3 perlre - Perl regular expressions
7 This page describes the syntax of regular expressions in Perl. For a
8 description of how to I<use> regular expressions in matching
9 operations, plus various examples of the same, see C<m//> and C<s///> in
12 The matching operations can have various modifiers. The modifiers
13 which relate to the interpretation of the regular expression inside
14 are listed below. For the modifiers that alter the behaviour of the
15 operation, see L<perlop/"m//"> and L<perlop/"s//">.
21 Do case-insensitive pattern matching.
23 If C<use locale> is in effect, the case map is taken from the current
24 locale. See L<perllocale>.
28 Treat string as multiple lines. That is, change "^" and "$" from matching
29 at only the very start or end of the string to the start or end of any
30 line anywhere within the string,
34 Treat string as single line. That is, change "." to match any character
35 whatsoever, even a newline, which it normally would not match.
39 Extend your pattern's legibility by permitting whitespace and comments.
43 These are usually written as "the C</x> modifier", even though the delimiter
44 in question might not actually be a slash. In fact, any of these
45 modifiers may also be embedded within the regular expression itself using
46 the new C<(?...)> construct. See below.
48 The C</x> modifier itself needs a little more explanation. It tells
49 the regular expression parser to ignore whitespace that is neither
50 backslashed nor within a character class. You can use this to break up
51 your regular expression into (slightly) more readable parts. The C<#>
52 character is also treated as a metacharacter introducing a comment,
53 just as in ordinary Perl code. This also means that if you want real
54 whitespace or C<#> characters in the pattern that you'll have to either
55 escape them or encode them using octal or hex escapes. Taken together,
56 these features go a long way towards making Perl's regular expressions
57 more readable. See the C comment deletion code in L<perlop>.
59 =head2 Regular Expressions
61 The patterns used in pattern matching are regular expressions such as
62 those supplied in the Version 8 regexp routines. (In fact, the
63 routines are derived (distantly) from Henry Spencer's freely
64 redistributable reimplementation of the V8 routines.)
65 See L<Version 8 Regular Expressions> for details.
67 In particular the following metacharacters have their standard I<egrep>-ish
70 \ Quote the next metacharacter
71 ^ Match the beginning of the line
72 . Match any character (except newline)
73 $ Match the end of the line (or before newline at the end)
78 By default, the "^" character is guaranteed to match at only the
79 beginning of the string, the "$" character at only the end (or before the
80 newline at the end) and Perl does certain optimizations with the
81 assumption that the string contains only one line. Embedded newlines
82 will not be matched by "^" or "$". You may, however, wish to treat a
83 string as a multi-line buffer, such that the "^" will match after any
84 newline within the string, and "$" will match before any newline. At the
85 cost of a little more overhead, you can do this by using the /m modifier
86 on the pattern match operator. (Older programs did this by setting C<$*>,
87 but this practice is now deprecated.)
89 To facilitate multi-line substitutions, the "." character never matches a
90 newline unless you use the C</s> modifier, which in effect tells Perl to pretend
91 the string is a single line--even if it isn't. The C</s> modifier also
92 overrides the setting of C<$*>, in case you have some (badly behaved) older
93 code that sets it in another module.
95 The following standard quantifiers are recognized:
97 * Match 0 or more times
98 + Match 1 or more times
100 {n} Match exactly n times
101 {n,} Match at least n times
102 {n,m} Match at least n but not more than m times
104 (If a curly bracket occurs in any other context, it is treated
105 as a regular character.) The "*" modifier is equivalent to C<{0,}>, the "+"
106 modifier to C<{1,}>, and the "?" modifier to C<{0,1}>. n and m are limited
107 to integral values less than 65536.
109 By default, a quantified subpattern is "greedy", that is, it will match as
110 many times as possible (given a particular starting location) while still
111 allowing the rest of the pattern to match. If you want it to match the
112 minimum number of times possible, follow the quantifier with a "?". Note
113 that the meanings don't change, just the "greediness":
115 *? Match 0 or more times
116 +? Match 1 or more times
118 {n}? Match exactly n times
119 {n,}? Match at least n times
120 {n,m}? Match at least n but not more than m times
122 Because patterns are processed as double quoted strings, the following
129 \a alarm (bell) (BEL)
130 \e escape (think troff) (ESC)
131 \033 octal char (think of a PDP-11)
134 \l lowercase next char (think vi)
135 \u uppercase next char (think vi)
136 \L lowercase till \E (think vi)
137 \U uppercase till \E (think vi)
138 \E end case modification (think vi)
139 \Q quote regexp metacharacters till \E
141 If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
142 and <\U> is taken from the current locale. See L<perllocale>.
144 In addition, Perl defines the following:
146 \w Match a "word" character (alphanumeric plus "_")
147 \W Match a non-word character
148 \s Match a whitespace character
149 \S Match a non-whitespace character
150 \d Match a digit character
151 \D Match a non-digit character
153 Note that C<\w> matches a single alphanumeric character, not a whole
154 word. To match a word you'd need to say C<\w+>. If C<use locale> is in
155 effect, the list of alphabetic characters generated by C<\w> is taken
156 from the current locale. See L<perllocale>. You may use C<\w>, C<\W>,
157 C<\s>, C<\S>, C<\d>, and C<\D> within character classes (though not as
158 either end of a range).
160 Perl defines the following zero-width assertions:
162 \b Match a word boundary
163 \B Match a non-(word boundary)
164 \A Match at only beginning of string
165 \Z Match at only end of string (or before newline at the end)
166 \G Match only where previous m//g left off
168 A word boundary (C<\b>) is defined as a spot between two characters that
169 has a C<\w> on one side of it and a C<\W> on the other side of it (in
170 either order), counting the imaginary characters off the beginning and
171 end of the string as matching a C<\W>. (Within character classes C<\b>
172 represents backspace rather than a word boundary.) The C<\A> and C<\Z> are
173 just like "^" and "$" except that they won't match multiple times when the
174 C</m> modifier is used, while "^" and "$" will match at every internal line
175 boundary. To match the actual end of the string, not ignoring newline,
176 you can use C<\Z(?!\n)>. The C<\G> assertion can be used to mix global
177 matches (using C<m//g>) and non-global ones, as described in
178 L<perlop/"Regexp Quote-Like Operators">.
179 It is also useful when writing C<lex>-like scanners, when you have several
180 regexps which you want to match against consequent substrings of your
181 string, see the previous reference.
182 The actual location where C<\G> will match can also be influenced
183 by using C<pos()> as an lvalue. See L<perlfunc/pos>.
185 When the bracketing construct C<( ... )> is used, \E<lt>digitE<gt> matches the
186 digit'th substring. Outside of the pattern, always use "$" instead of "\"
187 in front of the digit. (While the \E<lt>digitE<gt> notation can on rare occasion work
188 outside the current pattern, this should not be relied upon. See the
189 WARNING below.) The scope of $E<lt>digitE<gt> (and C<$`>, C<$&>, and C<$'>)
190 extends to the end of the enclosing BLOCK or eval string, or to the next
191 successful pattern match, whichever comes first. If you want to use
192 parentheses to delimit a subpattern (e.g., a set of alternatives) without
193 saving it as a subpattern, follow the ( with a ?:.
195 You may have as many parentheses as you wish. If you have more
196 than 9 substrings, the variables $10, $11, ... refer to the
197 corresponding substring. Within the pattern, \10, \11, etc. refer back
198 to substrings if there have been at least that many left parentheses before
199 the backreference. Otherwise (for backward compatibility) \10 is the
200 same as \010, a backspace, and \11 the same as \011, a tab. And so
201 on. (\1 through \9 are always backreferences.)
203 C<$+> returns whatever the last bracket match matched. C<$&> returns the
204 entire matched string. (C<$0> used to return the same thing, but not any
205 more.) C<$`> returns everything before the matched string. C<$'> returns
206 everything after the matched string. Examples:
208 s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words
210 if (/Time: (..):(..):(..)/) {
216 Once perl sees that you need one of C<$&>, C<$`> or C<$'> anywhere in
217 the program, it has to provide them on each and every pattern match.
218 This can slow your program down. The same mechanism that handles
219 these provides for the use of $1, $2, etc., so you pay the same price
220 for each regexp that contains capturing parentheses. But if you never
221 use $&, etc., in your script, then regexps I<without> capturing
222 parentheses won't be penalized. So avoid $&, $', and $` if you can,
223 but if you can't (and some algorithms really appreciate them), once
224 you've used them once, use them at will, because you've already paid
227 You will note that all backslashed metacharacters in Perl are
228 alphanumeric, such as C<\b>, C<\w>, C<\n>. Unlike some other regular expression
229 languages, there are no backslashed symbols that aren't alphanumeric.
230 So anything that looks like \\, \(, \), \E<lt>, \E<gt>, \{, or \} is always
231 interpreted as a literal character, not a metacharacter. This makes it
232 simple to quote a string that you want to use for a pattern but that
233 you are afraid might contain metacharacters. Quote simply all the
234 non-alphanumeric characters:
236 $pattern =~ s/(\W)/\\$1/g;
238 You can also use the builtin quotemeta() function to do this.
239 An even easier way to quote metacharacters right in the match operator
242 /$unquoted\Q$quoted\E$unquoted/
244 Perl defines a consistent extension syntax for regular expressions.
245 The syntax is a pair of parentheses with a question mark as the first
246 thing within the parentheses (this was a syntax error in older
247 versions of Perl). The character after the question mark gives the
248 function of the extension. Several extensions are already supported:
254 A comment. The text is ignored. If the C</x> switch is used to enable
255 whitespace formatting, a simple C<#> will suffice.
259 This groups things like "()" but doesn't make backreferences like "()" does. So
261 split(/\b(?:a|b|c)\b/)
267 but doesn't spit out extra fields.
271 A zero-width positive lookahead assertion. For example, C</\w+(?=\t)/>
272 matches a word followed by a tab, without including the tab in C<$&>.
276 A zero-width negative lookahead assertion. For example C</foo(?!bar)/>
277 matches any occurrence of "foo" that isn't followed by "bar". Note
278 however that lookahead and lookbehind are NOT the same thing. You cannot
279 use this for lookbehind: C</(?!foo)bar/> will not find an occurrence of
280 "bar" that is preceded by something which is not "foo". That's because
281 the C<(?!foo)> is just saying that the next thing cannot be "foo"--and
282 it's not, it's a "bar", so "foobar" will match. You would have to do
283 something like C</(?!foo)...bar/> for that. We say "like" because there's
284 the case of your "bar" not having three characters before it. You could
285 cover that this way: C</(?:(?!foo)...|^..?)bar/>. Sometimes it's still
288 if (/foo/ && $` =~ /bar$/)
293 One or more embedded pattern-match modifiers. This is particularly
294 useful for patterns that are specified in a table somewhere, some of
295 which want to be case sensitive, and some of which don't. The case
296 insensitive ones need to include merely C<(?i)> at the front of the
297 pattern. For example:
304 $pattern = "(?i)foobar";
309 The specific choice of question mark for this and the new minimal
310 matching construct was because 1) question mark is pretty rare in older
311 regular expressions, and 2) whenever you see one, you should stop
312 and "question" exactly what is going on. That's psychology...
316 A fundamental feature of regular expression matching involves the notion
317 called I<backtracking>. which is used (when needed) by all regular
318 expression quantifiers, namely C<*>, C<*?>, C<+>, C<+?>, C<{n,m}>, and
321 For a regular expression to match, the I<entire> regular expression must
322 match, not just part of it. So if the beginning of a pattern containing a
323 quantifier succeeds in a way that causes later parts in the pattern to
324 fail, the matching engine backs up and recalculates the beginning
325 part--that's why it's called backtracking.
327 Here is an example of backtracking: Let's say you want to find the
328 word following "foo" in the string "Food is on the foo table.":
330 $_ = "Food is on the foo table.";
331 if ( /\b(foo)\s+(\w+)/i ) {
332 print "$2 follows $1.\n";
335 When the match runs, the first part of the regular expression (C<\b(foo)>)
336 finds a possible match right at the beginning of the string, and loads up
337 $1 with "Foo". However, as soon as the matching engine sees that there's
338 no whitespace following the "Foo" that it had saved in $1, it realizes its
339 mistake and starts over again one character after where it had the
340 tentative match. This time it goes all the way until the next occurrence
341 of "foo". The complete regular expression matches this time, and you get
342 the expected output of "table follows foo."
344 Sometimes minimal matching can help a lot. Imagine you'd like to match
345 everything between "foo" and "bar". Initially, you write something
348 $_ = "The food is under the bar in the barn.";
349 if ( /foo(.*)bar/ ) {
353 Which perhaps unexpectedly yields:
355 got <d is under the bar in the >
357 That's because C<.*> was greedy, so you get everything between the
358 I<first> "foo" and the I<last> "bar". In this case, it's more effective
359 to use minimal matching to make sure you get the text between a "foo"
360 and the first "bar" thereafter.
362 if ( /foo(.*?)bar/ ) { print "got <$1>\n" }
363 got <d is under the >
365 Here's another example: let's say you'd like to match a number at the end
366 of a string, and you also want to keep the preceding part the match.
369 $_ = "I have 2 numbers: 53147";
370 if ( /(.*)(\d*)/ ) { # Wrong!
371 print "Beginning is <$1>, number is <$2>.\n";
374 That won't work at all, because C<.*> was greedy and gobbled up the
375 whole string. As C<\d*> can match on an empty string the complete
376 regular expression matched successfully.
378 Beginning is <I have 2 numbers: 53147>, number is <>.
380 Here are some variants, most of which don't work:
382 $_ = "I have 2 numbers: 53147";
395 printf "%-12s ", $pat;
405 (.*)(\d*) <I have 2 numbers: 53147> <>
406 (.*)(\d+) <I have 2 numbers: 5314> <7>
408 (.*?)(\d+) <I have > <2>
409 (.*)(\d+)$ <I have 2 numbers: 5314> <7>
410 (.*?)(\d+)$ <I have 2 numbers: > <53147>
411 (.*)\b(\d+)$ <I have 2 numbers: > <53147>
412 (.*\D)(\d+)$ <I have 2 numbers: > <53147>
414 As you see, this can be a bit tricky. It's important to realize that a
415 regular expression is merely a set of assertions that gives a definition
416 of success. There may be 0, 1, or several different ways that the
417 definition might succeed against a particular string. And if there are
418 multiple ways it might succeed, you need to understand backtracking to know which variety of success you will achieve.
420 When using lookahead assertions and negations, this can all get even
421 tricker. Imagine you'd like to find a sequence of non-digits not
422 followed by "123". You might try to write that as
425 if ( /^\D*(?!123)/ ) { # Wrong!
426 print "Yup, no 123 in $_\n";
429 But that isn't going to match; at least, not the way you're hoping. It
430 claims that there is no 123 in the string. Here's a clearer picture of
431 why it that pattern matches, contrary to popular expectations:
436 print "1: got $1\n" if $x =~ /^(ABC)(?!123)/ ;
437 print "2: got $1\n" if $y =~ /^(ABC)(?!123)/ ;
439 print "3: got $1\n" if $x =~ /^(\D*)(?!123)/ ;
440 print "4: got $1\n" if $y =~ /^(\D*)(?!123)/ ;
448 You might have expected test 3 to fail because it seems to a more
449 general purpose version of test 1. The important difference between
450 them is that test 3 contains a quantifier (C<\D*>) and so can use
451 backtracking, whereas test 1 will not. What's happening is
452 that you've asked "Is it true that at the start of $x, following 0 or more
453 non-digits, you have something that's not 123?" If the pattern matcher had
454 let C<\D*> expand to "ABC", this would have caused the whole pattern to
456 The search engine will initially match C<\D*> with "ABC". Then it will
457 try to match C<(?!123> with "123" which, of course, fails. But because
458 a quantifier (C<\D*>) has been used in the regular expression, the
459 search engine can backtrack and retry the match differently
460 in the hope of matching the complete regular expression.
463 the pattern really, I<really> wants to succeed, so it uses the
464 standard regexp back-off-and-retry and lets C<\D*> expand to just "AB" this
465 time. Now there's indeed something following "AB" that is not
466 "123". It's in fact "C123", which suffices.
468 We can deal with this by using both an assertion and a negation. We'll
469 say that the first part in $1 must be followed by a digit, and in fact, it
470 must also be followed by something that's not "123". Remember that the
471 lookaheads are zero-width expressions--they only look, but don't consume
472 any of the string in their match. So rewriting this way produces what
473 you'd expect; that is, case 5 will fail, but case 6 succeeds:
475 print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/ ;
476 print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/ ;
480 In other words, the two zero-width assertions next to each other work like
481 they're ANDed together, just as you'd use any builtin assertions: C</^$/>
482 matches only if you're at the beginning of the line AND the end of the
483 line simultaneously. The deeper underlying truth is that juxtaposition in
484 regular expressions always means AND, except when you write an explicit OR
485 using the vertical bar. C</ab/> means match "a" AND (then) match "b",
486 although the attempted matches are made at different positions because "a"
487 is not a zero-width assertion, but a one-width assertion.
489 One warning: particularly complicated regular expressions can take
490 exponential time to solve due to the immense number of possible ways they
491 can use backtracking to try match. For example this will take a very long
494 /((a{0,5}){0,5}){0,5}/
496 And if you used C<*>'s instead of limiting it to 0 through 5 matches, then
497 it would take literally forever--or until you ran out of stack space.
499 =head2 Version 8 Regular Expressions
501 In case you're not familiar with the "regular" Version 8 regexp
502 routines, here are the pattern-matching rules not described above.
504 Any single character matches itself, unless it is a I<metacharacter>
505 with a special meaning described here or above. You can cause
506 characters which normally function as metacharacters to be interpreted
507 literally by prefixing them with a "\" (e.g., "\." matches a ".", not any
508 character; "\\" matches a "\"). A series of characters matches that
509 series of characters in the target string, so the pattern C<blurfl>
510 would match "blurfl" in the target string.
512 You can specify a character class, by enclosing a list of characters
513 in C<[]>, which will match any one of the characters in the list. If the
514 first character after the "[" is "^", the class matches any character not
515 in the list. Within a list, the "-" character is used to specify a
516 range, so that C<a-z> represents all the characters between "a" and "z",
519 Characters may be specified using a metacharacter syntax much like that
520 used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return,
521 "\f" a form feed, etc. More generally, \I<nnn>, where I<nnn> is a string
522 of octal digits, matches the character whose ASCII value is I<nnn>.
523 Similarly, \xI<nn>, where I<nn> are hexadecimal digits, matches the
524 character whose ASCII value is I<nn>. The expression \cI<x> matches the
525 ASCII character control-I<x>. Finally, the "." metacharacter matches any
526 character except "\n" (unless you use C</s>).
528 You can specify a series of alternatives for a pattern using "|" to
529 separate them, so that C<fee|fie|foe> will match any of "fee", "fie",
530 or "foe" in the target string (as would C<f(e|i|o)e>). Note that the
531 first alternative includes everything from the last pattern delimiter
532 ("(", "[", or the beginning of the pattern) up to the first "|", and
533 the last alternative contains everything from the last "|" to the next
534 pattern delimiter. For this reason, it's common practice to include
535 alternatives in parentheses, to minimize confusion about where they
536 start and end. Note however that "|" is interpreted as a literal with
537 square brackets, so if you write C<[fee|fie|foe]> you're really only
540 Within a pattern, you may designate subpatterns for later reference by
541 enclosing them in parentheses, and you may refer back to the I<n>th
542 subpattern later in the pattern using the metacharacter \I<n>.
543 Subpatterns are numbered based on the left to right order of their
544 opening parenthesis. Note that a backreference matches whatever
545 actually matched the subpattern in the string being examined, not the
546 rules for that subpattern. Therefore, C<(0|0x)\d*\s\1\d*> will
547 match "0x1234 0x4321",but not "0x1234 01234", because subpattern 1
548 actually matched "0x", even though the rule C<0|0x> could
549 potentially match the leading 0 in the second number.
551 =head2 WARNING on \1 vs $1
553 Some people get too used to writing things like
555 $pattern =~ s/(\W)/\\\1/g;
557 This is grandfathered for the RHS of a substitute to avoid shocking the
558 B<sed> addicts, but it's a dirty habit to get into. That's because in
559 PerlThink, the righthand side of a C<s///> is a double-quoted string. C<\1> in
560 the usual double-quoted string means a control-A. The customary Unix
561 meaning of C<\1> is kludged in for C<s///>. However, if you get into the habit
562 of doing that, you get yourself into trouble if you then add an C</e>
571 You can't disambiguate that by saying C<\{1}000>, whereas you can fix it with
572 C<${1}000>. Basically, the operation of interpolation should not be confused
573 with the operation of matching a backreference. Certainly they mean two
574 different things on the I<left> side of the C<s///>.