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
46 delimitted, at both ends, by delimitter characters. These are often,
47 as in the example above, forward slashes, and the typical way a pattern
48 is written in documentation is with those slashes. In most cases, the
49 delimitter 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
295 the pattern match operator. (Older programs did this by setting $*,
296 but this option was removed in perl 5.10.)
297 To simplify multi-line substitutions, the "." character never matches a
299 newline unless you use the "/s" modifier, which in effect tells Perl to
300 pretend the string is a single line--even if it isn't.
301 Modifiers
303 Overview
304 The default behavior for matching can be changed, using various
306 modifiers. Modifiers that relate to the interpretation of the pattern
307 are listed just below. Modifiers that alter the way a pattern is used
308 by Perl are detailed in "Regexp Quote-Like Operators" in perlop and
309 "Gory details of parsing quoted constructs" in perlop.
310 "m" Treat the string being matched against as multiple lines. That is,
312 change "^" and "$" from matching the start of the string's first
313 line and the end of its last line to matching the start and end of
314 each line within the string.
315 "s" Treat the string as single line. That is, change "." to match any
317 character whatsoever, even a newline, which normally it would not
318 match.
319 Used together, as "/ms", they let the "." match any character
321 whatsoever, while still allowing "^" and "$" to match,
322 respectively, just after and just before newlines within the
323 string.
324 "i" Do case-insensitive pattern matching. For example, "A" will match
326 "a" under "/i".
327 If locale matching rules are in effect, the case map is taken from
329 the current locale for code points less than 255, and from Unicode
330 rules for larger code points. However, matches that would cross
331 the Unicode rules/non-Unicode rules boundary (ords 255/256) will
332 not succeed, unless the locale is a UTF-8 one. See perllocale.
333 There are a number of Unicode characters that match a sequence of
335 multiple characters under "/i". For example, "LATIN SMALL LIGATURE
336 FI" should match the sequence "fi". Perl is not currently able to
337 do this when the multiple characters are in the pattern and are
338 split between groupings, or when one or more are quantified. Thus
339 "\N{LATIN SMALL LIGATURE FI}" =~ /fi/i; # Matches
341 "\N{LATIN SMALL LIGATURE FI}" =~ /[fi][fi]/i; # Doesn't match!
342 "\N{LATIN SMALL LIGATURE FI}" =~ /fi*/i; # Doesn't match!
343 # The below doesn't match, and it isn't clear what $1 and $2 would
345 # be even if it did!!
346 "\N{LATIN SMALL LIGATURE FI}" =~ /(f)(i)/i; # Doesn't match!
347 Perl doesn't match multiple characters in a bracketed character
349 class unless the character that maps to them is explicitly
350 mentioned, and it doesn't match them at all if the character class
351 is inverted, which otherwise could be highly confusing. See
352 "Bracketed Character Classes" in perlrecharclass, and "Negation" in
353 perlrecharclass.
354 "x" and "xx"
356 Extend your pattern's legibility by permitting whitespace and
357 comments. Details in "/x and /xx"
358 "p" Preserve the string matched such that "${^PREMATCH}", "${^MATCH}",
360 and "${^POSTMATCH}" are available for use after matching.
361 In Perl 5.20 and higher this is ignored. Due to a new copy-on-write
363 mechanism, "${^PREMATCH}", "${^MATCH}", and "${^POSTMATCH}" will be
364 available after the match regardless of the modifier.
365 "a", "d", "l", and "u"
367 These modifiers, all new in 5.14, affect which character-set rules
368 (Unicode, etc.) are used, as described below in "Character set
369 modifiers".
370 "n" Prevent the grouping metacharacters "()" from capturing. This
372 modifier, new in 5.22, will stop $1, $2, etc... from being filled
373 in.
374 "hello" =~ /(hi|hello)/; # $1 is "hello"
376 "hello" =~ /(hi|hello)/n; # $1 is undef
377 This is equivalent to putting "?:" at the beginning of every
379 capturing group:
380 "hello" =~ /(?:hi|hello)/; # $1 is undef
382 "/n" can be negated on a per-group basis. Alternatively, named
384 captures may still be used.
385 "hello" =~ /(?-n:(hi|hello))/n; # $1 is "hello"
387 "hello" =~ /(?<greet>hi|hello)/n; # $1 is "hello", $+{greet} is
388 # "hello"
389 Other Modifiers
391 There are a number of flags that can be found at the end of regular
392 expression constructs that are not generic regular expression
393 flags, but apply to the operation being performed, like matching or
394 substitution ("m//" or "s///" respectively).
395 Flags described further in "Using regular expressions in Perl" in
397 perlretut are:
398 c - keep the current position during repeated matching
400 g - globally match the pattern repeatedly in the string
401 Substitution-specific modifiers described in
403 "s/PATTERN/REPLACEMENT/msixpodualngcer" in perlop are:
404 e - evaluate the right-hand side as an expression
406 ee - evaluate the right side as a string then eval the result
407 o - pretend to optimize your code, but actually introduce bugs
408 r - perform non-destructive substitution and return the new value
409 Regular expression modifiers are usually written in documentation as
411 e.g., "the "/x" modifier", even though the delimiter in question might
412 not really be a slash. The modifiers "/imnsxadlup" may also be
413 embedded within the regular expression itself using the "(?...)"
414 construct, see "Extended Patterns" below.
415 Details on some modifiers
417 Some of the modifiers require more explanation than given in the
419 "Overview" above.
420 "/x" and "/xx"
422 A single "/x" tells the regular expression parser to ignore most
424 whitespace that is neither backslashed nor within a bracketed character
425 class. You can use this to break up your regular expression into more
426 readable parts. Also, the "#" character is treated as a metacharacter
427 introducing a comment that runs up to the pattern's closing delimiter,
428 or to the end of the current line if the pattern extends onto the next
429 line. Hence, this is very much like an ordinary Perl code comment.
430 (You can include the closing delimiter within the comment only if you
431 precede it with a backslash, so be careful!)
432 Use of "/x" means that if you want real whitespace or "#" characters in
434 the pattern (outside a bracketed character class, which is unaffected
435 by "/x"), then you'll either have to escape them (using backslashes or
436 "\Q...\E") or encode them using octal, hex, or "\N{}" or "\p{name=...}"
437 escapes. It is ineffective to try to continue a comment onto the next
438 line by escaping the "\n" with a backslash or "\Q".
439 You can use "(?#text)" to create a comment that ends earlier than the
441 end of the current line, but "text" also can't contain the closing
442 delimiter unless escaped with a backslash.
443 A common pitfall is to forget that "#" characters begin a comment under
445 "/x" and are not matched literally. Just keep that in mind when trying
446 to puzzle out why a particular "/x" pattern isn't working as expected.
447 Starting in Perl v5.26, if the modifier has a second "x" within it, it
449 does everything that a single "/x" does, but additionally non-
450 backslashed SPACE and TAB characters within bracketed character classes
451 are also generally ignored, and hence can be added to make the classes
452 more readable.
453 / [d-e g-i 3-7]/xx
455 /[ ! @ " # $ % ^ & * () = ? <> ' ]/xx
456 may be easier to grasp than the squashed equivalents
458 /[d-eg-i3-7]/
460 /[!@"#$%^&*()=?<>']/
461 Taken together, these features go a long way towards making Perl's
463 regular expressions more readable. Here's an example:
464 # Delete (most) C comments.
466 $program =~ s {
467 /\* # Match the opening delimiter.
468 .*? # Match a minimal number of characters.
469 \*/ # Match the closing delimiter.
470 } []gsx;
471 Note that anything inside a "\Q...\E" stays unaffected by "/x". And
473 note that "/x" doesn't affect space interpretation within a single
474 multi-character construct. For example in "\x{...}", regardless of the
475 "/x" modifier, there can be no spaces. Same for a quantifier such as
476 "{3}" or "{5,}". Similarly, "(?:...)" can't have a space between the
477 "(", "?", and ":". Within any delimiters for such a construct, allowed
478 spaces are not affected by "/x", and depend on the construct. For
479 example, "\x{...}" can't have spaces because hexadecimal numbers don't
480 have spaces in them. But, Unicode properties can have spaces, so in
481 "\p{...}" there can be spaces that follow the Unicode rules, for which
482 see "Properties accessible through \p{} and \P{}" in perluniprops.
483 The set of characters that are deemed whitespace are those that Unicode
485 calls "Pattern White Space", namely:
486 U+0009 CHARACTER TABULATION
488 U+000A LINE FEED
489 U+000B LINE TABULATION
490 U+000C FORM FEED
491 U+000D CARRIAGE RETURN
492 U+0020 SPACE
493 U+0085 NEXT LINE
494 U+200E LEFT-TO-RIGHT MARK
495 U+200F RIGHT-TO-LEFT MARK
496 U+2028 LINE SEPARATOR
497 U+2029 PARAGRAPH SEPARATOR
498 Character set modifiers
500 "/d", "/u", "/a", and "/l", available starting in 5.14, are called the
502 character set modifiers; they affect the character set rules used for
503 the regular expression.
504 The "/d", "/u", and "/l" modifiers are not likely to be of much use to
506 you, and so you need not worry about them very much. They exist for
507 Perl's internal use, so that complex regular expression data structures
508 can be automatically serialized and later exactly reconstituted,
509 including all their nuances. But, since Perl can't keep a secret, and
510 there may be rare instances where they are useful, they are documented
511 here.
512 The "/a" modifier, on the other hand, may be useful. Its purpose is to
514 allow code that is to work mostly on ASCII data to not have to concern
515 itself with Unicode.
516 Briefly, "/l" sets the character set to that of whatever Locale is in
518 effect at the time of the execution of the pattern match.
519 "/u" sets the character set to Unicode.
521 "/a" also sets the character set to Unicode, BUT adds several
523 restrictions for ASCII-safe matching.
524 "/d" is the old, problematic, pre-5.14 Default character set behavior.
526 Its only use is to force that old behavior.
527 At any given time, exactly one of these modifiers is in effect. Their
529 existence allows Perl to keep the originally compiled behavior of a
530 regular expression, regardless of what rules are in effect when it is
531 actually executed. And if it is interpolated into a larger regex, the
532 original's rules continue to apply to it, and don't affect the other
533 parts.
534 The "/l" and "/u" modifiers are automatically selected for regular
536 expressions compiled within the scope of various pragmas, and we
537 recommend that in general, you use those pragmas instead of specifying
538 these modifiers explicitly. For one thing, the modifiers affect only
539 pattern matching, and do not extend to even any replacement done,
540 whereas using the pragmas gives consistent results for all appropriate
541 operations within their scopes. For example,
542 s/foo/\Ubar/il
544 will match "foo" using the locale's rules for case-insensitive
546 matching, but the "/l" does not affect how the "\U" operates. Most
547 likely you want both of them to use locale rules. To do this, instead
548 compile the regular expression within the scope of "use locale". This
549 both implicitly adds the "/l", and applies locale rules to the "\U".
550 The lesson is to "use locale", and not "/l" explicitly.
551 Similarly, it would be better to use "use feature 'unicode_strings'"
553 instead of,
554 s/foo/\Lbar/iu
556 to get Unicode rules, as the "\L" in the former (but not necessarily
558 the latter) would also use Unicode rules.
559 More detail on each of the modifiers follows. Most likely you don't
561 need to know this detail for "/l", "/u", and "/d", and can skip ahead
562 to /a.
563 /l
565 means to use the current locale's rules (see perllocale) when pattern
567 matching. For example, "\w" will match the "word" characters of that
568 locale, and "/i" case-insensitive matching will match according to the
569 locale's case folding rules. The locale used will be the one in effect
570 at the time of execution of the pattern match. This may not be the
571 same as the compilation-time locale, and can differ from one match to
572 another if there is an intervening call of the setlocale() function.
573 Prior to v5.20, Perl did not support multi-byte locales. Starting
575 then, UTF-8 locales are supported. No other multi byte locales are
576 ever likely to be supported. However, in all locales, one can have
577 code points above 255 and these will always be treated as Unicode no
578 matter what locale is in effect.
579 Under Unicode rules, there are a few case-insensitive matches that
581 cross the 255/256 boundary. Except for UTF-8 locales in Perls v5.20
582 and later, these are disallowed under "/l". For example, 0xFF (on
583 ASCII platforms) does not caselessly match the character at 0x178,
584 "LATIN CAPITAL LETTER Y WITH DIAERESIS", because 0xFF may not be "LATIN
585 SMALL LETTER Y WITH DIAERESIS" in the current locale, and Perl has no
586 way of knowing if that character even exists in the locale, much less
587 what code point it is.
588 In a UTF-8 locale in v5.20 and later, the only visible difference
590 between locale and non-locale in regular expressions should be tainting
591 (see perlsec).
592 This modifier may be specified to be the default by "use locale", but
594 see "Which character set modifier is in effect?".
595 /u
597 means to use Unicode rules when pattern matching. On ASCII platforms,
599 this means that the code points between 128 and 255 take on their
600 Latin-1 (ISO-8859-1) meanings (which are the same as Unicode's).
601 (Otherwise Perl considers their meanings to be undefined.) Thus, under
602 this modifier, the ASCII platform effectively becomes a Unicode
603 platform; and hence, for example, "\w" will match any of the more than
604 100_000 word characters in Unicode.
605 Unlike most locales, which are specific to a language and country pair,
607 Unicode classifies all the characters that are letters somewhere in the
608 world as "\w". For example, your locale might not think that "LATIN
609 SMALL LETTER ETH" is a letter (unless you happen to speak Icelandic),
610 but Unicode does. Similarly, all the characters that are decimal
611 digits somewhere in the world will match "\d"; this is hundreds, not
612 10, possible matches. And some of those digits look like some of the
613 10 ASCII digits, but mean a different number, so a human could easily
614 think a number is a different quantity than it really is. For example,
615 "BENGALI DIGIT FOUR" (U+09EA) looks very much like an "ASCII DIGIT
616 EIGHT" (U+0038), and "LEPCHA DIGIT SIX" (U+1C46) looks very much like
617 an "ASCII DIGIT FIVE" (U+0035). And, "\d+", may match strings of
618 digits that are a mixture from different writing systems, creating a
619 security issue. A fraudulent website, for example, could display the
620 price of something using U+1C46, and it would appear to the user that
621 something cost 500 units, but it really costs 600. A browser that
622 enforced script runs ("Script Runs") would prevent that fraudulent
623 display. "num()" in Unicode::UCD can also be used to sort this out.
624 Or the "/a" modifier can be used to force "\d" to match just the ASCII
625 0 through 9.
626 Also, under this modifier, case-insensitive matching works on the full
628 set of Unicode characters. The "KELVIN SIGN", for example matches the
629 letters "k" and "K"; and "LATIN SMALL LIGATURE FF" matches the sequence
630 "ff", which, if you're not prepared, might make it look like a
631 hexadecimal constant, presenting another potential security issue. See
632 <https://unicode.org/reports/tr36> for a detailed discussion of Unicode
633 security issues.
634 This modifier may be specified to be the default by "use feature
636 'unicode_strings", "use locale ':not_characters'", or "use 5.012" (or
637 higher), but see "Which character set modifier is in effect?".
638 /d
640 This modifier means to use the "Default" native rules of the platform
642 except when there is cause to use Unicode rules instead, as follows:
643 1. the target string is encoded in UTF-8; or
645 2. the pattern is encoded in UTF-8; or
647 3. the pattern explicitly mentions a code point that is above 255 (say
649 by "\x{100}"); or
650 4. the pattern uses a Unicode name ("\N{...}"); or
652 5. the pattern uses a Unicode property ("\p{...}" or "\P{...}"); or
654 6. the pattern uses a Unicode break ("\b{...}" or "\B{...}"); or
656 7. the pattern uses ""(?[ ])""
658 8. the pattern uses "(*script_run: ...)"
660 Another mnemonic for this modifier is "Depends", as the rules actually
662 used depend on various things, and as a result you can get unexpected
663 results. See "The "Unicode Bug"" in perlunicode. The Unicode Bug has
664 become rather infamous, leading to yet another (without swearing) name
665 for this modifier, "Dodgy".
666 Unless the pattern or string are encoded in UTF-8, only ASCII
668 characters can match positively.
669 Here are some examples of how that works on an ASCII platform:
671 $str = "\xDF"; # $str is not in UTF-8 format.
673 $str =~ /^\w/; # No match, as $str isn't in UTF-8 format.
674 $str .= "\x{0e0b}"; # Now $str is in UTF-8 format.
675 $str =~ /^\w/; # Match! $str is now in UTF-8 format.
676 chop $str;
677 $str =~ /^\w/; # Still a match! $str remains in UTF-8 format.
678 This modifier is automatically selected by default when none of the
680 others are, so yet another name for it is "Default".
681 Because of the unexpected behaviors associated with this modifier, you
683 probably should only explicitly use it to maintain weird backward
684 compatibilities.
685 /a (and /aa)
687 This modifier stands for ASCII-restrict (or ASCII-safe). This modifier
689 may be doubled-up to increase its effect.
690 When it appears singly, it causes the sequences "\d", "\s", "\w", and
692 the Posix character classes to match only in the ASCII range. They
693 thus revert to their pre-5.6, pre-Unicode meanings. Under "/a", "\d"
694 always means precisely the digits "0" to "9"; "\s" means the five
695 characters "[ \f\n\r\t]", and starting in Perl v5.18, the vertical tab;
696 "\w" means the 63 characters "[A-Za-z0-9_]"; and likewise, all the
697 Posix classes such as "[[:print:]]" match only the appropriate ASCII-
698 range characters.
699 This modifier is useful for people who only incidentally use Unicode,
701 and who do not wish to be burdened with its complexities and security
702 concerns.
703 With "/a", one can write "\d" with confidence that it will only match
705 ASCII characters, and should the need arise to match beyond ASCII, you
706 can instead use "\p{Digit}" (or "\p{Word}" for "\w"). There are
707 similar "\p{...}" constructs that can match beyond ASCII both white
708 space (see "Whitespace" in perlrecharclass), and Posix classes (see
709 "POSIX Character Classes" in perlrecharclass). Thus, this modifier
710 doesn't mean you can't use Unicode, it means that to get Unicode
711 matching you must explicitly use a construct ("\p{}", "\P{}") that
712 signals Unicode.
713 As you would expect, this modifier causes, for example, "\D" to mean
715 the same thing as "[^0-9]"; in fact, all non-ASCII characters match
716 "\D", "\S", and "\W". "\b" still means to match at the boundary
717 between "\w" and "\W", using the "/a" definitions of them (similarly
718 for "\B").
719 Otherwise, "/a" behaves like the "/u" modifier, in that case-
721 insensitive matching uses Unicode rules; for example, "k" will match
722 the Unicode "\N{KELVIN SIGN}" under "/i" matching, and code points in
723 the Latin1 range, above ASCII will have Unicode rules when it comes to
724 case-insensitive matching.
725 To forbid ASCII/non-ASCII matches (like "k" with "\N{KELVIN SIGN}"),
727 specify the "a" twice, for example "/aai" or "/aia". (The first
728 occurrence of "a" restricts the "\d", etc., and the second occurrence
729 adds the "/i" restrictions.) But, note that code points outside the
730 ASCII range will use Unicode rules for "/i" matching, so the modifier
731 doesn't really restrict things to just ASCII; it just forbids the
732 intermixing of ASCII and non-ASCII.
733 To summarize, this modifier provides protection for applications that
735 don't wish to be exposed to all of Unicode. Specifying it twice gives
736 added protection.
737 This modifier may be specified to be the default by "use re '/a'" or
739 "use re '/aa'". If you do so, you may actually have occasion to use
740 the "/u" modifier explicitly if there are a few regular expressions
741 where you do want full Unicode rules (but even here, it's best if
742 everything were under feature "unicode_strings", along with the "use re
743 '/aa'"). Also see "Which character set modifier is in effect?".
744 Which character set modifier is in effect?
746 Which of these modifiers is in effect at any given point in a regular
748 expression depends on a fairly complex set of interactions. These have
749 been designed so that in general you don't have to worry about it, but
750 this section gives the gory details. As explained below in "Extended
751 Patterns" it is possible to explicitly specify modifiers that apply
752 only to portions of a regular expression. The innermost always has
753 priority over any outer ones, and one applying to the whole expression
754 has priority over any of the default settings that are described in the
755 remainder of this section.
756 The "use re '/foo'" pragma can be used to set default modifiers
758 (including these) for regular expressions compiled within its scope.
759 This pragma has precedence over the other pragmas listed below that
760 also change the defaults.
761 Otherwise, "use locale" sets the default modifier to "/l"; and "use
763 feature 'unicode_strings", or "use 5.012" (or higher) set the default
764 to "/u" when not in the same scope as either "use locale" or "use
765 bytes". ("use locale ':not_characters'" also sets the default to "/u",
766 overriding any plain "use locale".) Unlike the mechanisms mentioned
767 above, these affect operations besides regular expressions pattern
768 matching, and so give more consistent results with other operators,
769 including using "\U", "\l", etc. in substitution replacements.
770 If none of the above apply, for backwards compatibility reasons, the
772 "/d" modifier is the one in effect by default. As this can lead to
773 unexpected results, it is best to specify which other rule set should
774 be used.
775 Character set modifier behavior prior to Perl 5.14
777 Prior to 5.14, there were no explicit modifiers, but "/l" was implied
779 for regexes compiled within the scope of "use locale", and "/d" was
780 implied otherwise. However, interpolating a regex into a larger regex
781 would ignore the original compilation in favor of whatever was in
782 effect at the time of the second compilation. There were a number of
783 inconsistencies (bugs) with the "/d" modifier, where Unicode rules
784 would be used when inappropriate, and vice versa. "\p{}" did not imply
785 Unicode rules, and neither did all occurrences of "\N{}", until 5.12.
786 Regular Expressions
788 Quantifiers
789 Quantifiers are used when a particular portion of a pattern needs to
791 match a certain number (or numbers) of times. If there isn't a
792 quantifier the number of times to match is exactly one. The following
793 standard quantifiers are recognized:
794 * Match 0 or more times
796 + Match 1 or more times
797 ? Match 1 or 0 times
798 {n} Match exactly n times
799 {n,} Match at least n times
800 {n,m} Match at least n but not more than m times
801 (If a non-escaped curly bracket occurs in a context other than one of
803 the quantifiers listed above, where it does not form part of a
804 backslashed sequence like "\x{...}", it is either a fatal syntax error,
805 or treated as a regular character, generally with a deprecation warning
806 raised. To escape it, you can precede it with a backslash ("\{") or
807 enclose it within square brackets ("[{]"). This change will allow for
808 future syntax extensions (like making the lower bound of a quantifier
809 optional), and better error checking of quantifiers).
810 The "*" quantifier is equivalent to "{0,}", the "+" quantifier to
812 "{1,}", and the "?" quantifier to "{0,1}". n and m are limited to non-
813 negative integral values less than a preset limit defined when perl is
814 built. This is usually 32766 on the most common platforms. The actual
815 limit can be seen in the error message generated by code such as this:
816 $_ **= $_ , / {$_} / for 2 .. 42;
818 By default, a quantified subpattern is "greedy", that is, it will match
820 as many times as possible (given a particular starting location) while
821 still allowing the rest of the pattern to match. If you want it to
822 match the minimum number of times possible, follow the quantifier with
823 a "?". Note that the meanings don't change, just the "greediness":
824 *? Match 0 or more times, not greedily
826 +? Match 1 or more times, not greedily
827 ?? Match 0 or 1 time, not greedily
828 {n}? Match exactly n times, not greedily (redundant)
829 {n,}? Match at least n times, not greedily
830 {n,m}? Match at least n but not more than m times, not greedily
831 Normally when a quantified subpattern does not allow the rest of the
833 overall pattern to match, Perl will backtrack. However, this behaviour
834 is sometimes undesirable. Thus Perl provides the "possessive"
835 quantifier form as well.
836 *+ Match 0 or more times and give nothing back
838 ++ Match 1 or more times and give nothing back
839 ?+ Match 0 or 1 time and give nothing back
840 {n}+ Match exactly n times and give nothing back (redundant)
841 {n,}+ Match at least n times and give nothing back
842 {n,m}+ Match at least n but not more than m times and give nothing back
843 For instance,
845 'aaaa' =~ /a++a/
847 will never match, as the "a++" will gobble up all the "a"'s in the
849 string and won't leave any for the remaining part of the pattern. This
850 feature can be extremely useful to give perl hints about where it
851 shouldn't backtrack. For instance, the typical "match a double-quoted
852 string" problem can be most efficiently performed when written as:
853 /"(?:[^"\\]++|\\.)*+"/
855 as we know that if the final quote does not match, backtracking will
857 not help. See the independent subexpression ""(?>pattern)"" for more
858 details; possessive quantifiers are just syntactic sugar for that
859 construct. For instance the above example could also be written as
860 follows:
861 /"(?>(?:(?>[^"\\]+)|\\.)*)"/
863 Note that the possessive quantifier modifier can not be combined with
865 the non-greedy modifier. This is because it would make no sense.
866 Consider the follow equivalency table:
867 Illegal Legal
869 ------------ ------
870 X??+ X{0}
871 X+?+ X{1}
872 X{min,max}?+ X{min}
873 Escape sequences
875 Because patterns are processed as double-quoted strings, the following
877 also work:
878 \t tab (HT, TAB)
880 \n newline (LF, NL)
881 \r return (CR)
882 \f form feed (FF)
883 \a alarm (bell) (BEL)
884 \e escape (think troff) (ESC)
885 \cK control char (example: VT)
886 \x{}, \x00 character whose ordinal is the given hexadecimal number
887 \N{name} named Unicode character or character sequence
888 \N{U+263D} Unicode character (example: FIRST QUARTER MOON)
889 \o{}, \000 character whose ordinal is the given octal number
890 \l lowercase next char (think vi)
891 \u uppercase next char (think vi)
892 \L lowercase until \E (think vi)
893 \U uppercase until \E (think vi)
894 \Q quote (disable) pattern metacharacters until \E
895 \E end either case modification or quoted section, think vi
896 Details are in "Quote and Quote-like Operators" in perlop.
898 Character Classes and other Special Escapes
900 In addition, Perl defines the following:
902 Sequence Note Description
904 [...] [1] Match a character according to the rules of the
905 bracketed character class defined by the "...".
906 Example: [a-z] matches "a" or "b" or "c" ... or "z"
907 [[:...:]] [2] Match a character according to the rules of the POSIX
908 character class "..." within the outer bracketed
909 character class. Example: [[:upper:]] matches any
910 uppercase character.
911 (?[...]) [8] Extended bracketed character class
912 \w [3] Match a "word" character (alphanumeric plus "_", plus
913 other connector punctuation chars plus Unicode
914 marks)
915 \W [3] Match a non-"word" character
916 \s [3] Match a whitespace character
917 \S [3] Match a non-whitespace character
918 \d [3] Match a decimal digit character
919 \D [3] Match a non-digit character
920 \pP [3] Match P, named property. Use \p{Prop} for longer names
921 \PP [3] Match non-P
922 \X [4] Match Unicode "eXtended grapheme cluster"
923 \1 [5] Backreference to a specific capture group or buffer.
924 '1' may actually be any positive integer.
925 \g1 [5] Backreference to a specific or previous group,
926 \g{-1} [5] The number may be negative indicating a relative
927 previous group and may optionally be wrapped in
928 curly brackets for safer parsing.
929 \g{name} [5] Named backreference
930 \k<name> [5] Named backreference
931 \K [6] Keep the stuff left of the \K, don't include it in $&
932 \N [7] Any character but \n. Not affected by /s modifier
933 \v [3] Vertical whitespace
934 \V [3] Not vertical whitespace
935 \h [3] Horizontal whitespace
936 \H [3] Not horizontal whitespace
937 \R [4] Linebreak
938 [1] See "Bracketed Character Classes" in perlrecharclass for details.
940 [2] See "POSIX Character Classes" in perlrecharclass for details.
942 [3] See "Unicode Character Properties" in perlunicode for details
944 [4] See "Misc" in perlrebackslash for details.
946 [5] See "Capture groups" below for details.
948 [6] See "Extended Patterns" below for details.
950 [7] Note that "\N" has two meanings. When of the form "\N{NAME}", it
952 matches the character or character sequence whose name is NAME; and
953 similarly when of the form "\N{U+hex}", it matches the character
954 whose Unicode code point is hex. Otherwise it matches any
955 character but "\n".
956 [8] See "Extended Bracketed Character Classes" in perlrecharclass for
958 details.
959 Assertions
961 Besides "^" and "$", Perl defines the following zero-width assertions:
963 \b{} Match at Unicode boundary of specified type
965 \B{} Match where corresponding \b{} doesn't match
966 \b Match a \w\W or \W\w boundary
967 \B Match except at a \w\W or \W\w boundary
968 \A Match only at beginning of string
969 \Z Match only at end of string, or before newline at the end
970 \z Match only at end of string
971 \G Match only at pos() (e.g. at the end-of-match position
972 of prior m//g)
973 A Unicode boundary ("\b{}"), available starting in v5.22, is a spot
975 between two characters, or before the first character in the string, or
976 after the final character in the string where certain criteria defined
977 by Unicode are met. See "\b{}, \b, \B{}, \B" in perlrebackslash for
978 details.
979 A word boundary ("\b") is a spot between two characters that has a "\w"
981 on one side of it and a "\W" on the other side of it (in either order),
982 counting the imaginary characters off the beginning and end of the
983 string as matching a "\W". (Within character classes "\b" represents
984 backspace rather than a word boundary, just as it normally does in any
985 double-quoted string.) The "\A" and "\Z" are just like "^" and "$",
986 except that they won't match multiple times when the "/m" modifier is
987 used, while "^" and "$" will match at every internal line boundary. To
988 match the actual end of the string and not ignore an optional trailing
989 newline, use "\z".
990 The "\G" assertion can be used to chain global matches (using "m//g"),
992 as described in "Regexp Quote-Like Operators" in perlop. It is also
993 useful when writing "lex"-like scanners, when you have several patterns
994 that you want to match against consequent substrings of your string;
995 see the previous reference. The actual location where "\G" will match
996 can also be influenced by using "pos()" as an lvalue: see "pos" in
997 perlfunc. Note that the rule for zero-length matches (see "Repeated
998 Patterns Matching a Zero-length Substring") is modified somewhat, in
999 that contents to the left of "\G" are not counted when determining the
1000 length of the match. Thus the following will not match forever:
1001 my $string = 'ABC';
1003 pos($string) = 1;
1004 while ($string =~ /(.\G)/g) {
1005 print $1;
1006 }
1007 It will print 'A' and then terminate, as it considers the match to be
1009 zero-width, and thus will not match at the same position twice in a
1010 row.
1011 It is worth noting that "\G" improperly used can result in an infinite
1013 loop. Take care when using patterns that include "\G" in an
1014 alternation.
1015 Note also that "s///" will refuse to overwrite part of a substitution
1017 that has already been replaced; so for example this will stop after the
1018 first iteration, rather than iterating its way backwards through the
1019 string:
1020 $_ = "123456789";
1022 pos = 6;
1023 s/.(?=.\G)/X/g;
1024 print; # prints 1234X6789, not XXXXX6789
1025 Capture groups
1027 The grouping construct "( ... )" creates capture groups (also referred
1029 to as capture buffers). To refer to the current contents of a group
1030 later on, within the same pattern, use "\g1" (or "\g{1}") for the
1031 first, "\g2" (or "\g{2}") for the second, and so on. This is called a
1032 backreference.
1033 There is no limit to the number of captured substrings that you may
1042 use. Groups are numbered with the leftmost open parenthesis being
1043 number 1, etc. If a group did not match, the associated backreference
1044 won't match either. (This can happen if the group is optional, or in a
1045 different branch of an alternation.) You can omit the "g", and write
1046 "\1", etc, but there are some issues with this form, described below.
1047 You can also refer to capture groups relatively, by using a negative
1049 number, so that "\g-1" and "\g{-1}" both refer to the immediately
1050 preceding capture group, and "\g-2" and "\g{-2}" both refer to the
1051 group before it. For example:
1052 /
1054 (Y) # group 1
1055 ( # group 2
1056 (X) # group 3
1057 \g{-1} # backref to group 3
1058 \g{-3} # backref to group 1
1059 )
1060 /x
1061 would match the same as "/(Y) ( (X) \g3 \g1 )/x". This allows you to
1063 interpolate regexes into larger regexes and not have to worry about the
1064 capture groups being renumbered.
1065 You can dispense with numbers altogether and create named capture
1067 groups. The notation is "(?<name>...)" to declare and "\g{name}" to
1068 reference. (To be compatible with .Net regular expressions, "\g{name}"
1069 may also be written as "\k{name}", "\k<name>" or "\k'name'".) name
1070 must not begin with a number, nor contain hyphens. When different
1071 groups within the same pattern have the same name, any reference to
1072 that name assumes the leftmost defined group. Named groups count in
1073 absolute and relative numbering, and so can also be referred to by
1074 those numbers. (It's possible to do things with named capture groups
1075 that would otherwise require "(??{})".)
1076 Capture group contents are dynamically scoped and available to you
1078 outside the pattern until the end of the enclosing block or until the
1079 next successful match, whichever comes first. (See "Compound
1080 Statements" in perlsyn.) You can refer to them by absolute number
1081 (using "$1" instead of "\g1", etc); or by name via the "%+" hash, using
1082 "$+{name}".
1083 Braces are required in referring to named capture groups, but are
1085 optional for absolute or relative numbered ones. Braces are safer when
1086 creating a regex by concatenating smaller strings. For example if you
1087 have "qr/$a$b/", and $a contained "\g1", and $b contained "37", you
1088 would get "/\g137/" which is probably not what you intended.
1089 The "\g" and "\k" notations were introduced in Perl 5.10.0. Prior to
1091 that there were no named nor relative numbered capture groups.
1092 Absolute numbered groups were referred to using "\1", "\2", etc., and
1093 this notation is still accepted (and likely always will be). But it
1094 leads to some ambiguities if there are more than 9 capture groups, as
1095 "\10" could mean either the tenth capture group, or the character whose
1096 ordinal in octal is 010 (a backspace in ASCII). Perl resolves this
1097 ambiguity by interpreting "\10" as a backreference only if at least 10
1098 left parentheses have opened before it. Likewise "\11" is a
1099 backreference only if at least 11 left parentheses have opened before
1100 it. And so on. "\1" through "\9" are always interpreted as
1101 backreferences. There are several examples below that illustrate these
1102 perils. You can avoid the ambiguity by always using "\g{}" or "\g" if
1103 you mean capturing groups; and for octal constants always using "\o{}",
1104 or for "\077" and below, using 3 digits padded with leading zeros,
1105 since a leading zero implies an octal constant.
1106 The "\digit" notation also works in certain circumstances outside the
1108 pattern. See "Warning on \1 Instead of $1" below for details.
1109 Examples:
1111 s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words
1113 /(.)\g1/ # find first doubled char
1115 and print "'$1' is the first doubled character\n";
1116 /(?<char>.)\k<char>/ # ... a different way
1118 and print "'$+{char}' is the first doubled character\n";
1119 /(?'char'.)\g1/ # ... mix and match
1121 and print "'$1' is the first doubled character\n";
1122 if (/Time: (..):(..):(..)/) { # parse out values
1124 $hours = $1;
1125 $minutes = $2;
1126 $seconds = $3;
1127 }
1128 /(.)(.)(.)(.)(.)(.)(.)(.)(.)\g10/ # \g10 is a backreference
1130 /(.)(.)(.)(.)(.)(.)(.)(.)(.)\10/ # \10 is octal
1131 /((.)(.)(.)(.)(.)(.)(.)(.)(.))\10/ # \10 is a backreference
1132 /((.)(.)(.)(.)(.)(.)(.)(.)(.))\010/ # \010 is octal
1133 $a = '(.)\1'; # Creates problems when concatenated.
1135 $b = '(.)\g{1}'; # Avoids the problems.
1136 "aa" =~ /${a}/; # True
1137 "aa" =~ /${b}/; # True
1138 "aa0" =~ /${a}0/; # False!
1139 "aa0" =~ /${b}0/; # True
1140 "aa\x08" =~ /${a}0/; # True!
1141 "aa\x08" =~ /${b}0/; # False
1142 Several special variables also refer back to portions of the previous
1144 match. $+ returns whatever the last bracket match matched. $& returns
1145 the entire matched string. (At one point $0 did also, but now it
1146 returns the name of the program.) "$`" returns everything before the
1147 matched string. "$'" returns everything after the matched string. And
1148 $^N contains whatever was matched by the most-recently closed group
1149 (submatch). $^N can be used in extended patterns (see below), for
1150 example to assign a submatch to a variable.
1151 These special variables, like the "%+" hash and the numbered match
1153 variables ($1, $2, $3, etc.) are dynamically scoped until the end of
1154 the enclosing block or until the next successful match, whichever comes
1155 first. (See "Compound Statements" in perlsyn.)
1156 NOTE: Failed matches in Perl do not reset the match variables, which
1158 makes it easier to write code that tests for a series of more specific
1159 cases and remembers the best match.
1160 WARNING: If your code is to run on Perl 5.16 or earlier, beware that
1162 once Perl sees that you need one of $&, "$`", or "$'" anywhere in the
1163 program, it has to provide them for every pattern match. This may
1164 substantially slow your program.
1165 Perl uses the same mechanism to produce $1, $2, etc, so you also pay a
1167 price for each pattern that contains capturing parentheses. (To avoid
1168 this cost while retaining the grouping behaviour, use the extended
1169 regular expression "(?: ... )" instead.) But if you never use $&, "$`"
1170 or "$'", then patterns without capturing parentheses will not be
1171 penalized. So avoid $&, "$'", and "$`" if you can, but if you can't
1172 (and some algorithms really appreciate them), once you've used them
1173 once, use them at will, because you've already paid the price.
1174 Perl 5.16 introduced a slightly more efficient mechanism that notes
1176 separately whether each of "$`", $&, and "$'" have been seen, and thus
1177 may only need to copy part of the string. Perl 5.20 introduced a much
1178 more efficient copy-on-write mechanism which eliminates any slowdown.
1179 As another workaround for this problem, Perl 5.10.0 introduced
1181 "${^PREMATCH}", "${^MATCH}" and "${^POSTMATCH}", which are equivalent
1182 to "$`", $& and "$'", except that they are only guaranteed to be
1183 defined after a successful match that was executed with the "/p"
1184 (preserve) modifier. The use of these variables incurs no global
1185 performance penalty, unlike their punctuation character equivalents,
1186 however at the trade-off that you have to tell perl when you want to
1187 use them. As of Perl 5.20, these three variables are equivalent to
1188 "$`", $& and "$'", and "/p" is ignored.
1189 Quoting metacharacters
1191 Backslashed metacharacters in Perl are alphanumeric, such as "\b",
1192 "\w", "\n". Unlike some other regular expression languages, there are
1193 no backslashed symbols that aren't alphanumeric. So anything that
1194 looks like "\\", "\(", "\)", "\[", "\]", "\{", or "\}" is always
1195 interpreted as a literal character, not a metacharacter. This was once
1196 used in a common idiom to disable or quote the special meanings of
1197 regular expression metacharacters in a string that you want to use for
1198 a pattern. Simply quote all non-"word" characters:
1199 $pattern =~ s/(\W)/\\$1/g;
1201 (If "use locale" is set, then this depends on the current locale.)
1203 Today it is more common to use the "quotemeta()" function or the "\Q"
1204 metaquoting escape sequence to disable all metacharacters' special
1205 meanings like this:
1206 /$unquoted\Q$quoted\E$unquoted/
1208 Beware that if you put literal backslashes (those not inside
1210 interpolated variables) between "\Q" and "\E", double-quotish backslash
1211 interpolation may lead to confusing results. If you need to use
1212 literal backslashes within "\Q...\E", consult "Gory details of parsing
1213 quoted constructs" in perlop.
1214 "quotemeta()" and "\Q" are fully described in "quotemeta" in perlfunc.
1216 Extended Patterns
1218 Perl also defines a consistent extension syntax for features not found
1219 in standard tools like awk and lex. The syntax for most of these is a
1220 pair of parentheses with a question mark as the first thing within the
1221 parentheses. The character after the question mark indicates the
1222 extension.
1223 A question mark was chosen for this and for the minimal-matching
1225 construct because 1) question marks are rare in older regular
1226 expressions, and 2) whenever you see one, you should stop and
1227 "question" exactly what is going on. That's psychology....
1228 "(?#text)"
1230 A comment. The text is ignored. Note that Perl closes the comment
1231 as soon as it sees a ")", so there is no way to put a literal ")"
1232 in the comment. The pattern's closing delimiter must be escaped by
1233 a backslash if it appears in the comment.
1234 See "/x" for another way to have comments in patterns.
1236 Note that a comment can go just about anywhere, except in the
1238 middle of an escape sequence. Examples:
1239 qr/foo(?#comment)bar/' # Matches 'foobar'
1241 # The pattern below matches 'abcd', 'abccd', or 'abcccd'
1243 qr/abc(?#comment between literal and its quantifier){1,3}d/
1244 # The pattern below generates a syntax error, because the '\p' must
1246 # be followed immediately by a '{'.
1247 qr/\p(?#comment between \p and its property name){Any}/
1248 # The pattern below generates a syntax error, because the initial
1250 # '\(' is a literal opening parenthesis, and so there is nothing
1251 # for the closing ')' to match
1252 qr/\(?#the backslash means this isn't a comment)p{Any}/
1253 # Comments can be used to fold long patterns into multiple lines
1255 qr/First part of a long regex(?#
1256 )remaining part/
1257 "(?adlupimnsx-imnsx)"
1259 "(?^alupimnsx)"
1260 Zero or more embedded pattern-match modifiers, to be turned on (or
1261 turned off if preceded by "-") for the remainder of the pattern or
1262 the remainder of the enclosing pattern group (if any).
1263 This is particularly useful for dynamically-generated patterns,
1265 such as those read in from a configuration file, taken from an
1266 argument, or specified in a table somewhere. Consider the case
1267 where some patterns want to be case-sensitive and some do not: The
1268 case-insensitive ones merely need to include "(?i)" at the front of
1269 the pattern. For example:
1270 $pattern = "foobar";
1272 if ( /$pattern/i ) { }
1273 # more flexible:
1275 $pattern = "(?i)foobar";
1277 if ( /$pattern/ ) { }
1278 These modifiers are restored at the end of the enclosing group. For
1280 example,
1281 ( (?i) blah ) \s+ \g1
1283 will match "blah" in any case, some spaces, and an exact (including
1285 the case!) repetition of the previous word, assuming the "/x"
1286 modifier, and no "/i" modifier outside this group.
1287 These modifiers do not carry over into named subpatterns called in
1289 the enclosing group. In other words, a pattern such as
1290 "((?i)(?&NAME))" does not change the case-sensitivity of the NAME
1291 pattern.
1292 A modifier is overridden by later occurrences of this construct in
1294 the same scope containing the same modifier, so that
1295 /((?im)foo(?-m)bar)/
1297 matches all of "foobar" case insensitively, but uses "/m" rules for
1299 only the "foo" portion. The "a" flag overrides "aa" as well;
1300 likewise "aa" overrides "a". The same goes for "x" and "xx".
1301 Hence, in
1302 /(?-x)foo/xx
1304 both "/x" and "/xx" are turned off during matching "foo". And in
1306 /(?x)foo/x
1308 "/x" but NOT "/xx" is turned on for matching "foo". (One might
1310 mistakenly think that since the inner "(?x)" is already in the
1311 scope of "/x", that the result would effectively be the sum of
1312 them, yielding "/xx". It doesn't work that way.) Similarly, doing
1313 something like "(?xx-x)foo" turns off all "x" behavior for matching
1314 "foo", it is not that you subtract 1 "x" from 2 to get 1 "x"
1315 remaining.
1316 Any of these modifiers can be set to apply globally to all regular
1318 expressions compiled within the scope of a "use re". See "'/flags'
1319 mode" in re.
1320 Starting in Perl 5.14, a "^" (caret or circumflex accent)
1322 immediately after the "?" is a shorthand equivalent to "d-imnsx".
1323 Flags (except "d") may follow the caret to override it. But a
1324 minus sign is not legal with it.
1325 Note that the "a", "d", "l", "p", and "u" modifiers are special in
1327 that they can only be enabled, not disabled, and the "a", "d", "l",
1328 and "u" modifiers are mutually exclusive: specifying one de-
1329 specifies the others, and a maximum of one (or two "a"'s) may
1330 appear in the construct. Thus, for example, "(?-p)" will warn when
1331 compiled under "use warnings"; "(?-d:...)" and "(?dl:...)" are
1332 fatal errors.
1333 Note also that the "p" modifier is special in that its presence
1335 anywhere in a pattern has a global effect.
1336 Having zero modifiers makes this a no-op (so why did you specify
1338 it, unless it's generated code), and starting in v5.30, warns under
1339 "use re 'strict'".
1340 "(?:pattern)"
1342 "(?adluimnsx-imnsx:pattern)"
1343 "(?^aluimnsx:pattern)"
1344 This is for clustering, not capturing; it groups subexpressions
1345 like "()", but doesn't make backreferences as "()" does. So
1346 @fields = split(/\b(?:a|b|c)\b/)
1348 matches the same field delimiters as
1350 @fields = split(/\b(a|b|c)\b/)
1352 but doesn't spit out the delimiters themselves as extra fields
1354 (even though that's the behaviour of "split" in perlfunc when its
1355 pattern contains capturing groups). It's also cheaper not to
1356 capture characters if you don't need to.
1357 Any letters between "?" and ":" act as flags modifiers as with
1359 "(?adluimnsx-imnsx)". For example,
1360 /(?s-i:more.*than).*million/i
1362 is equivalent to the more verbose
1364 /(?:(?s-i)more.*than).*million/i
1366 Note that any "()" constructs enclosed within this one will still
1368 capture unless the "/n" modifier is in effect.
1369 Like the "(?adlupimnsx-imnsx)" construct, "aa" and "a" override
1371 each other, as do "xx" and "x". They are not additive. So, doing
1372 something like "(?xx-x:foo)" turns off all "x" behavior for
1373 matching "foo".
1374 Starting in Perl 5.14, a "^" (caret or circumflex accent)
1376 immediately after the "?" is a shorthand equivalent to "d-imnsx".
1377 Any positive flags (except "d") may follow the caret, so
1378 (?^x:foo)
1380 is equivalent to
1382 (?x-imns:foo)
1384 The caret tells Perl that this cluster doesn't inherit the flags of
1386 any surrounding pattern, but uses the system defaults ("d-imnsx"),
1387 modified by any flags specified.
1388 The caret allows for simpler stringification of compiled regular
1390 expressions. These look like
1391 (?^:pattern)
1393 with any non-default flags appearing between the caret and the
1395 colon. A test that looks at such stringification thus doesn't need
1396 to have the system default flags hard-coded in it, just the caret.
1397 If new flags are added to Perl, the meaning of the caret's
1398 expansion will change to include the default for those flags, so
1399 the test will still work, unchanged.
1400 Specifying a negative flag after the caret is an error, as the flag
1402 is redundant.
1403 Mnemonic for "(?^...)": A fresh beginning since the usual use of a
1405 caret is to match at the beginning.
1406 "(?|pattern)"
1408 This is the "branch reset" pattern, which has the special property
1409 that the capture groups are numbered from the same starting point
1410 in each alternation branch. It is available starting from perl
1411 5.10.0.
1412 Capture groups are numbered from left to right, but inside this
1414 construct the numbering is restarted for each branch.
1415 The numbering within each branch will be as normal, and any groups
1417 following this construct will be numbered as though the construct
1418 contained only one branch, that being the one with the most capture
1419 groups in it.
1420 This construct is useful when you want to capture one of a number
1422 of alternative matches.
1423 Consider the following pattern. The numbers underneath show in
1425 which group the captured content will be stored.
1426 # before ---------------branch-reset----------- after
1428 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1429 # 1 2 2 3 2 3 4
1430 Be careful when using the branch reset pattern in combination with
1432 named captures. Named captures are implemented as being aliases to
1433 numbered groups holding the captures, and that interferes with the
1434 implementation of the branch reset pattern. If you are using named
1435 captures in a branch reset pattern, it's best to use the same
1436 names, in the same order, in each of the alternations:
1437 /(?| (?<a> x ) (?<b> y )
1439 | (?<a> z ) (?<b> w )) /x
1440 Not doing so may lead to surprises:
1442 "12" =~ /(?| (?<a> \d+ ) | (?<b> \D+))/x;
1444 say $+{a}; # Prints '12'
1445 say $+{b}; # *Also* prints '12'.
1446 The problem here is that both the group named "a" and the group
1448 named "b" are aliases for the group belonging to $1.
1449 Lookaround Assertions
1451 Lookaround assertions are zero-width patterns which match a
1452 specific pattern without including it in $&. Positive assertions
1453 match when their subpattern matches, negative assertions match when
1454 their subpattern fails. Lookbehind matches text up to the current
1455 match position, lookahead matches text following the current match
1456 position.
1457 "(?=pattern)"
1459 "(*pla:pattern)"
1460 "(*positive_lookahead:pattern)"
1461 A zero-width positive lookahead assertion. For example,
1462 "/\w+(?=\t)/" matches a word followed by a tab, without
1463 including the tab in $&.
1464 "(?!pattern)"
1466 "(*nla:pattern)"
1467 "(*negative_lookahead:pattern)"
1468 A zero-width negative lookahead assertion. For example
1469 "/foo(?!bar)/" matches any occurrence of "foo" that isn't
1470 followed by "bar". Note however that lookahead and lookbehind
1471 are NOT the same thing. You cannot use this for lookbehind.
1472 If you are looking for a "bar" that isn't preceded by a "foo",
1474 "/(?!foo)bar/" will not do what you want. That's because the
1475 "(?!foo)" is just saying that the next thing cannot be
1476 "foo"--and it's not, it's a "bar", so "foobar" will match. Use
1477 lookbehind instead (see below).
1478 "(?<=pattern)"
1480 "\K"
1481 "(*plb:pattern)"
1482 "(*positive_lookbehind:pattern)"
1483 A zero-width positive lookbehind assertion. For example,
1484 "/(?<=\t)\w+/" matches a word that follows a tab, without
1485 including the tab in $&.
1486 Prior to Perl 5.30, it worked only for fixed-width lookbehind,
1488 but starting in that release, it can handle variable lengths
1489 from 1 to 255 characters as an experimental feature. The
1490 feature is enabled automatically if you use a variable length
1491 lookbehind assertion, but will raise a warning at pattern
1492 compilation time, unless turned off, in the "experimental::vlb"
1493 category. This is to warn you that the exact behavior is
1494 subject to change should feedback from actual use in the field
1495 indicate to do so; or even complete removal if the problems
1496 found are not practically surmountable. You can achieve close
1497 to pre-5.30 behavior by fatalizing warnings in this category.
1498 There is a special form of this construct, called "\K"
1500 (available since Perl 5.10.0), which causes the regex engine to
1501 "keep" everything it had matched prior to the "\K" and not
1502 include it in $&. This effectively provides non-experimental
1503 variable-length lookbehind of any length.
1504 And, there is a technique that can be used to handle variable
1506 length lookbehinds on earlier releases, and longer than 255
1507 characters. It is described in
1508 <http://www.drregex.com/2019/02/variable-length-lookbehinds-actually.html>.
1509 Note that under "/i", a few single characters match two or
1511 three other characters. This makes them variable length, and
1512 the 255 length applies to the maximum number of characters in
1513 the match. For example "qr/\N{LATIN SMALL LETTER SHARP S}/i"
1514 matches the sequence "ss". Your lookbehind assertion could
1515 contain 127 Sharp S characters under "/i", but adding a 128th
1516 would generate a compilation error, as that could match 256 "s"
1517 characters in a row.
1518 The use of "\K" inside of another lookaround assertion is
1520 allowed, but the behaviour is currently not well defined.
1521 For various reasons "\K" may be significantly more efficient
1523 than the equivalent "(?<=...)" construct, and it is especially
1524 useful in situations where you want to efficiently remove
1525 something following something else in a string. For instance
1526 s/(foo)bar/$1/g;
1528 can be rewritten as the much more efficient
1530 s/foo\Kbar//g;
1532 Use of the non-greedy modifier "?" may not give you the
1534 expected results if it is within a capturing group within the
1535 construct.
1536 "(?<!pattern)"
1538 "(*nlb:pattern)"
1539 "(*negative_lookbehind:pattern)"
1540 A zero-width negative lookbehind assertion. For example
1541 "/(?<!bar)foo/" matches any occurrence of "foo" that does not
1542 follow "bar".
1543 Prior to Perl 5.30, it worked only for fixed-width lookbehind,
1545 but starting in that release, it can handle variable lengths
1546 from 1 to 255 characters as an experimental feature. The
1547 feature is enabled automatically if you use a variable length
1548 lookbehind assertion, but will raise a warning at pattern
1549 compilation time, unless turned off, in the "experimental::vlb"
1550 category. This is to warn you that the exact behavior is
1551 subject to change should feedback from actual use in the field
1552 indicate to do so; or even complete removal if the problems
1553 found are not practically surmountable. You can achieve close
1554 to pre-5.30 behavior by fatalizing warnings in this category.
1555 There is a technique that can be used to handle variable length
1557 lookbehinds on earlier releases, and longer than 255
1558 characters. It is described in
1559 <http://www.drregex.com/2019/02/variable-length-lookbehinds-actually.html>.
1560 Note that under "/i", a few single characters match two or
1562 three other characters. This makes them variable length, and
1563 the 255 length applies to the maximum number of characters in
1564 the match. For example "qr/\N{LATIN SMALL LETTER SHARP S}/i"
1565 matches the sequence "ss". Your lookbehind assertion could
1566 contain 127 Sharp S characters under "/i", but adding a 128th
1567 would generate a compilation error, as that could match 256 "s"
1568 characters in a row.
1569 Use of the non-greedy modifier "?" may not give you the
1571 expected results if it is within a capturing group within the
1572 construct.
1573 "(?<NAME>pattern)"
1575 "(?'NAME'pattern)"
1576 A named capture group. Identical in every respect to normal
1577 capturing parentheses "()" but for the additional fact that the
1578 group can be referred to by name in various regular expression
1579 constructs (like "\g{NAME}") and can be accessed by name after a
1580 successful match via "%+" or "%-". See perlvar for more details on
1581 the "%+" and "%-" hashes.
1582 If multiple distinct capture groups have the same name, then
1584 $+{NAME} will refer to the leftmost defined group in the match.
1585 The forms "(?'NAME'pattern)" and "(?<NAME>pattern)" are equivalent.
1587 NOTE: While the notation of this construct is the same as the
1589 similar function in .NET regexes, the behavior is not. In Perl the
1590 groups are numbered sequentially regardless of being named or not.
1591 Thus in the pattern
1592 /(x)(?<foo>y)(z)/
1594 $+{foo} will be the same as $2, and $3 will contain 'z' instead of
1596 the opposite which is what a .NET regex hacker might expect.
1597 Currently NAME is restricted to simple identifiers only. In other
1599 words, it must match "/^[_A-Za-z][_A-Za-z0-9]*\z/" or its Unicode
1600 extension (see utf8), though it isn't extended by the locale (see
1601 perllocale).
1602 NOTE: In order to make things easier for programmers with
1604 experience with the Python or PCRE regex engines, the pattern
1605 "(?P<NAME>pattern)" may be used instead of "(?<NAME>pattern)";
1606 however this form does not support the use of single quotes as a
1607 delimiter for the name.
1608 "\k<NAME>"
1610 "\k'NAME'"
1611 Named backreference. Similar to numeric backreferences, except that
1612 the group is designated by name and not number. If multiple groups
1613 have the same name then it refers to the leftmost defined group in
1614 the current match.
1615 It is an error to refer to a name not defined by a "(?<NAME>)"
1617 earlier in the pattern.
1618 Both forms are equivalent.
1620 NOTE: In order to make things easier for programmers with
1622 experience with the Python or PCRE regex engines, the pattern
1623 "(?P=NAME)" may be used instead of "\k<NAME>".
1624 "(?{ code })"
1626 WARNING: Using this feature safely requires that you understand its
1627 limitations. Code executed that has side effects may not perform
1628 identically from version to version due to the effect of future
1629 optimisations in the regex engine. For more information on this,
1630 see "Embedded Code Execution Frequency".
1631 This zero-width assertion executes any embedded Perl code. It
1633 always succeeds, and its return value is set as $^R.
1634 In literal patterns, the code is parsed at the same time as the
1636 surrounding code. While within the pattern, control is passed
1637 temporarily back to the perl parser, until the logically-balancing
1638 closing brace is encountered. This is similar to the way that an
1639 array index expression in a literal string is handled, for example
1640 "abc$array[ 1 + f('[') + g()]def"
1642 In particular, braces do not need to be balanced:
1644 s/abc(?{ f('{'); })/def/
1646 Even in a pattern that is interpolated and compiled at run-time,
1648 literal code blocks will be compiled once, at perl compile time;
1649 the following prints "ABCD":
1650 print "D";
1652 my $qr = qr/(?{ BEGIN { print "A" } })/;
1653 my $foo = "foo";
1654 /$foo$qr(?{ BEGIN { print "B" } })/;
1655 BEGIN { print "C" }
1656 In patterns where the text of the code is derived from run-time
1658 information rather than appearing literally in a source code
1659 /pattern/, the code is compiled at the same time that the pattern
1660 is compiled, and for reasons of security, "use re 'eval'" must be
1661 in scope. This is to stop user-supplied patterns containing code
1662 snippets from being executable.
1663 In situations where you need to enable this with "use re 'eval'",
1665 you should also have taint checking enabled. Better yet, use the
1666 carefully constrained evaluation within a Safe compartment. See
1667 perlsec for details about both these mechanisms.
1668 From the viewpoint of parsing, lexical variable scope and closures,
1670 /AAA(?{ BBB })CCC/
1672 behaves approximately like
1674 /AAA/ && do { BBB } && /CCC/
1676 Similarly,
1678 qr/AAA(?{ BBB })CCC/
1680 behaves approximately like
1682 sub { /AAA/ && do { BBB } && /CCC/ }
1684 In particular:
1686 { my $i = 1; $r = qr/(?{ print $i })/ }
1688 my $i = 2;
1689 /$r/; # prints "1"
1690 Inside a "(?{...})" block, $_ refers to the string the regular
1692 expression is matching against. You can also use "pos()" to know
1693 what is the current position of matching within this string.
1694 The code block introduces a new scope from the perspective of
1696 lexical variable declarations, but not from the perspective of
1697 "local" and similar localizing behaviours. So later code blocks
1698 within the same pattern will still see the values which were
1699 localized in earlier blocks. These accumulated localizations are
1700 undone either at the end of a successful match, or if the assertion
1701 is backtracked (compare "Backtracking"). For example,
1702 $_ = 'a' x 8;
1704 m<
1705 (?{ $cnt = 0 }) # Initialize $cnt.
1706 (
1707 a
1708 (?{
1709 local $cnt = $cnt + 1; # Update $cnt,
1710 # backtracking-safe.
1711 })
1712 )*
1713 aaaa
1714 (?{ $res = $cnt }) # On success copy to
1715 # non-localized location.
1716 >x;
1717 will initially increment $cnt up to 8; then during backtracking,
1719 its value will be unwound back to 4, which is the value assigned to
1720 $res. At the end of the regex execution, $cnt will be wound back
1721 to its initial value of 0.
1722 This assertion may be used as the condition in a
1724 (?(condition)yes-pattern|no-pattern)
1726 switch. If not used in this way, the result of evaluation of code
1728 is put into the special variable $^R. This happens immediately, so
1729 $^R can be used from other "(?{ code })" assertions inside the same
1730 regular expression.
1731 The assignment to $^R above is properly localized, so the old value
1733 of $^R is restored if the assertion is backtracked; compare
1734 "Backtracking".
1735 Note that the special variable $^N is particularly useful with
1737 code blocks to capture the results of submatches in variables
1738 without having to keep track of the number of nested parentheses.
1739 For example:
1740 $_ = "The brown fox jumps over the lazy dog";
1742 /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i;
1743 print "color = $color, animal = $animal\n";
1744 "(??{ code })"
1746 WARNING: Using this feature safely requires that you understand its
1747 limitations. Code executed that has side effects may not perform
1748 identically from version to version due to the effect of future
1749 optimisations in the regex engine. For more information on this,
1750 see "Embedded Code Execution Frequency".
1751 This is a "postponed" regular subexpression. It behaves in exactly
1753 the same way as a "(?{ code })" code block as described above,
1754 except that its return value, rather than being assigned to $^R, is
1755 treated as a pattern, compiled if it's a string (or used as-is if
1756 its a qr// object), then matched as if it were inserted instead of
1757 this construct.
1758 During the matching of this sub-pattern, it has its own set of
1760 captures which are valid during the sub-match, but are discarded
1761 once control returns to the main pattern. For example, the
1762 following matches, with the inner pattern capturing "B" and
1763 matching "BB", while the outer pattern captures "A";
1764 my $inner = '(.)\1';
1766 "ABBA" =~ /^(.)(??{ $inner })\1/;
1767 print $1; # prints "A";
1768 Note that this means that there is no way for the inner pattern to
1770 refer to a capture group defined outside. (The code block itself
1771 can use $1, etc., to refer to the enclosing pattern's capture
1772 groups.) Thus, although
1773 ('a' x 100)=~/(??{'(.)' x 100})/
1775 will match, it will not set $1 on exit.
1777 The following pattern matches a parenthesized group:
1779 $re = qr{
1781 \(
1782 (?:
1783 (?> [^()]+ ) # Non-parens without backtracking
1784 |
1785 (??{ $re }) # Group with matching parens
1786 )*
1787 \)
1788 }x;
1789 See also "(?PARNO)" for a different, more efficient way to
1791 accomplish the same task.
1792 Executing a postponed regular expression too many times without
1794 consuming any input string will also result in a fatal error. The
1795 depth at which that happens is compiled into perl, so it can be
1796 changed with a custom build.
1797 "(?PARNO)" "(?-PARNO)" "(?+PARNO)" "(?R)" "(?0)"
1799 Recursive subpattern. Treat the contents of a given capture buffer
1800 in the current pattern as an independent subpattern and attempt to
1801 match it at the current position in the string. Information about
1802 capture state from the caller for things like backreferences is
1803 available to the subpattern, but capture buffers set by the
1804 subpattern are not visible to the caller.
1805 Similar to "(??{ code })" except that it does not involve executing
1807 any code or potentially compiling a returned pattern string;
1808 instead it treats the part of the current pattern contained within
1809 a specified capture group as an independent pattern that must match
1810 at the current position. Also different is the treatment of capture
1811 buffers, unlike "(??{ code })" recursive patterns have access to
1812 their caller's match state, so one can use backreferences safely.
1813 PARNO is a sequence of digits (not starting with 0) whose value
1815 reflects the paren-number of the capture group to recurse to.
1816 "(?R)" recurses to the beginning of the whole pattern. "(?0)" is an
1817 alternate syntax for "(?R)". If PARNO is preceded by a plus or
1818 minus sign then it is assumed to be relative, with negative numbers
1819 indicating preceding capture groups and positive ones following.
1820 Thus "(?-1)" refers to the most recently declared group, and
1821 "(?+1)" indicates the next group to be declared. Note that the
1822 counting for relative recursion differs from that of relative
1823 backreferences, in that with recursion unclosed groups are
1824 included.
1825 The following pattern matches a function "foo()" which may contain
1827 balanced parentheses as the argument.
1828 $re = qr{ ( # paren group 1 (full function)
1830 foo
1831 ( # paren group 2 (parens)
1832 \(
1833 ( # paren group 3 (contents of parens)
1834 (?:
1835 (?> [^()]+ ) # Non-parens without backtracking
1836 |
1837 (?2) # Recurse to start of paren group 2
1838 )*
1839 )
1840 \)
1841 )
1842 )
1843 }x;
1844 If the pattern was used as follows
1846 'foo(bar(baz)+baz(bop))'=~/$re/
1848 and print "\$1 = $1\n",
1849 "\$2 = $2\n",
1850 "\$3 = $3\n";
1851 the output produced should be the following:
1853 $1 = foo(bar(baz)+baz(bop))
1855 $2 = (bar(baz)+baz(bop))
1856 $3 = bar(baz)+baz(bop)
1857 If there is no corresponding capture group defined, then it is a
1859 fatal error. Recursing deeply without consuming any input string
1860 will also result in a fatal error. The depth at which that happens
1861 is compiled into perl, so it can be changed with a custom build.
1862 The following shows how using negative indexing can make it easier
1864 to embed recursive patterns inside of a "qr//" construct for later
1865 use:
1866 my $parens = qr/(\((?:[^()]++|(?-1))*+\))/;
1868 if (/foo $parens \s+ \+ \s+ bar $parens/x) {
1869 # do something here...
1870 }
1871 Note that this pattern does not behave the same way as the
1873 equivalent PCRE or Python construct of the same form. In Perl you
1874 can backtrack into a recursed group, in PCRE and Python the
1875 recursed into group is treated as atomic. Also, modifiers are
1876 resolved at compile time, so constructs like "(?i:(?1))" or
1877 "(?:(?i)(?1))" do not affect how the sub-pattern will be processed.
1878 "(?&NAME)"
1880 Recurse to a named subpattern. Identical to "(?PARNO)" except that
1881 the parenthesis to recurse to is determined by name. If multiple
1882 parentheses have the same name, then it recurses to the leftmost.
1883 It is an error to refer to a name that is not declared somewhere in
1885 the pattern.
1886 NOTE: In order to make things easier for programmers with
1888 experience with the Python or PCRE regex engines the pattern
1889 "(?P>NAME)" may be used instead of "(?&NAME)".
1890 "(?(condition)yes-pattern|no-pattern)"
1892 "(?(condition)yes-pattern)"
1893 Conditional expression. Matches yes-pattern if condition yields a
1894 true value, matches no-pattern otherwise. A missing pattern always
1895 matches.
1896 "(condition)" should be one of:
1898 an integer in parentheses
1900 (which is valid if the corresponding pair of parentheses
1901 matched);
1902 a lookahead/lookbehind/evaluate zero-width assertion;
1904 a name in angle brackets or single quotes
1905 (which is valid if a group with the given name matched);
1906 the special symbol "(R)"
1908 (true when evaluated inside of recursion or eval).
1909 Additionally the "R" may be followed by a number, (which will
1910 be true when evaluated when recursing inside of the appropriate
1911 group), or by "&NAME", in which case it will be true only when
1912 evaluated during recursion in the named group.
1913 Here's a summary of the possible predicates:
1915 "(1)" "(2)" ...
1917 Checks if the numbered capturing group has matched something.
1918 Full syntax: "(?(1)then|else)"
1919 "(<NAME>)" "('NAME')"
1921 Checks if a group with the given name has matched something.
1922 Full syntax: "(?(<name>)then|else)"
1923 "(?=...)" "(?!...)" "(?<=...)" "(?<!...)"
1925 Checks whether the pattern matches (or does not match, for the
1926 "!" variants). Full syntax: "(?(?=lookahead)then|else)"
1927 "(?{ CODE })"
1929 Treats the return value of the code block as the condition.
1930 Full syntax: "(?(?{ code })then|else)"
1931 "(R)"
1933 Checks if the expression has been evaluated inside of
1934 recursion. Full syntax: "(?(R)then|else)"
1935 "(R1)" "(R2)" ...
1937 Checks if the expression has been evaluated while executing
1938 directly inside of the n-th capture group. This check is the
1939 regex equivalent of
1940 if ((caller(0))[3] eq 'subname') { ... }
1942 In other words, it does not check the full recursion stack.
1944 Full syntax: "(?(R1)then|else)"
1946 "(R&NAME)"
1948 Similar to "(R1)", this predicate checks to see if we're
1949 executing directly inside of the leftmost group with a given
1950 name (this is the same logic used by "(?&NAME)" to
1951 disambiguate). It does not check the full stack, but only the
1952 name of the innermost active recursion. Full syntax:
1953 "(?(R&name)then|else)"
1954 "(DEFINE)"
1956 In this case, the yes-pattern is never directly executed, and
1957 no no-pattern is allowed. Similar in spirit to "(?{0})" but
1958 more efficient. See below for details. Full syntax:
1959 "(?(DEFINE)definitions...)"
1960 For example:
1962 m{ ( \( )?
1964 [^()]+
1965 (?(1) \) )
1966 }x
1967 matches a chunk of non-parentheses, possibly included in
1969 parentheses themselves.
1970 A special form is the "(DEFINE)" predicate, which never executes
1972 its yes-pattern directly, and does not allow a no-pattern. This
1973 allows one to define subpatterns which will be executed only by the
1974 recursion mechanism. This way, you can define a set of regular
1975 expression rules that can be bundled into any pattern you choose.
1976 It is recommended that for this usage you put the DEFINE block at
1978 the end of the pattern, and that you name any subpatterns defined
1979 within it.
1980 Also, it's worth noting that patterns defined this way probably
1982 will not be as efficient, as the optimizer is not very clever about
1983 handling them.
1984 An example of how this might be used is as follows:
1986 /(?<NAME>(?&NAME_PAT))(?<ADDR>(?&ADDRESS_PAT))
1988 (?(DEFINE)
1989 (?<NAME_PAT>....)
1990 (?<ADDRESS_PAT>....)
1991 )/x
1992 Note that capture groups matched inside of recursion are not
1994 accessible after the recursion returns, so the extra layer of
1995 capturing groups is necessary. Thus $+{NAME_PAT} would not be
1996 defined even though $+{NAME} would be.
1997 Finally, keep in mind that subpatterns created inside a DEFINE
1999 block count towards the absolute and relative number of captures,
2000 so this:
2001 my @captures = "a" =~ /(.) # First capture
2003 (?(DEFINE)
2004 (?<EXAMPLE> 1 ) # Second capture
2005 )/x;
2006 say scalar @captures;
2007 Will output 2, not 1. This is particularly important if you intend
2009 to compile the definitions with the "qr//" operator, and later
2010 interpolate them in another pattern.
2011 "(?>pattern)"
2013 "(*atomic:pattern)"
2014 An "independent" subexpression, one which matches the substring
2015 that a standalone pattern would match if anchored at the given
2016 position, and it matches nothing other than this substring. This
2017 construct is useful for optimizations of what would otherwise be
2018 "eternal" matches, because it will not backtrack (see
2019 "Backtracking"). It may also be useful in places where the "grab
2020 all you can, and do not give anything back" semantic is desirable.
2021 For example: "^(?>a*)ab" will never match, since "(?>a*)" (anchored
2023 at the beginning of string, as above) will match all characters "a"
2024 at the beginning of string, leaving no "a" for "ab" to match. In
2025 contrast, "a*ab" will match the same as "a+b", since the match of
2026 the subgroup "a*" is influenced by the following group "ab" (see
2027 "Backtracking"). In particular, "a*" inside "a*ab" will match
2028 fewer characters than a standalone "a*", since this makes the tail
2029 match.
2030 "(?>pattern)" does not disable backtracking altogether once it has
2032 matched. It is still possible to backtrack past the construct, but
2033 not into it. So "((?>a*)|(?>b*))ar" will still match "bar".
2034 An effect similar to "(?>pattern)" may be achieved by writing
2036 "(?=(pattern))\g{-1}". This matches the same substring as a
2037 standalone "a+", and the following "\g{-1}" eats the matched
2038 string; it therefore makes a zero-length assertion into an analogue
2039 of "(?>...)". (The difference between these two constructs is that
2040 the second one uses a capturing group, thus shifting ordinals of
2041 backreferences in the rest of a regular expression.)
2042 Consider this pattern:
2044 m{ \(
2046 (
2047 [^()]+ # x+
2048 |
2049 \( [^()]* \)
2050 )+
2051 \)
2052 }x
2053 That will efficiently match a nonempty group with matching
2055 parentheses two levels deep or less. However, if there is no such
2056 group, it will take virtually forever on a long string. That's
2057 because there are so many different ways to split a long string
2058 into several substrings. This is what "(.+)+" is doing, and
2059 "(.+)+" is similar to a subpattern of the above pattern. Consider
2060 how the pattern above detects no-match on "((()aaaaaaaaaaaaaaaaaa"
2061 in several seconds, but that each extra letter doubles this time.
2062 This exponential performance will make it appear that your program
2063 has hung. However, a tiny change to this pattern
2064 m{ \(
2066 (
2067 (?> [^()]+ ) # change x+ above to (?> x+ )
2068 |
2069 \( [^()]* \)
2070 )+
2071 \)
2072 }x
2073 which uses "(?>...)" matches exactly when the one above does
2075 (verifying this yourself would be a productive exercise), but
2076 finishes in a fourth the time when used on a similar string with
2077 1000000 "a"s. Be aware, however, that, when this construct is
2078 followed by a quantifier, it currently triggers a warning message
2079 under the "use warnings" pragma or -w switch saying it "matches
2080 null string many times in regex".
2081 On simple groups, such as the pattern "(?> [^()]+ )", a comparable
2083 effect may be achieved by negative lookahead, as in "[^()]+ (?!
2084 [^()] )". This was only 4 times slower on a string with 1000000
2085 "a"s.
2086 The "grab all you can, and do not give anything back" semantic is
2088 desirable in many situations where on the first sight a simple
2089 "()*" looks like the correct solution. Suppose we parse text with
2090 comments being delimited by "#" followed by some optional
2091 (horizontal) whitespace. Contrary to its appearance, "#[ \t]*" is
2092 not the correct subexpression to match the comment delimiter,
2093 because it may "give up" some whitespace if the remainder of the
2094 pattern can be made to match that way. The correct answer is
2095 either one of these:
2096 (?>#[ \t]*)
2098 #[ \t]*(?![ \t])
2099 For example, to grab non-empty comments into $1, one should use
2101 either one of these:
2102 / (?> \# [ \t]* ) ( .+ ) /x;
2104 / \# [ \t]* ( [^ \t] .* ) /x;
2105 Which one you pick depends on which of these expressions better
2107 reflects the above specification of comments.
2108 In some literature this construct is called "atomic matching" or
2110 "possessive matching".
2111 Possessive quantifiers are equivalent to putting the item they are
2113 applied to inside of one of these constructs. The following
2114 equivalences apply:
2115 Quantifier Form Bracketing Form
2117 --------------- ---------------
2118 PAT*+ (?>PAT*)
2119 PAT++ (?>PAT+)
2120 PAT?+ (?>PAT?)
2121 PAT{min,max}+ (?>PAT{min,max})
2122 Nested "(?>...)" constructs are not no-ops, even if at first glance
2124 they might seem to be. This is because the nested "(?>...)" can
2125 restrict internal backtracking that otherwise might occur. For
2126 example,
2127 "abc" =~ /(?>a[bc]*c)/
2129 matches, but
2131 "abc" =~ /(?>a(?>[bc]*)c)/
2133 does not.
2135 "(?[ ])"
2137 See "Extended Bracketed Character Classes" in perlrecharclass.
2138 Note that this feature is currently experimental; using it yields a
2140 warning in the "experimental::regex_sets" category.
2141 Backtracking
2143 NOTE: This section presents an abstract approximation of regular
2144 expression behavior. For a more rigorous (and complicated) view of the
2145 rules involved in selecting a match among possible alternatives, see
2146 "Combining RE Pieces".
2147 A fundamental feature of regular expression matching involves the
2149 notion called backtracking, which is currently used (when needed) by
2150 all regular non-possessive expression quantifiers, namely "*", "*?",
2151 "+", "+?", "{n,m}", and "{n,m}?". Backtracking is often optimized
2152 internally, but the general principle outlined here is valid.
2153 For a regular expression to match, the entire regular expression must
2155 match, not just part of it. So if the beginning of a pattern
2156 containing a quantifier succeeds in a way that causes later parts in
2157 the pattern to fail, the matching engine backs up and recalculates the
2158 beginning part--that's why it's called backtracking.
2159 Here is an example of backtracking: Let's say you want to find the
2161 word following "foo" in the string "Food is on the foo table.":
2162 $_ = "Food is on the foo table.";
2164 if ( /\b(foo)\s+(\w+)/i ) {
2165 print "$2 follows $1.\n";
2166 }
2167 When the match runs, the first part of the regular expression
2169 ("\b(foo)") finds a possible match right at the beginning of the
2170 string, and loads up $1 with "Foo". However, as soon as the matching
2171 engine sees that there's no whitespace following the "Foo" that it had
2172 saved in $1, it realizes its mistake and starts over again one
2173 character after where it had the tentative match. This time it goes
2174 all the way until the next occurrence of "foo". The complete regular
2175 expression matches this time, and you get the expected output of "table
2176 follows foo."
2177 Sometimes minimal matching can help a lot. Imagine you'd like to match
2179 everything between "foo" and "bar". Initially, you write something
2180 like this:
2181 $_ = "The food is under the bar in the barn.";
2183 if ( /foo(.*)bar/ ) {
2184 print "got <$1>\n";
2185 }
2186 Which perhaps unexpectedly yields:
2188 got <d is under the bar in the >
2190 That's because ".*" was greedy, so you get everything between the first
2192 "foo" and the last "bar". Here it's more effective to use minimal
2193 matching to make sure you get the text between a "foo" and the first
2194 "bar" thereafter.
2195 if ( /foo(.*?)bar/ ) { print "got <$1>\n" }
2197 got <d is under the >
2198 Here's another example. Let's say you'd like to match a number at the
2200 end of a string, and you also want to keep the preceding part of the
2201 match. So you write this:
2202 $_ = "I have 2 numbers: 53147";
2204 if ( /(.*)(\d*)/ ) { # Wrong!
2205 print "Beginning is <$1>, number is <$2>.\n";
2206 }
2207 That won't work at all, because ".*" was greedy and gobbled up the
2209 whole string. As "\d*" can match on an empty string the complete
2210 regular expression matched successfully.
2211 Beginning is <I have 2 numbers: 53147>, number is <>.
2213 Here are some variants, most of which don't work:
2215 $_ = "I have 2 numbers: 53147";
2217 @pats = qw{
2218 (.*)(\d*)
2219 (.*)(\d+)
2220 (.*?)(\d*)
2221 (.*?)(\d+)
2222 (.*)(\d+)$
2223 (.*?)(\d+)$
2224 (.*)\b(\d+)$
2225 (.*\D)(\d+)$
2226 };
2227 for $pat (@pats) {
2229 printf "%-12s ", $pat;
2230 if ( /$pat/ ) {
2231 print "<$1> <$2>\n";
2232 } else {
2233 print "FAIL\n";
2234 }
2235 }
2236 That will print out:
2238 (.*)(\d*) <I have 2 numbers: 53147> <>
2240 (.*)(\d+) <I have 2 numbers: 5314> <7>
2241 (.*?)(\d*) <> <>
2242 (.*?)(\d+) <I have > <2>
2243 (.*)(\d+)$ <I have 2 numbers: 5314> <7>
2244 (.*?)(\d+)$ <I have 2 numbers: > <53147>
2245 (.*)\b(\d+)$ <I have 2 numbers: > <53147>
2246 (.*\D)(\d+)$ <I have 2 numbers: > <53147>
2247 As you see, this can be a bit tricky. It's important to realize that a
2249 regular expression is merely a set of assertions that gives a
2250 definition of success. There may be 0, 1, or several different ways
2251 that the definition might succeed against a particular string. And if
2252 there are multiple ways it might succeed, you need to understand
2253 backtracking to know which variety of success you will achieve.
2254 When using lookahead assertions and negations, this can all get even
2256 trickier. Imagine you'd like to find a sequence of non-digits not
2257 followed by "123". You might try to write that as
2258 $_ = "ABC123";
2260 if ( /^\D*(?!123)/ ) { # Wrong!
2261 print "Yup, no 123 in $_\n";
2262 }
2263 But that isn't going to match; at least, not the way you're hoping. It
2265 claims that there is no 123 in the string. Here's a clearer picture of
2266 why that pattern matches, contrary to popular expectations:
2267 $x = 'ABC123';
2269 $y = 'ABC445';
2270 print "1: got $1\n" if $x =~ /^(ABC)(?!123)/;
2272 print "2: got $1\n" if $y =~ /^(ABC)(?!123)/;
2273 print "3: got $1\n" if $x =~ /^(\D*)(?!123)/;
2275 print "4: got $1\n" if $y =~ /^(\D*)(?!123)/;
2276 This prints
2278 2: got ABC
2280 3: got AB
2281 4: got ABC
2282 You might have expected test 3 to fail because it seems to a more
2284 general purpose version of test 1. The important difference between
2285 them is that test 3 contains a quantifier ("\D*") and so can use
2286 backtracking, whereas test 1 will not. What's happening is that you've
2287 asked "Is it true that at the start of $x, following 0 or more non-
2288 digits, you have something that's not 123?" If the pattern matcher had
2289 let "\D*" expand to "ABC", this would have caused the whole pattern to
2290 fail.
2291 The search engine will initially match "\D*" with "ABC". Then it will
2293 try to match "(?!123)" with "123", which fails. But because a
2294 quantifier ("\D*") has been used in the regular expression, the search
2295 engine can backtrack and retry the match differently in the hope of
2296 matching the complete regular expression.
2297 The pattern really, really wants to succeed, so it uses the standard
2299 pattern back-off-and-retry and lets "\D*" expand to just "AB" this
2300 time. Now there's indeed something following "AB" that is not "123".
2301 It's "C123", which suffices.
2302 We can deal with this by using both an assertion and a negation. We'll
2304 say that the first part in $1 must be followed both by a digit and by
2305 something that's not "123". Remember that the lookaheads are zero-
2306 width expressions--they only look, but don't consume any of the string
2307 in their match. So rewriting this way produces what you'd expect; that
2308 is, case 5 will fail, but case 6 succeeds:
2309 print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/;
2311 print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/;
2312 6: got ABC
2314 In other words, the two zero-width assertions next to each other work
2316 as though they're ANDed together, just as you'd use any built-in
2317 assertions: "/^$/" matches only if you're at the beginning of the line
2318 AND the end of the line simultaneously. The deeper underlying truth is
2319 that juxtaposition in regular expressions always means AND, except when
2320 you write an explicit OR using the vertical bar. "/ab/" means match
2321 "a" AND (then) match "b", although the attempted matches are made at
2322 different positions because "a" is not a zero-width assertion, but a
2323 one-width assertion.
2324 WARNING: Particularly complicated regular expressions can take
2326 exponential time to solve because of the immense number of possible
2327 ways they can use backtracking to try for a match. For example,
2328 without internal optimizations done by the regular expression engine,
2329 this will take a painfully long time to run:
2330 'aaaaaaaaaaaa' =~ /((a{0,5}){0,5})*[c]/
2332 And if you used "*"'s in the internal groups instead of limiting them
2334 to 0 through 5 matches, then it would take forever--or until you ran
2335 out of stack space. Moreover, these internal optimizations are not
2336 always applicable. For example, if you put "{0,5}" instead of "*" on
2337 the external group, no current optimization is applicable, and the
2338 match takes a long time to finish.
2339 A powerful tool for optimizing such beasts is what is known as an
2341 "independent group", which does not backtrack (see ""(?>pattern)"").
2342 Note also that zero-length lookahead/lookbehind assertions will not
2343 backtrack to make the tail match, since they are in "logical" context:
2344 only whether they match is considered relevant. For an example where
2345 side-effects of lookahead might have influenced the following match,
2346 see ""(?>pattern)"".
2347 Script Runs
2349 A script run is basically a sequence of characters, all from the same
2350 Unicode script (see "Scripts" in perlunicode), such as Latin or Greek.
2351 In most places a single word would never be written in multiple
2352 scripts, unless it is a spoofing attack. An infamous example, is
2353 paypal.com
2355 Those letters could all be Latin (as in the example just above), or
2357 they could be all Cyrillic (except for the dot), or they could be a
2358 mixture of the two. In the case of an internet address the ".com"
2359 would be in Latin, And any Cyrillic ones would cause it to be a
2360 mixture, not a script run. Someone clicking on such a link would not
2361 be directed to the real Paypal website, but an attacker would craft a
2362 look-alike one to attempt to gather sensitive information from the
2363 person.
2364 Starting in Perl 5.28, it is now easy to detect strings that aren't
2366 script runs. Simply enclose just about any pattern like either of
2367 these:
2368 (*script_run:pattern)
2370 (*sr:pattern)
2371 What happens is that after pattern succeeds in matching, it is
2373 subjected to the additional criterion that every character in it must
2374 be from the same script (see exceptions below). If this isn't true,
2375 backtracking occurs until something all in the same script is found
2376 that matches, or all possibilities are exhausted. This can cause a lot
2377 of backtracking, but generally, only malicious input will result in
2378 this, though the slow down could cause a denial of service attack. If
2379 your needs permit, it is best to make the pattern atomic to cut down on
2380 the amount of backtracking. This is so likely to be what you want,
2381 that instead of writing this:
2382 (*script_run:(?>pattern))
2384 you can write either of these:
2386 (*atomic_script_run:pattern)
2388 (*asr:pattern)
2389 (See ""(?>pattern)"".)
2391 In Taiwan, Japan, and Korea, it is common for text to have a mixture of
2393 characters from their native scripts and base Chinese. Perl follows
2394 Unicode's UTS 39 (<https://unicode.org/reports/tr39/>) Unicode Security
2395 Mechanisms in allowing such mixtures. For example, the Japanese
2396 scripts Katakana and Hiragana are commonly mixed together in practice,
2397 along with some Chinese characters, and hence are treated as being in a
2398 single script run by Perl.
2399 The rules used for matching decimal digits are slightly stricter. Many
2401 scripts have their own sets of digits equivalent to the Western 0
2402 through 9 ones. A few, such as Arabic, have more than one set. For a
2403 string to be considered a script run, all digits in it must come from
2404 the same set of ten, as determined by the first digit encountered. As
2405 an example,
2406 qr/(*script_run: \d+ \b )/x
2408 guarantees that the digits matched will all be from the same set of 10.
2410 You won't get a look-alike digit from a different script that has a
2411 different value than what it appears to be.
2412 Unicode has three pseudo scripts that are handled specially.
2414 "Unknown" is applied to code points whose meaning has yet to be
2416 determined. Perl currently will match as a script run, any single
2417 character string consisting of one of these code points. But any
2418 string longer than one code point containing one of these will not be
2419 considered a script run.
2420 "Inherited" is applied to characters that modify another, such as an
2422 accent of some type. These are considered to be in the script of the
2423 master character, and so never cause a script run to not match.
2424 The other one is "Common". This consists of mostly punctuation, emoji,
2426 and characters used in mathematics and music, the ASCII digits 0
2427 through 9, and full-width forms of these digits. These characters can
2428 appear intermixed in text in many of the world's scripts. These also
2429 don't cause a script run to not match. But like other scripts, all
2430 digits in a run must come from the same set of 10.
2431 This construct is non-capturing. You can add parentheses to pattern to
2433 capture, if desired. You will have to do this if you plan to use
2434 "(*ACCEPT) (*ACCEPT:arg)" and not have it bypass the script run
2435 checking.
2436 The "Script_Extensions" property as modified by UTS 39
2438 (<https://unicode.org/reports/tr39/>) is used as the basis for this
2439 feature.
2440 To summarize,
2442 • All length 0 or length 1 sequences are script runs.
2444 • A longer sequence is a script run if and only if all of the
2446 following conditions are met:
2447 1. No code point in the sequence has the "Script_Extension"
2451 property of "Unknown".
2452 This currently means that all code points in the sequence have
2454 been assigned by Unicode to be characters that aren't private
2455 use nor surrogate code points.
2456 2. All characters in the sequence come from the Common script
2458 and/or the Inherited script and/or a single other script.
2459 The script of a character is determined by the
2461 "Script_Extensions" property as modified by UTS 39
2462 (<https://unicode.org/reports/tr39/>), as described above.
2463 3. All decimal digits in the sequence come from the same block of
2465 10 consecutive digits.
2466 Special Backtracking Control Verbs
2468 These special patterns are generally of the form "(*VERB:arg)". Unless
2469 otherwise stated the arg argument is optional; in some cases, it is
2470 mandatory.
2471 Any pattern containing a special backtracking verb that allows an
2473 argument has the special behaviour that when executed it sets the
2474 current package's $REGERROR and $REGMARK variables. When doing so the
2475 following rules apply:
2476 On failure, the $REGERROR variable will be set to the arg value of the
2478 verb pattern, if the verb was involved in the failure of the match. If
2479 the arg part of the pattern was omitted, then $REGERROR will be set to
2480 the name of the last "(*MARK:NAME)" pattern executed, or to TRUE if
2481 there was none. Also, the $REGMARK variable will be set to FALSE.
2482 On a successful match, the $REGERROR variable will be set to FALSE, and
2484 the $REGMARK variable will be set to the name of the last
2485 "(*MARK:NAME)" pattern executed. See the explanation for the
2486 "(*MARK:NAME)" verb below for more details.
2487 NOTE: $REGERROR and $REGMARK are not magic variables like $1 and most
2489 other regex-related variables. They are not local to a scope, nor
2490 readonly, but instead are volatile package variables similar to
2491 $AUTOLOAD. They are set in the package containing the code that
2492 executed the regex (rather than the one that compiled it, where those
2493 differ). If necessary, you can use "local" to localize changes to
2494 these variables to a specific scope before executing a regex.
2495 If a pattern does not contain a special backtracking verb that allows
2497 an argument, then $REGERROR and $REGMARK are not touched at all.
2498 Verbs
2500 "(*PRUNE)" "(*PRUNE:NAME)"
2501 This zero-width pattern prunes the backtracking tree at the
2502 current point when backtracked into on failure. Consider the
2503 pattern "/A (*PRUNE) B/", where A and B are complex patterns.
2504 Until the "(*PRUNE)" verb is reached, A may backtrack as
2505 necessary to match. Once it is reached, matching continues in B,
2506 which may also backtrack as necessary; however, should B not
2507 match, then no further backtracking will take place, and the
2508 pattern will fail outright at the current starting position.
2509 The following example counts all the possible matching strings
2511 in a pattern (without actually matching any of them).
2512 'aaab' =~ /a+b?(?{print "$&\n"; $count++})(*FAIL)/;
2514 print "Count=$count\n";
2515 which produces:
2517 aaab
2519 aaa
2520 aa
2521 a
2522 aab
2523 aa
2524 a
2525 ab
2526 a
2527 Count=9
2528 If we add a "(*PRUNE)" before the count like the following
2530 'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/;
2532 print "Count=$count\n";
2533 we prevent backtracking and find the count of the longest
2535 matching string at each matching starting point like so:
2536 aaab
2538 aab
2539 ab
2540 Count=3
2541 Any number of "(*PRUNE)" assertions may be used in a pattern.
2543 See also "(?>pattern)" and possessive quantifiers for other ways
2545 to control backtracking. In some cases, the use of "(*PRUNE)"
2546 can be replaced with a "(?>pattern)" with no functional
2547 difference; however, "(*PRUNE)" can be used to handle cases that
2548 cannot be expressed using a "(?>pattern)" alone.
2549 "(*SKIP)" "(*SKIP:NAME)"
2551 This zero-width pattern is similar to "(*PRUNE)", except that on
2552 failure it also signifies that whatever text that was matched
2553 leading up to the "(*SKIP)" pattern being executed cannot be
2554 part of any match of this pattern. This effectively means that
2555 the regex engine "skips" forward to this position on failure and
2556 tries to match again, (assuming that there is sufficient room to
2557 match).
2558 The name of the "(*SKIP:NAME)" pattern has special significance.
2560 If a "(*MARK:NAME)" was encountered while matching, then it is
2561 that position which is used as the "skip point". If no "(*MARK)"
2562 of that name was encountered, then the "(*SKIP)" operator has no
2563 effect. When used without a name the "skip point" is where the
2564 match point was when executing the "(*SKIP)" pattern.
2565 Compare the following to the examples in "(*PRUNE)"; note the
2567 string is twice as long:
2568 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/;
2570 print "Count=$count\n";
2571 outputs
2573 aaab
2575 aaab
2576 Count=2
2577 Once the 'aaab' at the start of the string has matched, and the
2579 "(*SKIP)" executed, the next starting point will be where the
2580 cursor was when the "(*SKIP)" was executed.
2581 "(*MARK:NAME)" "(*:NAME)"
2583 This zero-width pattern can be used to mark the point reached in
2584 a string when a certain part of the pattern has been
2585 successfully matched. This mark may be given a name. A later
2586 "(*SKIP)" pattern will then skip forward to that point if
2587 backtracked into on failure. Any number of "(*MARK)" patterns
2588 are allowed, and the NAME portion may be duplicated.
2589 In addition to interacting with the "(*SKIP)" pattern,
2591 "(*MARK:NAME)" can be used to "label" a pattern branch, so that
2592 after matching, the program can determine which branches of the
2593 pattern were involved in the match.
2594 When a match is successful, the $REGMARK variable will be set to
2596 the name of the most recently executed "(*MARK:NAME)" that was
2597 involved in the match.
2598 This can be used to determine which branch of a pattern was
2600 matched without using a separate capture group for each branch,
2601 which in turn can result in a performance improvement, as perl
2602 cannot optimize "/(?:(x)|(y)|(z))/" as efficiently as something
2603 like "/(?:x(*MARK:x)|y(*MARK:y)|z(*MARK:z))/".
2604 When a match has failed, and unless another verb has been
2606 involved in failing the match and has provided its own name to
2607 use, the $REGERROR variable will be set to the name of the most
2608 recently executed "(*MARK:NAME)".
2609 See "(*SKIP)" for more details.
2611 As a shortcut "(*MARK:NAME)" can be written "(*:NAME)".
2613 "(*THEN)" "(*THEN:NAME)"
2615 This is similar to the "cut group" operator "::" from Raku.
2616 Like "(*PRUNE)", this verb always matches, and when backtracked
2617 into on failure, it causes the regex engine to try the next
2618 alternation in the innermost enclosing group (capturing or
2619 otherwise) that has alternations. The two branches of a
2620 "(?(condition)yes-pattern|no-pattern)" do not count as an
2621 alternation, as far as "(*THEN)" is concerned.
2622 Its name comes from the observation that this operation combined
2624 with the alternation operator ("|") can be used to create what
2625 is essentially a pattern-based if/then/else block:
2626 ( COND (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ )
2628 Note that if this operator is used and NOT inside of an
2630 alternation then it acts exactly like the "(*PRUNE)" operator.
2631 / A (*PRUNE) B /
2633 is the same as
2635 / A (*THEN) B /
2637 but
2639 / ( A (*THEN) B | C ) /
2641 is not the same as
2643 / ( A (*PRUNE) B | C ) /
2645 as after matching the A but failing on the B the "(*THEN)" verb
2647 will backtrack and try C; but the "(*PRUNE)" verb will simply
2648 fail.
2649 "(*COMMIT)" "(*COMMIT:arg)"
2651 This is the Raku "commit pattern" "<commit>" or ":::". It's a
2652 zero-width pattern similar to "(*SKIP)", except that when
2653 backtracked into on failure it causes the match to fail
2654 outright. No further attempts to find a valid match by advancing
2655 the start pointer will occur again. For example,
2656 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/;
2658 print "Count=$count\n";
2659 outputs
2661 aaab
2663 Count=1
2664 In other words, once the "(*COMMIT)" has been entered, and if
2666 the pattern does not match, the regex engine will not try any
2667 further matching on the rest of the string.
2668 "(*FAIL)" "(*F)" "(*FAIL:arg)"
2670 This pattern matches nothing and always fails. It can be used to
2671 force the engine to backtrack. It is equivalent to "(?!)", but
2672 easier to read. In fact, "(?!)" gets optimised into "(*FAIL)"
2673 internally. You can provide an argument so that if the match
2674 fails because of this "FAIL" directive the argument can be
2675 obtained from $REGERROR.
2676 It is probably useful only when combined with "(?{})" or
2678 "(??{})".
2679 "(*ACCEPT)" "(*ACCEPT:arg)"
2681 This pattern matches nothing and causes the end of successful
2682 matching at the point at which the "(*ACCEPT)" pattern was
2683 encountered, regardless of whether there is actually more to
2684 match in the string. When inside of a nested pattern, such as
2685 recursion, or in a subpattern dynamically generated via
2686 "(??{})", only the innermost pattern is ended immediately.
2687 If the "(*ACCEPT)" is inside of capturing groups then the groups
2689 are marked as ended at the point at which the "(*ACCEPT)" was
2690 encountered. For instance:
2691 'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x;
2693 will match, and $1 will be "AB" and $2 will be "B", $3 will not
2695 be set. If another branch in the inner parentheses was matched,
2696 such as in the string 'ACDE', then the "D" and "E" would have to
2697 be matched as well.
2698 You can provide an argument, which will be available in the var
2700 $REGMARK after the match completes.
2701 Warning on "\1" Instead of $1
2703 Some people get too used to writing things like:
2704 $pattern =~ s/(\W)/\\\1/g;
2706 This is grandfathered (for \1 to \9) for the RHS of a substitute to
2708 avoid shocking the sed addicts, but it's a dirty habit to get into.
2709 That's because in PerlThink, the righthand side of an "s///" is a
2710 double-quoted string. "\1" in the usual double-quoted string means a
2711 control-A. The customary Unix meaning of "\1" is kludged in for
2712 "s///". However, if you get into the habit of doing that, you get
2713 yourself into trouble if you then add an "/e" modifier.
2714 s/(\d+)/ \1 + 1 /eg; # causes warning under -w
2716 Or if you try to do
2718 s/(\d+)/\1000/;
2720 You can't disambiguate that by saying "\{1}000", whereas you can fix it
2722 with "${1}000". The operation of interpolation should not be confused
2723 with the operation of matching a backreference. Certainly they mean
2724 two different things on the left side of the "s///".
2725 Repeated Patterns Matching a Zero-length Substring
2727 WARNING: Difficult material (and prose) ahead. This section needs a
2728 rewrite.
2729 Regular expressions provide a terse and powerful programming language.
2731 As with most other power tools, power comes together with the ability
2732 to wreak havoc.
2733 A common abuse of this power stems from the ability to make infinite
2735 loops using regular expressions, with something as innocuous as:
2736 'foo' =~ m{ ( o? )* }x;
2738 The "o?" matches at the beginning of ""foo"", and since the position in
2740 the string is not moved by the match, "o?" would match again and again
2741 because of the "*" quantifier. Another common way to create a similar
2742 cycle is with the looping modifier "/g":
2743 @matches = ( 'foo' =~ m{ o? }xg );
2745 or
2747 print "match: <$&>\n" while 'foo' =~ m{ o? }xg;
2749 or the loop implied by "split()".
2751 However, long experience has shown that many programming tasks may be
2753 significantly simplified by using repeated subexpressions that may
2754 match zero-length substrings. Here's a simple example being:
2755 @chars = split //, $string; # // is not magic in split
2757 ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// /
2758 Thus Perl allows such constructs, by forcefully breaking the infinite
2760 loop. The rules for this are different for lower-level loops given by
2761 the greedy quantifiers "*+{}", and for higher-level ones like the "/g"
2762 modifier or "split()" operator.
2763 The lower-level loops are interrupted (that is, the loop is broken)
2765 when Perl detects that a repeated expression matched a zero-length
2766 substring. Thus
2767 m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x;
2769 is made equivalent to
2771 m{ (?: NON_ZERO_LENGTH )* (?: ZERO_LENGTH )? }x;
2773 For example, this program
2775 #!perl -l
2777 "aaaaab" =~ /
2778 (?:
2779 a # non-zero
2780 | # or
2781 (?{print "hello"}) # print hello whenever this
2782 # branch is tried
2783 (?=(b)) # zero-width assertion
2784 )* # any number of times
2785 /x;
2786 print $&;
2787 print $1;
2788 prints
2790 hello
2792 aaaaa
2793 b
2794 Notice that "hello" is only printed once, as when Perl sees that the
2796 sixth iteration of the outermost "(?:)*" matches a zero-length string,
2797 it stops the "*".
2798 The higher-level loops preserve an additional state between iterations:
2800 whether the last match was zero-length. To break the loop, the
2801 following match after a zero-length match is prohibited to have a
2802 length of zero. This prohibition interacts with backtracking (see
2803 "Backtracking"), and so the second best match is chosen if the best
2804 match is of zero length.
2805 For example:
2807 $_ = 'bar';
2809 s/\w??/<$&>/g;
2810 results in "<><b><><a><><r><>". At each position of the string the
2812 best match given by non-greedy "??" is the zero-length match, and the
2813 second best match is what is matched by "\w". Thus zero-length matches
2814 alternate with one-character-long matches.
2815 Similarly, for repeated "m/()/g" the second-best match is the match at
2817 the position one notch further in the string.
2818 The additional state of being matched with zero-length is associated
2820 with the matched string, and is reset by each assignment to "pos()".
2821 Zero-length matches at the end of the previous match are ignored during
2822 "split".
2823 Combining RE Pieces
2825 Each of the elementary pieces of regular expressions which were
2826 described before (such as "ab" or "\Z") could match at most one
2827 substring at the given position of the input string. However, in a
2828 typical regular expression these elementary pieces are combined into
2829 more complicated patterns using combining operators "ST", "S|T", "S*"
2830 etc. (in these examples "S" and "T" are regular subexpressions).
2831 Such combinations can include alternatives, leading to a problem of
2833 choice: if we match a regular expression "a|ab" against "abc", will it
2834 match substring "a" or "ab"? One way to describe which substring is
2835 actually matched is the concept of backtracking (see "Backtracking").
2836 However, this description is too low-level and makes you think in terms
2837 of a particular implementation.
2838 Another description starts with notions of "better"/"worse". All the
2840 substrings which may be matched by the given regular expression can be
2841 sorted from the "best" match to the "worst" match, and it is the "best"
2842 match which is chosen. This substitutes the question of "what is
2843 chosen?" by the question of "which matches are better, and which are
2844 worse?".
2845 Again, for elementary pieces there is no such question, since at most
2847 one match at a given position is possible. This section describes the
2848 notion of better/worse for combining operators. In the description
2849 below "S" and "T" are regular subexpressions.
2850 "ST"
2852 Consider two possible matches, "AB" and "A'B'", "A" and "A'" are
2853 substrings which can be matched by "S", "B" and "B'" are substrings
2854 which can be matched by "T".
2855 If "A" is a better match for "S" than "A'", "AB" is a better match
2857 than "A'B'".
2858 If "A" and "A'" coincide: "AB" is a better match than "AB'" if "B"
2860 is a better match for "T" than "B'".
2861 "S|T"
2863 When "S" can match, it is a better match than when only "T" can
2864 match.
2865 Ordering of two matches for "S" is the same as for "S". Similar
2867 for two matches for "T".
2868 "S{REPEAT_COUNT}"
2870 Matches as "SSS...S" (repeated as many times as necessary).
2871 "S{min,max}"
2873 Matches as "S{max}|S{max-1}|...|S{min+1}|S{min}".
2874 "S{min,max}?"
2876 Matches as "S{min}|S{min+1}|...|S{max-1}|S{max}".
2877 "S?", "S*", "S+"
2879 Same as "S{0,1}", "S{0,BIG_NUMBER}", "S{1,BIG_NUMBER}"
2880 respectively.
2881 "S??", "S*?", "S+?"
2883 Same as "S{0,1}?", "S{0,BIG_NUMBER}?", "S{1,BIG_NUMBER}?"
2884 respectively.
2885 "(?>S)"
2887 Matches the best match for "S" and only that.
2888 "(?=S)", "(?<=S)"
2890 Only the best match for "S" is considered. (This is important only
2891 if "S" has capturing parentheses, and backreferences are used
2892 somewhere else in the whole regular expression.)
2893 "(?!S)", "(?<!S)"
2895 For this grouping operator there is no need to describe the
2896 ordering, since only whether or not "S" can match is important.
2897 "(??{ EXPR })", "(?PARNO)"
2899 The ordering is the same as for the regular expression which is the
2900 result of EXPR, or the pattern contained by capture group PARNO.
2901 "(?(condition)yes-pattern|no-pattern)"
2903 Recall that which of yes-pattern or no-pattern actually matches is
2904 already determined. The ordering of the matches is the same as for
2905 the chosen subexpression.
2906 The above recipes describe the ordering of matches at a given position.
2908 One more rule is needed to understand how a match is determined for the
2909 whole regular expression: a match at an earlier position is always
2910 better than a match at a later position.
2911 Creating Custom RE Engines
2913 As of Perl 5.10.0, one can create custom regular expression engines.
2914 This is not for the faint of heart, as they have to plug in at the C
2915 level. See perlreapi for more details.
2916 As an alternative, overloaded constants (see overload) provide a simple
2918 way to extend the functionality of the RE engine, by substituting one
2919 pattern for another.
2920 Suppose that we want to enable a new RE escape-sequence "\Y|" which
2922 matches at a boundary between whitespace characters and non-whitespace
2923 characters. Note that "(?=\S)(?<!\S)|(?!\S)(?<=\S)" matches exactly at
2924 these positions, so we want to have each "\Y|" in the place of the more
2925 complicated version. We can create a module "customre" to do this:
2926 package customre;
2928 use overload;
2929 sub import {
2931 shift;
2932 die "No argument to customre::import allowed" if @_;
2933 overload::constant 'qr' => \&convert;
2934 }
2935 sub invalid { die "/$_[0]/: invalid escape '\\$_[1]'"}
2937 # We must also take care of not escaping the legitimate \\Y|
2939 # sequence, hence the presence of '\\' in the conversion rules.
2940 my %rules = ( '\\' => '\\\\',
2941 'Y|' => qr/(?=\S)(?<!\S)|(?!\S)(?<=\S)/ );
2942 sub convert {
2943 my $re = shift;
2944 $re =~ s{
2945 \\ ( \\ | Y . )
2946 }
2947 { $rules{$1} or invalid($re,$1) }sgex;
2948 return $re;
2949 }
2950 Now "use customre" enables the new escape in constant regular
2952 expressions, i.e., those without any runtime variable interpolations.
2953 As documented in overload, this conversion will work only over literal
2954 parts of regular expressions. For "\Y|$re\Y|" the variable part of
2955 this regular expression needs to be converted explicitly (but only if
2956 the special meaning of "\Y|" should be enabled inside $re):
2957 use customre;
2959 $re = <>;
2960 chomp $re;
2961 $re = customre::convert $re;
2962 /\Y|$re\Y|/;
2963 Embedded Code Execution Frequency
2965 The exact rules for how often "(??{})" and "(?{})" are executed in a
2966 pattern are unspecified. In the case of a successful match you can
2967 assume that they DWIM and will be executed in left to right order the
2968 appropriate number of times in the accepting path of the pattern as
2969 would any other meta-pattern. How non-accepting pathways and match
2970 failures affect the number of times a pattern is executed is
2971 specifically unspecified and may vary depending on what optimizations
2972 can be applied to the pattern and is likely to change from version to
2973 version.
2974 For instance in
2976 "aaabcdeeeee"=~/a(?{print "a"})b(?{print "b"})cde/;
2978 the exact number of times "a" or "b" are printed out is unspecified for
2980 failure, but you may assume they will be printed at least once during a
2981 successful match, additionally you may assume that if "b" is printed,
2982 it will be preceded by at least one "a".
2983 In the case of branching constructs like the following:
2985 /a(b|(?{ print "a" }))c(?{ print "c" })/;
2987 you can assume that the input "ac" will output "ac", and that "abc"
2989 will output only "c".
2990 When embedded code is quantified, successful matches will call the code
2992 once for each matched iteration of the quantifier. For example:
2993 "good" =~ /g(?:o(?{print "o"}))*d/;
2995 will output "o" twice.
2997 PCRE/Python Support
2999 As of Perl 5.10.0, Perl supports several Python/PCRE-specific
3000 extensions to the regex syntax. While Perl programmers are encouraged
3001 to use the Perl-specific syntax, the following are also accepted:
3002 "(?P<NAME>pattern)"
3004 Define a named capture group. Equivalent to "(?<NAME>pattern)".
3005 "(?P=NAME)"
3007 Backreference to a named capture group. Equivalent to "\g{NAME}".
3008 "(?P>NAME)"
3010 Subroutine call to a named capture group. Equivalent to "(?&NAME)".
3011
3013 There are a number of issues with regard to case-insensitive matching
3014 in Unicode rules. See "i" under "Modifiers" above.
3015 This document varies from difficult to understand to completely and
3017 utterly opaque. The wandering prose riddled with jargon is hard to
3018 fathom in several places.
3019 This document needs a rewrite that separates the tutorial content from
3021 the reference content.
3022
3024 The syntax of patterns used in Perl pattern matching evolved from those
3025 supplied in the Bell Labs Research Unix 8th Edition (Version 8) regex
3026 routines. (The code is actually derived (distantly) from Henry
3027 Spencer's freely redistributable reimplementation of those V8
3028 routines.)
3029 perlrequick.
3031 perlretut.
3033 "Regexp Quote-Like Operators" in perlop.
3035 "Gory details of parsing quoted constructs" in perlop.
3037 perlfaq6.
3039 "pos" in perlfunc.
3041 perllocale.
3043 perlebcdic.
3045 Mastering Regular Expressions by Jeffrey Friedl, published by O'Reilly
3047 and Associates.
3048perl v5.32.1 2021-05-31 PERLRE(1)