whether there is actually more to
2775 match in the string. When inside of a nested pattern, such as
2776 recursion, or in a subpattern dynamically generated via
2777 "(??{})", only the innermost pattern is ended immediately.
2778 If the "(*ACCEPT)" is inside of capturing groups then the groups
2780 are marked as ended at the point at which the "(*ACCEPT)" was
2781 encountered. For instance:
2782 'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x;
2784 will match, and $1 will be "AB" and $2 will be "B", $3 will not
2786 be set. If another branch in the inner parentheses was matched,
2787 such as in the string 'ACDE', then the "D" and "E" would have to
2788 be matched as well.
2789 You can provide an argument, which will be available in the var
2791 $REGMARK after the match completes.
2792 Warning on "\1" Instead of $1
2794 Some people get too used to writing things like:
2795 $pattern =~ s/(\W)/\\\1/g;
2797 This is grandfathered (for \1 to \9) for the RHS of a substitute to
2799 avoid shocking the sed addicts, but it's a dirty habit to get into.
2800 That's because in PerlThink, the righthand side of an "s///" is a
2801 double-quoted string. "\1" in the usual double-quoted string means a
2802 control-A. The customary Unix meaning of "\1" is kludged in for
2803 "s///". However, if you get into the habit of doing that, you get
2804 yourself into trouble if you then add an "/e" modifier.
2805 s/(\d+)/ \1 + 1 /eg; # causes warning under -w
2807 Or if you try to do
2809 s/(\d+)/\1000/;
2811 You can't disambiguate that by saying "\{1}000", whereas you can fix it
2813 with "${1}000". The operation of interpolation should not be confused
2814 with the operation of matching a backreference. Certainly they mean
2815 two different things on the left side of the "s///".
2816 Repeated Patterns Matching a Zero-length Substring
2818 WARNING: Difficult material (and prose) ahead. This section needs a
2819 rewrite.
2820 Regular expressions provide a terse and powerful programming language.
2822 As with most other power tools, power comes together with the ability
2823 to wreak havoc.
2824 A common abuse of this power stems from the ability to make infinite
2826 loops using regular expressions, with something as innocuous as:
2827 'foo' =~ m{ ( o? )* }x;
2829 The "o?" matches at the beginning of ""foo"", and since the position in
2831 the string is not moved by the match, "o?" would match again and again
2832 because of the "*" quantifier. Another common way to create a similar
2833 cycle is with the looping modifier "/g":
2834 @matches = ( 'foo' =~ m{ o? }xg );
2836 or
2838 print "match: <$&>\n" while 'foo' =~ m{ o? }xg;
2840 or the loop implied by "split()".
2842 However, long experience has shown that many programming tasks may be
2844 significantly simplified by using repeated subexpressions that may
2845 match zero-length substrings. Here's a simple example being:
2846 @chars = split //, $string; # // is not magic in split
2848 ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// /
2849 Thus Perl allows such constructs, by forcefully breaking the infinite
2851 loop. The rules for this are different for lower-level loops given by
2852 the greedy quantifiers "*+{}", and for higher-level ones like the "/g"
2853 modifier or "split()" operator.
2854 The lower-level loops are interrupted (that is, the loop is broken)
2856 when Perl detects that a repeated expression matched a zero-length
2857 substring. Thus
2858 m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x;
2860 is made equivalent to
2862 m{ (?: NON_ZERO_LENGTH )* (?: ZERO_LENGTH )? }x;
2864 For example, this program
2866 #!perl -l
2868 "aaaaab" =~ /
2869 (?:
2870 a # non-zero
2871 | # or
2872 (?{print "hello"}) # print hello whenever this
2873 # branch is tried
2874 (?=(b)) # zero-width assertion
2875 )* # any number of times
2876 /x;
2877 print $&;
2878 print $1;
2879 prints
2881 hello
2883 aaaaa
2884 b
2885 Notice that "hello" is only printed once, as when Perl sees that the
2887 sixth iteration of the outermost "(?:)*" matches a zero-length string,
2888 it stops the "*".
2889 The higher-level loops preserve an additional state between iterations:
2891 whether the last match was zero-length. To break the loop, the
2892 following match after a zero-length match is prohibited to have a
2893 length of zero. This prohibition interacts with backtracking (see
2894 "Backtracking"), and so the second best match is chosen if the best
2895 match is of zero length.
2896 For example:
2898 $_ = 'bar';
2900 s/\w??/<$&>/g;
2901 results in "<><b><><a><><r><>". At each position of the string the
2903 best match given by non-greedy "??" is the zero-length match, and the
2904 second best match is what is matched by "\w". Thus zero-length matches
2905 alternate with one-character-long matches.
2906 Similarly, for repeated "m/()/g" the second-best match is the match at
2908 the position one notch further in the string.
2909 The additional state of being matched with zero-length is associated
2911 with the matched string, and is reset by each assignment to "pos()".
2912 Zero-length matches at the end of the previous match are ignored during
2913 "split".
2914 Combining RE Pieces
2916 Each of the elementary pieces of regular expressions which were
2917 described before (such as "ab" or "\Z") could match at most one
2918 substring at the given position of the input string. However, in a
2919 typical regular expression these elementary pieces are combined into
2920 more complicated patterns using combining operators "ST", "S|T", "S*"
2921 etc. (in these examples "S" and "T" are regular subexpressions).
2922 Such combinations can include alternatives, leading to a problem of
2924 choice: if we match a regular expression "a|ab" against "abc", will it
2925 match substring "a" or "ab"? One way to describe which substring is
2926 actually matched is the concept of backtracking (see "Backtracking").
2927 However, this description is too low-level and makes you think in terms
2928 of a particular implementation.
2929 Another description starts with notions of "better"/"worse". All the
2931 substrings which may be matched by the given regular expression can be
2932 sorted from the "best" match to the "worst" match, and it is the "best"
2933 match which is chosen. This substitutes the question of "what is
2934 chosen?" by the question of "which matches are better, and which are
2935 worse?".
2936 Again, for elementary pieces there is no such question, since at most
2938 one match at a given position is possible. This section describes the
2939 notion of better/worse for combining operators. In the description
2940 below "S" and "T" are regular subexpressions.
2941 "ST"
2943 Consider two possible matches, "AB" and "A'B'", "A" and "A'" are
2944 substrings which can be matched by "S", "B" and "B'" are substrings
2945 which can be matched by "T".
2946 If "A" is a better match for "S" than "A'", "AB" is a better match
2948 than "A'B'".
2949 If "A" and "A'" coincide: "AB" is a better match than "AB'" if "B"
2951 is a better match for "T" than "B'".
2952 "S|T"
2954 When "S" can match, it is a better match than when only "T" can
2955 match.
2956 Ordering of two matches for "S" is the same as for "S". Similar
2958 for two matches for "T".
2959 "S{REPEAT_COUNT}"
2961 Matches as "SSS...S" (repeated as many times as necessary).
2962 "S{min,max}"
2964 Matches as "S{max}|S{max-1}|...|S{min+1}|S{min}".
2965 "S{min,max}?"
2967 Matches as "S{min}|S{min+1}|...|S{max-1}|S{max}".
2968 "S?", "S*", "S+"
2970 Same as "S{0,1}", "S{0,BIG_NUMBER}", "S{1,BIG_NUMBER}"
2971 respectively.
2972 "S??", "S*?", "S+?"
2974 Same as "S{0,1}?", "S{0,BIG_NUMBER}?", "S{1,BIG_NUMBER}?"
2975 respectively.
2976 "(?>S)"
2978 Matches the best match for "S" and only that.
2979 "(?=S)", "(?<=S)"
2981 Only the best match for "S" is considered. (This is important only
2982 if "S" has capturing parentheses, and backreferences are used
2983 somewhere else in the whole regular expression.)
2984 "(?!S)", "(?<!S)"
2986 For this grouping operator there is no need to describe the
2987 ordering, since only whether or not "S" can match is important.
2988 "(??{ EXPR })", "(?PARNO)"
2990 The ordering is the same as for the regular expression which is the
2991 result of EXPR, or the pattern contained by capture group PARNO.
2992 "(?(condition)yes-pattern|no-pattern)"
2994 Recall that which of yes-pattern or no-pattern actually matches is
2995 already determined. The ordering of the matches is the same as for
2996 the chosen subexpression.
2997 The above recipes describe the ordering of matches at a given position.
2999 One more rule is needed to understand how a match is determined for the
3000 whole regular expression: a match at an earlier position is always
3001 better than a match at a later position.
3002 Creating Custom RE Engines
3004 As of Perl 5.10.0, one can create custom regular expression engines.
3005 This is not for the faint of heart, as they have to plug in at the C
3006 level. See perlreapi for more details.
3007 As an alternative, overloaded constants (see overload) provide a simple
3009 way to extend the functionality of the RE engine, by substituting one
3010 pattern for another.
3011 Suppose that we want to enable a new RE escape-sequence "\Y|" which
3013 matches at a boundary between whitespace characters and non-whitespace
3014 characters. Note that "(?=\S)(?<!\S)|(?!\S)(?<=\S)" matches exactly at
3015 these positions, so we want to have each "\Y|" in the place of the more
3016 complicated version. We can create a module "customre" to do this:
3017 package customre;
3019 use overload;
3020 sub import {
3022 shift;
3023 die "No argument to customre::import allowed" if @_;
3024 overload::constant 'qr' => \&convert;
3025 }
3026 sub invalid { die "/$_[0]/: invalid escape '\\$_[1]'"}
3028 # We must also take care of not escaping the legitimate \\Y|
3030 # sequence, hence the presence of '\\' in the conversion rules.
3031 my %rules = ( '\\' => '\\\\',
3032 'Y|' => qr/(?=\S)(?<!\S)|(?!\S)(?<=\S)/ );
3033 sub convert {
3034 my $re = shift;
3035 $re =~ s{
3036 \\ ( \\ | Y . )
3037 }
3038 { $rules{$1} or invalid($re,$1) }sgex;
3039 return $re;
3040 }
3041 Now "use customre" enables the new escape in constant regular
3043 expressions, i.e., those without any runtime variable interpolations.
3044 As documented in overload, this conversion will work only over literal
3045 parts of regular expressions. For "\Y|$re\Y|" the variable part of
3046 this regular expression needs to be converted explicitly (but only if
3047 the special meaning of "\Y|" should be enabled inside $re):
3048 use customre;
3050 $re = <>;
3051 chomp $re;
3052 $re = customre::convert $re;
3053 /\Y|$re\Y|/;
3054 Embedded Code Execution Frequency
3056 The exact rules for how often "(??{})" and "(?{})" are executed in a
3057 pattern are unspecified. In the case of a successful match you can
3058 assume that they DWIM and will be executed in left to right order the
3059 appropriate number of times in the accepting path of the pattern as
3060 would any other meta-pattern. How non-accepting pathways and match
3061 failures affect the number of times a pattern is executed is
3062 specifically unspecified and may vary depending on what optimizations
3063 can be applied to the pattern and is likely to change from version to
3064 version.
3065 For instance in
3067 "aaabcdeeeee"=~/a(?{print "a"})b(?{print "b"})cde/;
3069 the exact number of times "a" or "b" are printed out is unspecified for
3071 failure, but you may assume they will be printed at least once during a
3072 successful match, additionally you may assume that if "b" is printed,
3073 it will be preceded by at least one "a".
3074 In the case of branching constructs like the following:
3076 /a(b|(?{ print "a" }))c(?{ print "c" })/;
3078 you can assume that the input "ac" will output "ac", and that "abc"
3080 will output only "c".
3081 When embedded code is quantified, successful matches will call the code
3083 once for each matched iteration of the quantifier. For example:
3084 "good" =~ /g(?:o(?{print "o"}))*d/;
3086 will output "o" twice.
3088 PCRE/Python Support
3090 As of Perl 5.10.0, Perl supports several Python/PCRE-specific
3091 extensions to the regex syntax. While Perl programmers are encouraged
3092 to use the Perl-specific syntax, the following are also accepted:
3093 "(?P<NAME>pattern)"
3095 Define a named capture group. Equivalent to "(?<NAME>pattern)".
3096 "(?P=NAME)"
3098 Backreference to a named capture group. Equivalent to "\g{NAME}".
3099 "(?P>NAME)"
3101 Subroutine call to a named capture group. Equivalent to "(?&NAME)".
3102
3104 There are a number of issues with regard to case-insensitive matching
3105 in Unicode rules. See "i" under "Modifiers" above.
3106 This document varies from difficult to understand to completely and
3108 utterly opaque. The wandering prose riddled with jargon is hard to
3109 fathom in several places.
3110 This document needs a rewrite that separates the tutorial content from
3112 the reference content.
3113
3115 The syntax of patterns used in Perl pattern matching evolved from those
3116 supplied in the Bell Labs Research Unix 8th Edition (Version 8) regex
3117 routines. (The code is actually derived (distantly) from Henry
3118 Spencer's freely redistributable reimplementation of those V8
3119 routines.)
3120 perlrequick.
3122 perlretut.
3124 "Regexp Quote-Like Operators" in perlop.
3126 "Gory details of parsing quoted constructs" in perlop.
3128 perlfaq6.
3130 "pos" in perlfunc.
3132 perllocale.
3134 perlebcdic.
3136 Mastering Regular Expressions by Jeffrey Friedl, published by O'Reilly
3138 and Associates.
3139perl v5.36.3 2023-11-30 PERLRE(1)