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{}" escapes. It is
437 ineffective to try to continue a comment onto the next line by escaping
438 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 <http://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 The alphabetic forms are experimental; using them yields a
1466 warning in the "experimental::alpha_assertions" category.
1467 "(?!pattern)"
1469 "(*nla:pattern)"
1470 "(*negative_lookahead:pattern)"
1471 A zero-width negative lookahead assertion. For example
1472 "/foo(?!bar)/" matches any occurrence of "foo" that isn't
1473 followed by "bar". Note however that lookahead and lookbehind
1474 are NOT the same thing. You cannot use this for lookbehind.
1475 If you are looking for a "bar" that isn't preceded by a "foo",
1477 "/(?!foo)bar/" will not do what you want. That's because the
1478 "(?!foo)" is just saying that the next thing cannot be
1479 "foo"--and it's not, it's a "bar", so "foobar" will match. Use
1480 lookbehind instead (see below).
1481 The alphabetic forms are experimental; using them yields a
1483 warning in the "experimental::alpha_assertions" category.
1484 "(?<=pattern)"
1486 "\K"
1487 "(*plb:pattern)"
1488 "(*positive_lookbehind:pattern)"
1489 A zero-width positive lookbehind assertion. For example,
1490 "/(?<=\t)\w+/" matches a word that follows a tab, without
1491 including the tab in $&.
1492 Prior to Perl 5.30, it worked only for fixed-width lookbehind,
1494 but starting in that release, it can handle variable lengths
1495 from 1 to 255 characters as an experimental feature. The
1496 feature is enabled automatically if you use a variable length
1497 lookbehind assertion, but will raise a warning at pattern
1498 compilation time, unless turned off, in the "experimental::vlb"
1499 category. This is to warn you that the exact behavior is
1500 subject to change should feedback from actual use in the field
1501 indicate to do so; or even complete removal if the problems
1502 found are not practically surmountable. You can achieve close
1503 to pre-5.30 behavior by fatalizing warnings in this category.
1504 There is a special form of this construct, called "\K"
1506 (available since Perl 5.10.0), which causes the regex engine to
1507 "keep" everything it had matched prior to the "\K" and not
1508 include it in $&. This effectively provides non-experimental
1509 variable-length lookbehind of any length.
1510 And, there is a technique that can be used to handle variable
1512 length lookbehinds on earlier releases, and longer than 255
1513 characters. It is described in
1514 <http://www.drregex.com/2019/02/variable-length-lookbehinds-actually.html>.
1515 Note that under "/i", a few single characters match two or
1517 three other characters. This makes them variable length, and
1518 the 255 length applies to the maximum number of characters in
1519 the match. For example "qr/\N{LATIN SMALL LETTER SHARP S}/i"
1520 matches the sequence "ss". Your lookbehind assertion could
1521 contain 127 Sharp S characters under "/i", but adding a 128th
1522 would generate a compilation error, as that could match 256 "s"
1523 characters in a row.
1524 The use of "\K" inside of another lookaround assertion is
1526 allowed, but the behaviour is currently not well defined.
1527 For various reasons "\K" may be significantly more efficient
1529 than the equivalent "(?<=...)" construct, and it is especially
1530 useful in situations where you want to efficiently remove
1531 something following something else in a string. For instance
1532 s/(foo)bar/$1/g;
1534 can be rewritten as the much more efficient
1536 s/foo\Kbar//g;
1538 Use of the non-greedy modifier "?" may not give you the
1540 expected results if it is within a capturing group within the
1541 construct.
1542 The alphabetic forms (not including "\K" are experimental;
1544 using them yields a warning in the
1545 "experimental::alpha_assertions" category.
1546 "(?<!pattern)"
1548 "(*nlb:pattern)"
1549 "(*negative_lookbehind:pattern)"
1550 A zero-width negative lookbehind assertion. For example
1551 "/(?<!bar)foo/" matches any occurrence of "foo" that does not
1552 follow "bar".
1553 Prior to Perl 5.30, it worked only for fixed-width lookbehind,
1555 but starting in that release, it can handle variable lengths
1556 from 1 to 255 characters as an experimental feature. The
1557 feature is enabled automatically if you use a variable length
1558 lookbehind assertion, but will raise a warning at pattern
1559 compilation time, unless turned off, in the "experimental::vlb"
1560 category. This is to warn you that the exact behavior is
1561 subject to change should feedback from actual use in the field
1562 indicate to do so; or even complete removal if the problems
1563 found are not practically surmountable. You can achieve close
1564 to pre-5.30 behavior by fatalizing warnings in this category.
1565 There is a technique that can be used to handle variable length
1567 lookbehinds on earlier releases, and longer than 255
1568 characters. It is described in
1569 <http://www.drregex.com/2019/02/variable-length-lookbehinds-actually.html>.
1570 Note that under "/i", a few single characters match two or
1572 three other characters. This makes them variable length, and
1573 the 255 length applies to the maximum number of characters in
1574 the match. For example "qr/\N{LATIN SMALL LETTER SHARP S}/i"
1575 matches the sequence "ss". Your lookbehind assertion could
1576 contain 127 Sharp S characters under "/i", but adding a 128th
1577 would generate a compilation error, as that could match 256 "s"
1578 characters in a row.
1579 Use of the non-greedy modifier "?" may not give you the
1581 expected results if it is within a capturing group within the
1582 construct.
1583 The alphabetic forms are experimental; using them yields a
1585 warning in the "experimental::alpha_assertions" category.
1586 "(?<NAME>pattern)"
1588 "(?'NAME'pattern)"
1589 A named capture group. Identical in every respect to normal
1590 capturing parentheses "()" but for the additional fact that the
1591 group can be referred to by name in various regular expression
1592 constructs (like "\g{NAME}") and can be accessed by name after a
1593 successful match via "%+" or "%-". See perlvar for more details on
1594 the "%+" and "%-" hashes.
1595 If multiple distinct capture groups have the same name, then
1597 $+{NAME} will refer to the leftmost defined group in the match.
1598 The forms "(?'NAME'pattern)" and "(?<NAME>pattern)" are equivalent.
1600 NOTE: While the notation of this construct is the same as the
1602 similar function in .NET regexes, the behavior is not. In Perl the
1603 groups are numbered sequentially regardless of being named or not.
1604 Thus in the pattern
1605 /(x)(?<foo>y)(z)/
1607 $+{foo} will be the same as $2, and $3 will contain 'z' instead of
1609 the opposite which is what a .NET regex hacker might expect.
1610 Currently NAME is restricted to simple identifiers only. In other
1612 words, it must match "/^[_A-Za-z][_A-Za-z0-9]*\z/" or its Unicode
1613 extension (see utf8), though it isn't extended by the locale (see
1614 perllocale).
1615 NOTE: In order to make things easier for programmers with
1617 experience with the Python or PCRE regex engines, the pattern
1618 "(?P<NAME>pattern)" may be used instead of "(?<NAME>pattern)";
1619 however this form does not support the use of single quotes as a
1620 delimiter for the name.
1621 "\k<NAME>"
1623 "\k'NAME'"
1624 Named backreference. Similar to numeric backreferences, except that
1625 the group is designated by name and not number. If multiple groups
1626 have the same name then it refers to the leftmost defined group in
1627 the current match.
1628 It is an error to refer to a name not defined by a "(?<NAME>)"
1630 earlier in the pattern.
1631 Both forms are equivalent.
1633 NOTE: In order to make things easier for programmers with
1635 experience with the Python or PCRE regex engines, the pattern
1636 "(?P=NAME)" may be used instead of "\k<NAME>".
1637 "(?{ code })"
1639 WARNING: Using this feature safely requires that you understand its
1640 limitations. Code executed that has side effects may not perform
1641 identically from version to version due to the effect of future
1642 optimisations in the regex engine. For more information on this,
1643 see "Embedded Code Execution Frequency".
1644 This zero-width assertion executes any embedded Perl code. It
1646 always succeeds, and its return value is set as $^R.
1647 In literal patterns, the code is parsed at the same time as the
1649 surrounding code. While within the pattern, control is passed
1650 temporarily back to the perl parser, until the logically-balancing
1651 closing brace is encountered. This is similar to the way that an
1652 array index expression in a literal string is handled, for example
1653 "abc$array[ 1 + f('[') + g()]def"
1655 In particular, braces do not need to be balanced:
1657 s/abc(?{ f('{'); })/def/
1659 Even in a pattern that is interpolated and compiled at run-time,
1661 literal code blocks will be compiled once, at perl compile time;
1662 the following prints "ABCD":
1663 print "D";
1665 my $qr = qr/(?{ BEGIN { print "A" } })/;
1666 my $foo = "foo";
1667 /$foo$qr(?{ BEGIN { print "B" } })/;
1668 BEGIN { print "C" }
1669 In patterns where the text of the code is derived from run-time
1671 information rather than appearing literally in a source code
1672 /pattern/, the code is compiled at the same time that the pattern
1673 is compiled, and for reasons of security, "use re 'eval'" must be
1674 in scope. This is to stop user-supplied patterns containing code
1675 snippets from being executable.
1676 In situations where you need to enable this with "use re 'eval'",
1678 you should also have taint checking enabled. Better yet, use the
1679 carefully constrained evaluation within a Safe compartment. See
1680 perlsec for details about both these mechanisms.
1681 From the viewpoint of parsing, lexical variable scope and closures,
1683 /AAA(?{ BBB })CCC/
1685 behaves approximately like
1687 /AAA/ && do { BBB } && /CCC/
1689 Similarly,
1691 qr/AAA(?{ BBB })CCC/
1693 behaves approximately like
1695 sub { /AAA/ && do { BBB } && /CCC/ }
1697 In particular:
1699 { my $i = 1; $r = qr/(?{ print $i })/ }
1701 my $i = 2;
1702 /$r/; # prints "1"
1703 Inside a "(?{...})" block, $_ refers to the string the regular
1705 expression is matching against. You can also use "pos()" to know
1706 what is the current position of matching within this string.
1707 The code block introduces a new scope from the perspective of
1709 lexical variable declarations, but not from the perspective of
1710 "local" and similar localizing behaviours. So later code blocks
1711 within the same pattern will still see the values which were
1712 localized in earlier blocks. These accumulated localizations are
1713 undone either at the end of a successful match, or if the assertion
1714 is backtracked (compare "Backtracking"). For example,
1715 $_ = 'a' x 8;
1717 m<
1718 (?{ $cnt = 0 }) # Initialize $cnt.
1719 (
1720 a
1721 (?{
1722 local $cnt = $cnt + 1; # Update $cnt,
1723 # backtracking-safe.
1724 })
1725 )*
1726 aaaa
1727 (?{ $res = $cnt }) # On success copy to
1728 # non-localized location.
1729 >x;
1730 will initially increment $cnt up to 8; then during backtracking,
1732 its value will be unwound back to 4, which is the value assigned to
1733 $res. At the end of the regex execution, $cnt will be wound back
1734 to its initial value of 0.
1735 This assertion may be used as the condition in a
1737 (?(condition)yes-pattern|no-pattern)
1739 switch. If not used in this way, the result of evaluation of code
1741 is put into the special variable $^R. This happens immediately, so
1742 $^R can be used from other "(?{ code })" assertions inside the same
1743 regular expression.
1744 The assignment to $^R above is properly localized, so the old value
1746 of $^R is restored if the assertion is backtracked; compare
1747 "Backtracking".
1748 Note that the special variable $^N is particularly useful with
1750 code blocks to capture the results of submatches in variables
1751 without having to keep track of the number of nested parentheses.
1752 For example:
1753 $_ = "The brown fox jumps over the lazy dog";
1755 /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i;
1756 print "color = $color, animal = $animal\n";
1757 "(??{ code })"
1759 WARNING: Using this feature safely requires that you understand its
1760 limitations. Code executed that has side effects may not perform
1761 identically from version to version due to the effect of future
1762 optimisations in the regex engine. For more information on this,
1763 see "Embedded Code Execution Frequency".
1764 This is a "postponed" regular subexpression. It behaves in exactly
1766 the same way as a "(?{ code })" code block as described above,
1767 except that its return value, rather than being assigned to $^R, is
1768 treated as a pattern, compiled if it's a string (or used as-is if
1769 its a qr// object), then matched as if it were inserted instead of
1770 this construct.
1771 During the matching of this sub-pattern, it has its own set of
1773 captures which are valid during the sub-match, but are discarded
1774 once control returns to the main pattern. For example, the
1775 following matches, with the inner pattern capturing "B" and
1776 matching "BB", while the outer pattern captures "A";
1777 my $inner = '(.)\1';
1779 "ABBA" =~ /^(.)(??{ $inner })\1/;
1780 print $1; # prints "A";
1781 Note that this means that there is no way for the inner pattern to
1783 refer to a capture group defined outside. (The code block itself
1784 can use $1, etc., to refer to the enclosing pattern's capture
1785 groups.) Thus, although
1786 ('a' x 100)=~/(??{'(.)' x 100})/
1788 will match, it will not set $1 on exit.
1790 The following pattern matches a parenthesized group:
1792 $re = qr{
1794 \(
1795 (?:
1796 (?> [^()]+ ) # Non-parens without backtracking
1797 |
1798 (??{ $re }) # Group with matching parens
1799 )*
1800 \)
1801 }x;
1802 See also "(?PARNO)" for a different, more efficient way to
1804 accomplish the same task.
1805 Executing a postponed regular expression too many times without
1807 consuming any input string will also result in a fatal error. The
1808 depth at which that happens is compiled into perl, so it can be
1809 changed with a custom build.
1810 "(?PARNO)" "(?-PARNO)" "(?+PARNO)" "(?R)" "(?0)"
1812 Recursive subpattern. Treat the contents of a given capture buffer
1813 in the current pattern as an independent subpattern and attempt to
1814 match it at the current position in the string. Information about
1815 capture state from the caller for things like backreferences is
1816 available to the subpattern, but capture buffers set by the
1817 subpattern are not visible to the caller.
1818 Similar to "(??{ code })" except that it does not involve executing
1820 any code or potentially compiling a returned pattern string;
1821 instead it treats the part of the current pattern contained within
1822 a specified capture group as an independent pattern that must match
1823 at the current position. Also different is the treatment of capture
1824 buffers, unlike "(??{ code })" recursive patterns have access to
1825 their caller's match state, so one can use backreferences safely.
1826 PARNO is a sequence of digits (not starting with 0) whose value
1828 reflects the paren-number of the capture group to recurse to.
1829 "(?R)" recurses to the beginning of the whole pattern. "(?0)" is an
1830 alternate syntax for "(?R)". If PARNO is preceded by a plus or
1831 minus sign then it is assumed to be relative, with negative numbers
1832 indicating preceding capture groups and positive ones following.
1833 Thus "(?-1)" refers to the most recently declared group, and
1834 "(?+1)" indicates the next group to be declared. Note that the
1835 counting for relative recursion differs from that of relative
1836 backreferences, in that with recursion unclosed groups are
1837 included.
1838 The following pattern matches a function "foo()" which may contain
1840 balanced parentheses as the argument.
1841 $re = qr{ ( # paren group 1 (full function)
1843 foo
1844 ( # paren group 2 (parens)
1845 \(
1846 ( # paren group 3 (contents of parens)
1847 (?:
1848 (?> [^()]+ ) # Non-parens without backtracking
1849 |
1850 (?2) # Recurse to start of paren group 2
1851 )*
1852 )
1853 \)
1854 )
1855 )
1856 }x;
1857 If the pattern was used as follows
1859 'foo(bar(baz)+baz(bop))'=~/$re/
1861 and print "\$1 = $1\n",
1862 "\$2 = $2\n",
1863 "\$3 = $3\n";
1864 the output produced should be the following:
1866 $1 = foo(bar(baz)+baz(bop))
1868 $2 = (bar(baz)+baz(bop))
1869 $3 = bar(baz)+baz(bop)
1870 If there is no corresponding capture group defined, then it is a
1872 fatal error. Recursing deeply without consuming any input string
1873 will also result in a fatal error. The depth at which that happens
1874 is compiled into perl, so it can be changed with a custom build.
1875 The following shows how using negative indexing can make it easier
1877 to embed recursive patterns inside of a "qr//" construct for later
1878 use:
1879 my $parens = qr/(\((?:[^()]++|(?-1))*+\))/;
1881 if (/foo $parens \s+ \+ \s+ bar $parens/x) {
1882 # do something here...
1883 }
1884 Note that this pattern does not behave the same way as the
1886 equivalent PCRE or Python construct of the same form. In Perl you
1887 can backtrack into a recursed group, in PCRE and Python the
1888 recursed into group is treated as atomic. Also, modifiers are
1889 resolved at compile time, so constructs like "(?i:(?1))" or
1890 "(?:(?i)(?1))" do not affect how the sub-pattern will be processed.
1891 "(?&NAME)"
1893 Recurse to a named subpattern. Identical to "(?PARNO)" except that
1894 the parenthesis to recurse to is determined by name. If multiple
1895 parentheses have the same name, then it recurses to the leftmost.
1896 It is an error to refer to a name that is not declared somewhere in
1898 the pattern.
1899 NOTE: In order to make things easier for programmers with
1901 experience with the Python or PCRE regex engines the pattern
1902 "(?P>NAME)" may be used instead of "(?&NAME)".
1903 "(?(condition)yes-pattern|no-pattern)"
1905 "(?(condition)yes-pattern)"
1906 Conditional expression. Matches yes-pattern if condition yields a
1907 true value, matches no-pattern otherwise. A missing pattern always
1908 matches.
1909 "(condition)" should be one of:
1911 an integer in parentheses
1913 (which is valid if the corresponding pair of parentheses
1914 matched);
1915 a lookahead/lookbehind/evaluate zero-width assertion;
1917 a name in angle brackets or single quotes
1918 (which is valid if a group with the given name matched);
1919 the special symbol "(R)"
1921 (true when evaluated inside of recursion or eval).
1922 Additionally the "R" may be followed by a number, (which will
1923 be true when evaluated when recursing inside of the appropriate
1924 group), or by "&NAME", in which case it will be true only when
1925 evaluated during recursion in the named group.
1926 Here's a summary of the possible predicates:
1928 "(1)" "(2)" ...
1930 Checks if the numbered capturing group has matched something.
1931 Full syntax: "(?(1)then|else)"
1932 "(<NAME>)" "('NAME')"
1934 Checks if a group with the given name has matched something.
1935 Full syntax: "(?(<name>)then|else)"
1936 "(?=...)" "(?!...)" "(?<=...)" "(?<!...)"
1938 Checks whether the pattern matches (or does not match, for the
1939 "!" variants). Full syntax: "(?(?=lookahead)then|else)"
1940 "(?{ CODE })"
1942 Treats the return value of the code block as the condition.
1943 Full syntax: "(?(?{ code })then|else)"
1944 "(R)"
1946 Checks if the expression has been evaluated inside of
1947 recursion. Full syntax: "(?(R)then|else)"
1948 "(R1)" "(R2)" ...
1950 Checks if the expression has been evaluated while executing
1951 directly inside of the n-th capture group. This check is the
1952 regex equivalent of
1953 if ((caller(0))[3] eq 'subname') { ... }
1955 In other words, it does not check the full recursion stack.
1957 Full syntax: "(?(R1)then|else)"
1959 "(R&NAME)"
1961 Similar to "(R1)", this predicate checks to see if we're
1962 executing directly inside of the leftmost group with a given
1963 name (this is the same logic used by "(?&NAME)" to
1964 disambiguate). It does not check the full stack, but only the
1965 name of the innermost active recursion. Full syntax:
1966 "(?(R&name)then|else)"
1967 "(DEFINE)"
1969 In this case, the yes-pattern is never directly executed, and
1970 no no-pattern is allowed. Similar in spirit to "(?{0})" but
1971 more efficient. See below for details. Full syntax:
1972 "(?(DEFINE)definitions...)"
1973 For example:
1975 m{ ( \( )?
1977 [^()]+
1978 (?(1) \) )
1979 }x
1980 matches a chunk of non-parentheses, possibly included in
1982 parentheses themselves.
1983 A special form is the "(DEFINE)" predicate, which never executes
1985 its yes-pattern directly, and does not allow a no-pattern. This
1986 allows one to define subpatterns which will be executed only by the
1987 recursion mechanism. This way, you can define a set of regular
1988 expression rules that can be bundled into any pattern you choose.
1989 It is recommended that for this usage you put the DEFINE block at
1991 the end of the pattern, and that you name any subpatterns defined
1992 within it.
1993 Also, it's worth noting that patterns defined this way probably
1995 will not be as efficient, as the optimizer is not very clever about
1996 handling them.
1997 An example of how this might be used is as follows:
1999 /(?<NAME>(?&NAME_PAT))(?<ADDR>(?&ADDRESS_PAT))
2001 (?(DEFINE)
2002 (?<NAME_PAT>....)
2003 (?<ADDRESS_PAT>....)
2004 )/x
2005 Note that capture groups matched inside of recursion are not
2007 accessible after the recursion returns, so the extra layer of
2008 capturing groups is necessary. Thus $+{NAME_PAT} would not be
2009 defined even though $+{NAME} would be.
2010 Finally, keep in mind that subpatterns created inside a DEFINE
2012 block count towards the absolute and relative number of captures,
2013 so this:
2014 my @captures = "a" =~ /(.) # First capture
2016 (?(DEFINE)
2017 (?<EXAMPLE> 1 ) # Second capture
2018 )/x;
2019 say scalar @captures;
2020 Will output 2, not 1. This is particularly important if you intend
2022 to compile the definitions with the "qr//" operator, and later
2023 interpolate them in another pattern.
2024 "(?>pattern)"
2026 "(*atomic:pattern)"
2027 An "independent" subexpression, one which matches the substring
2028 that a standalone pattern would match if anchored at the given
2029 position, and it matches nothing other than this substring. This
2030 construct is useful for optimizations of what would otherwise be
2031 "eternal" matches, because it will not backtrack (see
2032 "Backtracking"). It may also be useful in places where the "grab
2033 all you can, and do not give anything back" semantic is desirable.
2034 For example: "^(?>a*)ab" will never match, since "(?>a*)" (anchored
2036 at the beginning of string, as above) will match all characters "a"
2037 at the beginning of string, leaving no "a" for "ab" to match. In
2038 contrast, "a*ab" will match the same as "a+b", since the match of
2039 the subgroup "a*" is influenced by the following group "ab" (see
2040 "Backtracking"). In particular, "a*" inside "a*ab" will match
2041 fewer characters than a standalone "a*", since this makes the tail
2042 match.
2043 "(?>pattern)" does not disable backtracking altogether once it has
2045 matched. It is still possible to backtrack past the construct, but
2046 not into it. So "((?>a*)|(?>b*))ar" will still match "bar".
2047 An effect similar to "(?>pattern)" may be achieved by writing
2049 "(?=(pattern))\g{-1}". This matches the same substring as a
2050 standalone "a+", and the following "\g{-1}" eats the matched
2051 string; it therefore makes a zero-length assertion into an analogue
2052 of "(?>...)". (The difference between these two constructs is that
2053 the second one uses a capturing group, thus shifting ordinals of
2054 backreferences in the rest of a regular expression.)
2055 Consider this pattern:
2057 m{ \(
2059 (
2060 [^()]+ # x+
2061 |
2062 \( [^()]* \)
2063 )+
2064 \)
2065 }x
2066 That will efficiently match a nonempty group with matching
2068 parentheses two levels deep or less. However, if there is no such
2069 group, it will take virtually forever on a long string. That's
2070 because there are so many different ways to split a long string
2071 into several substrings. This is what "(.+)+" is doing, and
2072 "(.+)+" is similar to a subpattern of the above pattern. Consider
2073 how the pattern above detects no-match on "((()aaaaaaaaaaaaaaaaaa"
2074 in several seconds, but that each extra letter doubles this time.
2075 This exponential performance will make it appear that your program
2076 has hung. However, a tiny change to this pattern
2077 m{ \(
2079 (
2080 (?> [^()]+ ) # change x+ above to (?> x+ )
2081 |
2082 \( [^()]* \)
2083 )+
2084 \)
2085 }x
2086 which uses "(?>...)" matches exactly when the one above does
2088 (verifying this yourself would be a productive exercise), but
2089 finishes in a fourth the time when used on a similar string with
2090 1000000 "a"s. Be aware, however, that, when this construct is
2091 followed by a quantifier, it currently triggers a warning message
2092 under the "use warnings" pragma or -w switch saying it "matches
2093 null string many times in regex".
2094 On simple groups, such as the pattern "(?> [^()]+ )", a comparable
2096 effect may be achieved by negative lookahead, as in "[^()]+ (?!
2097 [^()] )". This was only 4 times slower on a string with 1000000
2098 "a"s.
2099 The "grab all you can, and do not give anything back" semantic is
2101 desirable in many situations where on the first sight a simple
2102 "()*" looks like the correct solution. Suppose we parse text with
2103 comments being delimited by "#" followed by some optional
2104 (horizontal) whitespace. Contrary to its appearance, "#[ \t]*" is
2105 not the correct subexpression to match the comment delimiter,
2106 because it may "give up" some whitespace if the remainder of the
2107 pattern can be made to match that way. The correct answer is
2108 either one of these:
2109 (?>#[ \t]*)
2111 #[ \t]*(?![ \t])
2112 For example, to grab non-empty comments into $1, one should use
2114 either one of these:
2115 / (?> \# [ \t]* ) ( .+ ) /x;
2117 / \# [ \t]* ( [^ \t] .* ) /x;
2118 Which one you pick depends on which of these expressions better
2120 reflects the above specification of comments.
2121 In some literature this construct is called "atomic matching" or
2123 "possessive matching".
2124 Possessive quantifiers are equivalent to putting the item they are
2126 applied to inside of one of these constructs. The following
2127 equivalences apply:
2128 Quantifier Form Bracketing Form
2130 --------------- ---------------
2131 PAT*+ (?>PAT*)
2132 PAT++ (?>PAT+)
2133 PAT?+ (?>PAT?)
2134 PAT{min,max}+ (?>PAT{min,max})
2135 Nested "(?>...)" constructs are not no-ops, even if at first glance
2137 they might seem to be. This is because the nested "(?>...)" can
2138 restrict internal backtracking that otherwise might occur. For
2139 example,
2140 "abc" =~ /(?>a[bc]*c)/
2142 matches, but
2144 "abc" =~ /(?>a(?>[bc]*)c)/
2146 does not.
2148 The alphabetic form ("(*atomic:...)") is experimental; using it
2150 yields a warning in the "experimental::alpha_assertions" category.
2151 "(?[ ])"
2153 See "Extended Bracketed Character Classes" in perlrecharclass.
2154 Note that this feature is currently experimental; using it yields a
2156 warning in the "experimental::regex_sets" category.
2157 Backtracking
2159 NOTE: This section presents an abstract approximation of regular
2160 expression behavior. For a more rigorous (and complicated) view of the
2161 rules involved in selecting a match among possible alternatives, see
2162 "Combining RE Pieces".
2163 A fundamental feature of regular expression matching involves the
2165 notion called backtracking, which is currently used (when needed) by
2166 all regular non-possessive expression quantifiers, namely "*", "*?",
2167 "+", "+?", "{n,m}", and "{n,m}?". Backtracking is often optimized
2168 internally, but the general principle outlined here is valid.
2169 For a regular expression to match, the entire regular expression must
2171 match, not just part of it. So if the beginning of a pattern
2172 containing a quantifier succeeds in a way that causes later parts in
2173 the pattern to fail, the matching engine backs up and recalculates the
2174 beginning part--that's why it's called backtracking.
2175 Here is an example of backtracking: Let's say you want to find the
2177 word following "foo" in the string "Food is on the foo table.":
2178 $_ = "Food is on the foo table.";
2180 if ( /\b(foo)\s+(\w+)/i ) {
2181 print "$2 follows $1.\n";
2182 }
2183 When the match runs, the first part of the regular expression
2185 ("\b(foo)") finds a possible match right at the beginning of the
2186 string, and loads up $1 with "Foo". However, as soon as the matching
2187 engine sees that there's no whitespace following the "Foo" that it had
2188 saved in $1, it realizes its mistake and starts over again one
2189 character after where it had the tentative match. This time it goes
2190 all the way until the next occurrence of "foo". The complete regular
2191 expression matches this time, and you get the expected output of "table
2192 follows foo."
2193 Sometimes minimal matching can help a lot. Imagine you'd like to match
2195 everything between "foo" and "bar". Initially, you write something
2196 like this:
2197 $_ = "The food is under the bar in the barn.";
2199 if ( /foo(.*)bar/ ) {
2200 print "got <$1>\n";
2201 }
2202 Which perhaps unexpectedly yields:
2204 got <d is under the bar in the >
2206 That's because ".*" was greedy, so you get everything between the first
2208 "foo" and the last "bar". Here it's more effective to use minimal
2209 matching to make sure you get the text between a "foo" and the first
2210 "bar" thereafter.
2211 if ( /foo(.*?)bar/ ) { print "got <$1>\n" }
2213 got <d is under the >
2214 Here's another example. Let's say you'd like to match a number at the
2216 end of a string, and you also want to keep the preceding part of the
2217 match. So you write this:
2218 $_ = "I have 2 numbers: 53147";
2220 if ( /(.*)(\d*)/ ) { # Wrong!
2221 print "Beginning is <$1>, number is <$2>.\n";
2222 }
2223 That won't work at all, because ".*" was greedy and gobbled up the
2225 whole string. As "\d*" can match on an empty string the complete
2226 regular expression matched successfully.
2227 Beginning is <I have 2 numbers: 53147>, number is <>.
2229 Here are some variants, most of which don't work:
2231 $_ = "I have 2 numbers: 53147";
2233 @pats = qw{
2234 (.*)(\d*)
2235 (.*)(\d+)
2236 (.*?)(\d*)
2237 (.*?)(\d+)
2238 (.*)(\d+)$
2239 (.*?)(\d+)$
2240 (.*)\b(\d+)$
2241 (.*\D)(\d+)$
2242 };
2243 for $pat (@pats) {
2245 printf "%-12s ", $pat;
2246 if ( /$pat/ ) {
2247 print "<$1> <$2>\n";
2248 } else {
2249 print "FAIL\n";
2250 }
2251 }
2252 That will print out:
2254 (.*)(\d*) <I have 2 numbers: 53147> <>
2256 (.*)(\d+) <I have 2 numbers: 5314> <7>
2257 (.*?)(\d*) <> <>
2258 (.*?)(\d+) <I have > <2>
2259 (.*)(\d+)$ <I have 2 numbers: 5314> <7>
2260 (.*?)(\d+)$ <I have 2 numbers: > <53147>
2261 (.*)\b(\d+)$ <I have 2 numbers: > <53147>
2262 (.*\D)(\d+)$ <I have 2 numbers: > <53147>
2263 As you see, this can be a bit tricky. It's important to realize that a
2265 regular expression is merely a set of assertions that gives a
2266 definition of success. There may be 0, 1, or several different ways
2267 that the definition might succeed against a particular string. And if
2268 there are multiple ways it might succeed, you need to understand
2269 backtracking to know which variety of success you will achieve.
2270 When using lookahead assertions and negations, this can all get even
2272 trickier. Imagine you'd like to find a sequence of non-digits not
2273 followed by "123". You might try to write that as
2274 $_ = "ABC123";
2276 if ( /^\D*(?!123)/ ) { # Wrong!
2277 print "Yup, no 123 in $_\n";
2278 }
2279 But that isn't going to match; at least, not the way you're hoping. It
2281 claims that there is no 123 in the string. Here's a clearer picture of
2282 why that pattern matches, contrary to popular expectations:
2283 $x = 'ABC123';
2285 $y = 'ABC445';
2286 print "1: got $1\n" if $x =~ /^(ABC)(?!123)/;
2288 print "2: got $1\n" if $y =~ /^(ABC)(?!123)/;
2289 print "3: got $1\n" if $x =~ /^(\D*)(?!123)/;
2291 print "4: got $1\n" if $y =~ /^(\D*)(?!123)/;
2292 This prints
2294 2: got ABC
2296 3: got AB
2297 4: got ABC
2298 You might have expected test 3 to fail because it seems to a more
2300 general purpose version of test 1. The important difference between
2301 them is that test 3 contains a quantifier ("\D*") and so can use
2302 backtracking, whereas test 1 will not. What's happening is that you've
2303 asked "Is it true that at the start of $x, following 0 or more non-
2304 digits, you have something that's not 123?" If the pattern matcher had
2305 let "\D*" expand to "ABC", this would have caused the whole pattern to
2306 fail.
2307 The search engine will initially match "\D*" with "ABC". Then it will
2309 try to match "(?!123)" with "123", which fails. But because a
2310 quantifier ("\D*") has been used in the regular expression, the search
2311 engine can backtrack and retry the match differently in the hope of
2312 matching the complete regular expression.
2313 The pattern really, really wants to succeed, so it uses the standard
2315 pattern back-off-and-retry and lets "\D*" expand to just "AB" this
2316 time. Now there's indeed something following "AB" that is not "123".
2317 It's "C123", which suffices.
2318 We can deal with this by using both an assertion and a negation. We'll
2320 say that the first part in $1 must be followed both by a digit and by
2321 something that's not "123". Remember that the lookaheads are zero-
2322 width expressions--they only look, but don't consume any of the string
2323 in their match. So rewriting this way produces what you'd expect; that
2324 is, case 5 will fail, but case 6 succeeds:
2325 print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/;
2327 print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/;
2328 6: got ABC
2330 In other words, the two zero-width assertions next to each other work
2332 as though they're ANDed together, just as you'd use any built-in
2333 assertions: "/^$/" matches only if you're at the beginning of the line
2334 AND the end of the line simultaneously. The deeper underlying truth is
2335 that juxtaposition in regular expressions always means AND, except when
2336 you write an explicit OR using the vertical bar. "/ab/" means match
2337 "a" AND (then) match "b", although the attempted matches are made at
2338 different positions because "a" is not a zero-width assertion, but a
2339 one-width assertion.
2340 WARNING: Particularly complicated regular expressions can take
2342 exponential time to solve because of the immense number of possible
2343 ways they can use backtracking to try for a match. For example,
2344 without internal optimizations done by the regular expression engine,
2345 this will take a painfully long time to run:
2346 'aaaaaaaaaaaa' =~ /((a{0,5}){0,5})*[c]/
2348 And if you used "*"'s in the internal groups instead of limiting them
2350 to 0 through 5 matches, then it would take forever--or until you ran
2351 out of stack space. Moreover, these internal optimizations are not
2352 always applicable. For example, if you put "{0,5}" instead of "*" on
2353 the external group, no current optimization is applicable, and the
2354 match takes a long time to finish.
2355 A powerful tool for optimizing such beasts is what is known as an
2357 "independent group", which does not backtrack (see ""(?>pattern)"").
2358 Note also that zero-length lookahead/lookbehind assertions will not
2359 backtrack to make the tail match, since they are in "logical" context:
2360 only whether they match is considered relevant. For an example where
2361 side-effects of lookahead might have influenced the following match,
2362 see ""(?>pattern)"".
2363 Script Runs
2365 A script run is basically a sequence of characters, all from the same
2366 Unicode script (see "Scripts" in perlunicode), such as Latin or Greek.
2367 In most places a single word would never be written in multiple
2368 scripts, unless it is a spoofing attack. An infamous example, is
2369 paypal.com
2371 Those letters could all be Latin (as in the example just above), or
2373 they could be all Cyrillic (except for the dot), or they could be a
2374 mixture of the two. In the case of an internet address the ".com"
2375 would be in Latin, And any Cyrillic ones would cause it to be a
2376 mixture, not a script run. Someone clicking on such a link would not
2377 be directed to the real Paypal website, but an attacker would craft a
2378 look-alike one to attempt to gather sensitive information from the
2379 person.
2380 Starting in Perl 5.28, it is now easy to detect strings that aren't
2382 script runs. Simply enclose just about any pattern like either of
2383 these:
2384 (*script_run:pattern)
2386 (*sr:pattern)
2387 What happens is that after pattern succeeds in matching, it is
2389 subjected to the additional criterion that every character in it must
2390 be from the same script (see exceptions below). If this isn't true,
2391 backtracking occurs until something all in the same script is found
2392 that matches, or all possibilities are exhausted. This can cause a lot
2393 of backtracking, but generally, only malicious input will result in
2394 this, though the slow down could cause a denial of service attack. If
2395 your needs permit, it is best to make the pattern atomic to cut down on
2396 the amount of backtracking. This is so likely to be what you want,
2397 that instead of writing this:
2398 (*script_run:(?>pattern))
2400 you can write either of these:
2402 (*atomic_script_run:pattern)
2404 (*asr:pattern)
2405 (See ""(?>pattern)"".)
2407 In Taiwan, Japan, and Korea, it is common for text to have a mixture of
2409 characters from their native scripts and base Chinese. Perl follows
2410 Unicode's UTS 39 (<http://unicode.org/reports/tr39/>) Unicode Security
2411 Mechanisms in allowing such mixtures. For example, the Japanese
2412 scripts Katakana and Hiragana are commonly mixed together in practice,
2413 along with some Chinese characters, and hence are treated as being in a
2414 single script run by Perl.
2415 The rules used for matching decimal digits are slightly stricter. Many
2417 scripts have their own sets of digits equivalent to the Western 0
2418 through 9 ones. A few, such as Arabic, have more than one set. For a
2419 string to be considered a script run, all digits in it must come from
2420 the same set of ten, as determined by the first digit encountered. As
2421 an example,
2422 qr/(*script_run: \d+ \b )/x
2424 guarantees that the digits matched will all be from the same set of 10.
2426 You won't get a look-alike digit from a different script that has a
2427 different value than what it appears to be.
2428 Unicode has three pseudo scripts that are handled specially.
2430 "Unknown" is applied to code points whose meaning has yet to be
2432 determined. Perl currently will match as a script run, any single
2433 character string consisting of one of these code points. But any
2434 string longer than one code point containing one of these will not be
2435 considered a script run.
2436 "Inherited" is applied to characters that modify another, such as an
2438 accent of some type. These are considered to be in the script of the
2439 master character, and so never cause a script run to not match.
2440 The other one is "Common". This consists of mostly punctuation, emoji,
2442 and characters used in mathematics and music, the ASCII digits 0
2443 through 9, and full-width forms of these digits. These characters can
2444 appear intermixed in text in many of the world's scripts. These also
2445 don't cause a script run to not match. But like other scripts, all
2446 digits in a run must come from the same set of 10.
2447 This construct is non-capturing. You can add parentheses to pattern to
2449 capture, if desired. You will have to do this if you plan to use
2450 "(*ACCEPT) (*ACCEPT:arg)" and not have it bypass the script run
2451 checking.
2452 This feature is experimental, and the exact syntax and details of
2454 operation are subject to change; using it yields a warning in the
2455 "experimental::script_run" category.
2456 The "Script_Extensions" property as modified by UTS 39
2458 (<http://unicode.org/reports/tr39/>) is used as the basis for this
2459 feature.
2460 To summarize,
2462 · All length 0 or length 1 sequences are script runs.
2464 · A longer sequence is a script run if and only if all of the
2466 following conditions are met:
2467 1. No code point in the sequence has the "Script_Extension"
2471 property of "Unknown".
2472 This currently means that all code points in the sequence have
2474 been assigned by Unicode to be characters that aren't private
2475 use nor surrogate code points.
2476 2. All characters in the sequence come from the Common script
2478 and/or the Inherited script and/or a single other script.
2479 The script of a character is determined by the
2481 "Script_Extensions" property as modified by UTS 39
2482 (<http://unicode.org/reports/tr39/>), as described above.
2483 3. All decimal digits in the sequence come from the same block of
2485 10 consecutive digits.
2486 Special Backtracking Control Verbs
2488 These special patterns are generally of the form "(*VERB:arg)". Unless
2489 otherwise stated the arg argument is optional; in some cases, it is
2490 mandatory.
2491 Any pattern containing a special backtracking verb that allows an
2493 argument has the special behaviour that when executed it sets the
2494 current package's $REGERROR and $REGMARK variables. When doing so the
2495 following rules apply:
2496 On failure, the $REGERROR variable will be set to the arg value of the
2498 verb pattern, if the verb was involved in the failure of the match. If
2499 the arg part of the pattern was omitted, then $REGERROR will be set to
2500 the name of the last "(*MARK:NAME)" pattern executed, or to TRUE if
2501 there was none. Also, the $REGMARK variable will be set to FALSE.
2502 On a successful match, the $REGERROR variable will be set to FALSE, and
2504 the $REGMARK variable will be set to the name of the last
2505 "(*MARK:NAME)" pattern executed. See the explanation for the
2506 "(*MARK:NAME)" verb below for more details.
2507 NOTE: $REGERROR and $REGMARK are not magic variables like $1 and most
2509 other regex-related variables. They are not local to a scope, nor
2510 readonly, but instead are volatile package variables similar to
2511 $AUTOLOAD. They are set in the package containing the code that
2512 executed the regex (rather than the one that compiled it, where those
2513 differ). If necessary, you can use "local" to localize changes to
2514 these variables to a specific scope before executing a regex.
2515 If a pattern does not contain a special backtracking verb that allows
2517 an argument, then $REGERROR and $REGMARK are not touched at all.
2518 Verbs
2520 "(*PRUNE)" "(*PRUNE:NAME)"
2521 This zero-width pattern prunes the backtracking tree at the
2522 current point when backtracked into on failure. Consider the
2523 pattern "/A (*PRUNE) B/", where A and B are complex patterns.
2524 Until the "(*PRUNE)" verb is reached, A may backtrack as
2525 necessary to match. Once it is reached, matching continues in B,
2526 which may also backtrack as necessary; however, should B not
2527 match, then no further backtracking will take place, and the
2528 pattern will fail outright at the current starting position.
2529 The following example counts all the possible matching strings
2531 in a pattern (without actually matching any of them).
2532 'aaab' =~ /a+b?(?{print "$&\n"; $count++})(*FAIL)/;
2534 print "Count=$count\n";
2535 which produces:
2537 aaab
2539 aaa
2540 aa
2541 a
2542 aab
2543 aa
2544 a
2545 ab
2546 a
2547 Count=9
2548 If we add a "(*PRUNE)" before the count like the following
2550 'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/;
2552 print "Count=$count\n";
2553 we prevent backtracking and find the count of the longest
2555 matching string at each matching starting point like so:
2556 aaab
2558 aab
2559 ab
2560 Count=3
2561 Any number of "(*PRUNE)" assertions may be used in a pattern.
2563 See also "(?>pattern)" and possessive quantifiers for other ways
2565 to control backtracking. In some cases, the use of "(*PRUNE)"
2566 can be replaced with a "(?>pattern)" with no functional
2567 difference; however, "(*PRUNE)" can be used to handle cases that
2568 cannot be expressed using a "(?>pattern)" alone.
2569 "(*SKIP)" "(*SKIP:NAME)"
2571 This zero-width pattern is similar to "(*PRUNE)", except that on
2572 failure it also signifies that whatever text that was matched
2573 leading up to the "(*SKIP)" pattern being executed cannot be
2574 part of any match of this pattern. This effectively means that
2575 the regex engine "skips" forward to this position on failure and
2576 tries to match again, (assuming that there is sufficient room to
2577 match).
2578 The name of the "(*SKIP:NAME)" pattern has special significance.
2580 If a "(*MARK:NAME)" was encountered while matching, then it is
2581 that position which is used as the "skip point". If no "(*MARK)"
2582 of that name was encountered, then the "(*SKIP)" operator has no
2583 effect. When used without a name the "skip point" is where the
2584 match point was when executing the "(*SKIP)" pattern.
2585 Compare the following to the examples in "(*PRUNE)"; note the
2587 string is twice as long:
2588 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/;
2590 print "Count=$count\n";
2591 outputs
2593 aaab
2595 aaab
2596 Count=2
2597 Once the 'aaab' at the start of the string has matched, and the
2599 "(*SKIP)" executed, the next starting point will be where the
2600 cursor was when the "(*SKIP)" was executed.
2601 "(*MARK:NAME)" "(*:NAME)"
2603 This zero-width pattern can be used to mark the point reached in
2604 a string when a certain part of the pattern has been
2605 successfully matched. This mark may be given a name. A later
2606 "(*SKIP)" pattern will then skip forward to that point if
2607 backtracked into on failure. Any number of "(*MARK)" patterns
2608 are allowed, and the NAME portion may be duplicated.
2609 In addition to interacting with the "(*SKIP)" pattern,
2611 "(*MARK:NAME)" can be used to "label" a pattern branch, so that
2612 after matching, the program can determine which branches of the
2613 pattern were involved in the match.
2614 When a match is successful, the $REGMARK variable will be set to
2616 the name of the most recently executed "(*MARK:NAME)" that was
2617 involved in the match.
2618 This can be used to determine which branch of a pattern was
2620 matched without using a separate capture group for each branch,
2621 which in turn can result in a performance improvement, as perl
2622 cannot optimize "/(?:(x)|(y)|(z))/" as efficiently as something
2623 like "/(?:x(*MARK:x)|y(*MARK:y)|z(*MARK:z))/".
2624 When a match has failed, and unless another verb has been
2626 involved in failing the match and has provided its own name to
2627 use, the $REGERROR variable will be set to the name of the most
2628 recently executed "(*MARK:NAME)".
2629 See "(*SKIP)" for more details.
2631 As a shortcut "(*MARK:NAME)" can be written "(*:NAME)".
2633 "(*THEN)" "(*THEN:NAME)"
2635 This is similar to the "cut group" operator "::" from Perl 6.
2636 Like "(*PRUNE)", this verb always matches, and when backtracked
2637 into on failure, it causes the regex engine to try the next
2638 alternation in the innermost enclosing group (capturing or
2639 otherwise) that has alternations. The two branches of a
2640 "(?(condition)yes-pattern|no-pattern)" do not count as an
2641 alternation, as far as "(*THEN)" is concerned.
2642 Its name comes from the observation that this operation combined
2644 with the alternation operator ("|") can be used to create what
2645 is essentially a pattern-based if/then/else block:
2646 ( COND (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ )
2648 Note that if this operator is used and NOT inside of an
2650 alternation then it acts exactly like the "(*PRUNE)" operator.
2651 / A (*PRUNE) B /
2653 is the same as
2655 / A (*THEN) B /
2657 but
2659 / ( A (*THEN) B | C ) /
2661 is not the same as
2663 / ( A (*PRUNE) B | C ) /
2665 as after matching the A but failing on the B the "(*THEN)" verb
2667 will backtrack and try C; but the "(*PRUNE)" verb will simply
2668 fail.
2669 "(*COMMIT)" "(*COMMIT:arg)"
2671 This is the Perl 6 "commit pattern" "<commit>" or ":::". It's a
2672 zero-width pattern similar to "(*SKIP)", except that when
2673 backtracked into on failure it causes the match to fail
2674 outright. No further attempts to find a valid match by advancing
2675 the start pointer will occur again. For example,
2676 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/;
2678 print "Count=$count\n";
2679 outputs
2681 aaab
2683 Count=1
2684 In other words, once the "(*COMMIT)" has been entered, and if
2686 the pattern does not match, the regex engine will not try any
2687 further matching on the rest of the string.
2688 "(*FAIL)" "(*F)" "(*FAIL:arg)"
2690 This pattern matches nothing and always fails. It can be used to
2691 force the engine to backtrack. It is equivalent to "(?!)", but
2692 easier to read. In fact, "(?!)" gets optimised into "(*FAIL)"
2693 internally. You can provide an argument so that if the match
2694 fails because of this "FAIL" directive the argument can be
2695 obtained from $REGERROR.
2696 It is probably useful only when combined with "(?{})" or
2698 "(??{})".
2699 "(*ACCEPT)" "(*ACCEPT:arg)"
2701 This pattern matches nothing and causes the end of successful
2702 matching at the point at which the "(*ACCEPT)" pattern was
2703 encountered, regardless of whether there is actually more to
2704 match in the string. When inside of a nested pattern, such as
2705 recursion, or in a subpattern dynamically generated via
2706 "(??{})", only the innermost pattern is ended immediately.
2707 If the "(*ACCEPT)" is inside of capturing groups then the groups
2709 are marked as ended at the point at which the "(*ACCEPT)" was
2710 encountered. For instance:
2711 'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x;
2713 will match, and $1 will be "AB" and $2 will be "B", $3 will not
2715 be set. If another branch in the inner parentheses was matched,
2716 such as in the string 'ACDE', then the "D" and "E" would have to
2717 be matched as well.
2718 You can provide an argument, which will be available in the var
2720 $REGMARK after the match completes.
2721 Warning on "\1" Instead of $1
2723 Some people get too used to writing things like:
2724 $pattern =~ s/(\W)/\\\1/g;
2726 This is grandfathered (for \1 to \9) for the RHS of a substitute to
2728 avoid shocking the sed addicts, but it's a dirty habit to get into.
2729 That's because in PerlThink, the righthand side of an "s///" is a
2730 double-quoted string. "\1" in the usual double-quoted string means a
2731 control-A. The customary Unix meaning of "\1" is kludged in for
2732 "s///". However, if you get into the habit of doing that, you get
2733 yourself into trouble if you then add an "/e" modifier.
2734 s/(\d+)/ \1 + 1 /eg; # causes warning under -w
2736 Or if you try to do
2738 s/(\d+)/\1000/;
2740 You can't disambiguate that by saying "\{1}000", whereas you can fix it
2742 with "${1}000". The operation of interpolation should not be confused
2743 with the operation of matching a backreference. Certainly they mean
2744 two different things on the left side of the "s///".
2745 Repeated Patterns Matching a Zero-length Substring
2747 WARNING: Difficult material (and prose) ahead. This section needs a
2748 rewrite.
2749 Regular expressions provide a terse and powerful programming language.
2751 As with most other power tools, power comes together with the ability
2752 to wreak havoc.
2753 A common abuse of this power stems from the ability to make infinite
2755 loops using regular expressions, with something as innocuous as:
2756 'foo' =~ m{ ( o? )* }x;
2758 The "o?" matches at the beginning of ""foo"", and since the position in
2760 the string is not moved by the match, "o?" would match again and again
2761 because of the "*" quantifier. Another common way to create a similar
2762 cycle is with the looping modifier "/g":
2763 @matches = ( 'foo' =~ m{ o? }xg );
2765 or
2767 print "match: <$&>\n" while 'foo' =~ m{ o? }xg;
2769 or the loop implied by "split()".
2771 However, long experience has shown that many programming tasks may be
2773 significantly simplified by using repeated subexpressions that may
2774 match zero-length substrings. Here's a simple example being:
2775 @chars = split //, $string; # // is not magic in split
2777 ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// /
2778 Thus Perl allows such constructs, by forcefully breaking the infinite
2780 loop. The rules for this are different for lower-level loops given by
2781 the greedy quantifiers "*+{}", and for higher-level ones like the "/g"
2782 modifier or "split()" operator.
2783 The lower-level loops are interrupted (that is, the loop is broken)
2785 when Perl detects that a repeated expression matched a zero-length
2786 substring. Thus
2787 m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x;
2789 is made equivalent to
2791 m{ (?: NON_ZERO_LENGTH )* (?: ZERO_LENGTH )? }x;
2793 For example, this program
2795 #!perl -l
2797 "aaaaab" =~ /
2798 (?:
2799 a # non-zero
2800 | # or
2801 (?{print "hello"}) # print hello whenever this
2802 # branch is tried
2803 (?=(b)) # zero-width assertion
2804 )* # any number of times
2805 /x;
2806 print $&;
2807 print $1;
2808 prints
2810 hello
2812 aaaaa
2813 b
2814 Notice that "hello" is only printed once, as when Perl sees that the
2816 sixth iteration of the outermost "(?:)*" matches a zero-length string,
2817 it stops the "*".
2818 The higher-level loops preserve an additional state between iterations:
2820 whether the last match was zero-length. To break the loop, the
2821 following match after a zero-length match is prohibited to have a
2822 length of zero. This prohibition interacts with backtracking (see
2823 "Backtracking"), and so the second best match is chosen if the best
2824 match is of zero length.
2825 For example:
2827 $_ = 'bar';
2829 s/\w??/<$&>/g;
2830 results in "<><b><><a><><r><>". At each position of the string the
2832 best match given by non-greedy "??" is the zero-length match, and the
2833 second best match is what is matched by "\w". Thus zero-length matches
2834 alternate with one-character-long matches.
2835 Similarly, for repeated "m/()/g" the second-best match is the match at
2837 the position one notch further in the string.
2838 The additional state of being matched with zero-length is associated
2840 with the matched string, and is reset by each assignment to "pos()".
2841 Zero-length matches at the end of the previous match are ignored during
2842 "split".
2843 Combining RE Pieces
2845 Each of the elementary pieces of regular expressions which were
2846 described before (such as "ab" or "\Z") could match at most one
2847 substring at the given position of the input string. However, in a
2848 typical regular expression these elementary pieces are combined into
2849 more complicated patterns using combining operators "ST", "S|T", "S*"
2850 etc. (in these examples "S" and "T" are regular subexpressions).
2851 Such combinations can include alternatives, leading to a problem of
2853 choice: if we match a regular expression "a|ab" against "abc", will it
2854 match substring "a" or "ab"? One way to describe which substring is
2855 actually matched is the concept of backtracking (see "Backtracking").
2856 However, this description is too low-level and makes you think in terms
2857 of a particular implementation.
2858 Another description starts with notions of "better"/"worse". All the
2860 substrings which may be matched by the given regular expression can be
2861 sorted from the "best" match to the "worst" match, and it is the "best"
2862 match which is chosen. This substitutes the question of "what is
2863 chosen?" by the question of "which matches are better, and which are
2864 worse?".
2865 Again, for elementary pieces there is no such question, since at most
2867 one match at a given position is possible. This section describes the
2868 notion of better/worse for combining operators. In the description
2869 below "S" and "T" are regular subexpressions.
2870 "ST"
2872 Consider two possible matches, "AB" and "A'B'", "A" and "A'" are
2873 substrings which can be matched by "S", "B" and "B'" are substrings
2874 which can be matched by "T".
2875 If "A" is a better match for "S" than "A'", "AB" is a better match
2877 than "A'B'".
2878 If "A" and "A'" coincide: "AB" is a better match than "AB'" if "B"
2880 is a better match for "T" than "B'".
2881 "S|T"
2883 When "S" can match, it is a better match than when only "T" can
2884 match.
2885 Ordering of two matches for "S" is the same as for "S". Similar
2887 for two matches for "T".
2888 "S{REPEAT_COUNT}"
2890 Matches as "SSS...S" (repeated as many times as necessary).
2891 "S{min,max}"
2893 Matches as "S{max}|S{max-1}|...|S{min+1}|S{min}".
2894 "S{min,max}?"
2896 Matches as "S{min}|S{min+1}|...|S{max-1}|S{max}".
2897 "S?", "S*", "S+"
2899 Same as "S{0,1}", "S{0,BIG_NUMBER}", "S{1,BIG_NUMBER}"
2900 respectively.
2901 "S??", "S*?", "S+?"
2903 Same as "S{0,1}?", "S{0,BIG_NUMBER}?", "S{1,BIG_NUMBER}?"
2904 respectively.
2905 "(?>S)"
2907 Matches the best match for "S" and only that.
2908 "(?=S)", "(?<=S)"
2910 Only the best match for "S" is considered. (This is important only
2911 if "S" has capturing parentheses, and backreferences are used
2912 somewhere else in the whole regular expression.)
2913 "(?!S)", "(?<!S)"
2915 For this grouping operator there is no need to describe the
2916 ordering, since only whether or not "S" can match is important.
2917 "(??{ EXPR })", "(?PARNO)"
2919 The ordering is the same as for the regular expression which is the
2920 result of EXPR, or the pattern contained by capture group PARNO.
2921 "(?(condition)yes-pattern|no-pattern)"
2923 Recall that which of yes-pattern or no-pattern actually matches is
2924 already determined. The ordering of the matches is the same as for
2925 the chosen subexpression.
2926 The above recipes describe the ordering of matches at a given position.
2928 One more rule is needed to understand how a match is determined for the
2929 whole regular expression: a match at an earlier position is always
2930 better than a match at a later position.
2931 Creating Custom RE Engines
2933 As of Perl 5.10.0, one can create custom regular expression engines.
2934 This is not for the faint of heart, as they have to plug in at the C
2935 level. See perlreapi for more details.
2936 As an alternative, overloaded constants (see overload) provide a simple
2938 way to extend the functionality of the RE engine, by substituting one
2939 pattern for another.
2940 Suppose that we want to enable a new RE escape-sequence "\Y|" which
2942 matches at a boundary between whitespace characters and non-whitespace
2943 characters. Note that "(?=\S)(?<!\S)|(?!\S)(?<=\S)" matches exactly at
2944 these positions, so we want to have each "\Y|" in the place of the more
2945 complicated version. We can create a module "customre" to do this:
2946 package customre;
2948 use overload;
2949 sub import {
2951 shift;
2952 die "No argument to customre::import allowed" if @_;
2953 overload::constant 'qr' => \&convert;
2954 }
2955 sub invalid { die "/$_[0]/: invalid escape '\\$_[1]'"}
2957 # We must also take care of not escaping the legitimate \\Y|
2959 # sequence, hence the presence of '\\' in the conversion rules.
2960 my %rules = ( '\\' => '\\\\',
2961 'Y|' => qr/(?=\S)(?<!\S)|(?!\S)(?<=\S)/ );
2962 sub convert {
2963 my $re = shift;
2964 $re =~ s{
2965 \\ ( \\ | Y . )
2966 }
2967 { $rules{$1} or invalid($re,$1) }sgex;
2968 return $re;
2969 }
2970 Now "use customre" enables the new escape in constant regular
2972 expressions, i.e., those without any runtime variable interpolations.
2973 As documented in overload, this conversion will work only over literal
2974 parts of regular expressions. For "\Y|$re\Y|" the variable part of
2975 this regular expression needs to be converted explicitly (but only if
2976 the special meaning of "\Y|" should be enabled inside $re):
2977 use customre;
2979 $re = <>;
2980 chomp $re;
2981 $re = customre::convert $re;
2982 /\Y|$re\Y|/;
2983 Embedded Code Execution Frequency
2985 The exact rules for how often "(??{})" and "(?{})" are executed in a
2986 pattern are unspecified. In the case of a successful match you can
2987 assume that they DWIM and will be executed in left to right order the
2988 appropriate number of times in the accepting path of the pattern as
2989 would any other meta-pattern. How non-accepting pathways and match
2990 failures affect the number of times a pattern is executed is
2991 specifically unspecified and may vary depending on what optimizations
2992 can be applied to the pattern and is likely to change from version to
2993 version.
2994 For instance in
2996 "aaabcdeeeee"=~/a(?{print "a"})b(?{print "b"})cde/;
2998 the exact number of times "a" or "b" are printed out is unspecified for
3000 failure, but you may assume they will be printed at least once during a
3001 successful match, additionally you may assume that if "b" is printed,
3002 it will be preceded by at least one "a".
3003 In the case of branching constructs like the following:
3005 /a(b|(?{ print "a" }))c(?{ print "c" })/;
3007 you can assume that the input "ac" will output "ac", and that "abc"
3009 will output only "c".
3010 When embedded code is quantified, successful matches will call the code
3012 once for each matched iteration of the quantifier. For example:
3013 "good" =~ /g(?:o(?{print "o"}))*d/;
3015 will output "o" twice.
3017 PCRE/Python Support
3019 As of Perl 5.10.0, Perl supports several Python/PCRE-specific
3020 extensions to the regex syntax. While Perl programmers are encouraged
3021 to use the Perl-specific syntax, the following are also accepted:
3022 "(?P<NAME>pattern)"
3024 Define a named capture group. Equivalent to "(?<NAME>pattern)".
3025 "(?P=NAME)"
3027 Backreference to a named capture group. Equivalent to "\g{NAME}".
3028 "(?P>NAME)"
3030 Subroutine call to a named capture group. Equivalent to "(?&NAME)".
3031
3033 There are a number of issues with regard to case-insensitive matching
3034 in Unicode rules. See "i" under "Modifiers" above.
3035 This document varies from difficult to understand to completely and
3037 utterly opaque. The wandering prose riddled with jargon is hard to
3038 fathom in several places.
3039 This document needs a rewrite that separates the tutorial content from
3041 the reference content.
3042
3044 The syntax of patterns used in Perl pattern matching evolved from those
3045 supplied in the Bell Labs Research Unix 8th Edition (Version 8) regex
3046 routines. (The code is actually derived (distantly) from Henry
3047 Spencer's freely redistributable reimplementation of those V8
3048 routines.)
3049 perlrequick.
3051 perlretut.
3053 "Regexp Quote-Like Operators" in perlop.
3055 "Gory details of parsing quoted constructs" in perlop.
3057 perlfaq6.
3059 "pos" in perlfunc.
3061 perllocale.
3063 perlebcdic.
3065 Mastering Regular Expressions by Jeffrey Friedl, published by O'Reilly
3067 and Associates.
3068perl v5.30.2 2020-03-27 PERLRE(1)