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
13 have various modifiers, some of which relate to the interpretation of
14 the regular expression inside. These are:
20 Do case-insensitive pattern matching.
22 If C<use locale> is in effect, the case map is taken from the current
23 locale. See L<perllocale>.
27 Treat string as multiple lines. That is, change "^" and "$" from matching
28 at only the very start or end of the string to the start or end of any
29 line anywhere within the string,
33 Treat string as single line. That is, change "." to match any character
34 whatsoever, even a newline, which it normally would not match.
38 Extend your pattern's legibility by permitting whitespace and comments.
42 These are usually written as "the C</x> modifier", even though the delimiter
43 in question might not actually be a slash. In fact, any of these
44 modifiers may also be embedded within the regular expression itself using
45 the new C<(?...)> construct. See below.
47 The C</x> modifier itself needs a little more explanation. It tells
48 the regular expression parser to ignore whitespace that is neither
49 backslashed nor within a character class. You can use this to break up
50 your regular expression into (slightly) more readable parts. The C<#>
51 character is also treated as a meta-character introducing a comment,
52 just as in ordinary Perl code. This also means that if you want real
53 whitespace or C<#> characters in the pattern that you'll have to either
54 escape them or encode them using octal or hex escapes. Taken together,
55 these features go a long way towards making Perl's regular expressions
56 more readable. See the C comment deletion code in L<perlop>.
58 =head2 Regular Expressions
60 The patterns used in pattern matching are regular expressions such as
61 those supplied in the Version 8 regexp routines. (In fact, the
62 routines are derived (distantly) from Henry Spencer's freely
63 redistributable reimplementation of the V8 routines.)
64 See L<Version 8 Regular Expressions> for details.
66 In particular the following metacharacters have their standard I<egrep>-ish
69 \ Quote the next meta-character
70 ^ Match the beginning of the line
71 . Match any character (except newline)
72 $ Match the end of the line (or before newline at the end)
77 By default, the "^" character is guaranteed to match at only the
78 beginning of the string, the "$" character at only the end (or before the
79 newline at the end) and Perl does certain optimizations with the
80 assumption that the string contains only one line. Embedded newlines
81 will not be matched by "^" or "$". You may, however, wish to treat a
82 string as a multi-line buffer, such that the "^" will match after any
83 newline within the string, and "$" will match before any newline. At the
84 cost of a little more overhead, you can do this by using the /m modifier
85 on the pattern match operator. (Older programs did this by setting C<$*>,
86 but this practice is now deprecated.)
88 To facilitate multi-line substitutions, the "." character never matches a
89 newline unless you use the C</s> modifier, which in effect tells Perl to pretend
90 the string is a single line--even if it isn't. The C</s> modifier also
91 overrides the setting of C<$*>, in case you have some (badly behaved) older
92 code that sets it in another module.
94 The following standard quantifiers are recognized:
96 * Match 0 or more times
97 + Match 1 or more times
99 {n} Match exactly n times
100 {n,} Match at least n times
101 {n,m} Match at least n but not more than m times
103 (If a curly bracket occurs in any other context, it is treated
104 as a regular character.) The "*" modifier is equivalent to C<{0,}>, the "+"
105 modifier to C<{1,}>, and the "?" modifier to C<{0,1}>. n and m are limited
106 to integral values less than 65536.
108 By default, a quantified sub-pattern is "greedy", that is, it will match as
109 many times as possible without causing the rest of the pattern not to match.
110 The standard quantifiers are all "greedy", in that they match as many
111 occurrences as possible (given a particular starting location) without
112 causing the pattern to fail. If you want it to match the minimum number
113 of times possible, follow the quantifier with a "?" after any of them.
114 Note that the meanings don't change, just the "gravity":
116 *? Match 0 or more times
117 +? Match 1 or more times
119 {n}? Match exactly n times
120 {n,}? Match at least n times
121 {n,m}? Match at least n but not more than m times
123 Because patterns are processed as double quoted strings, the following
130 \a alarm (bell) (BEL)
131 \e escape (think troff) (ESC)
132 \033 octal char (think of a PDP-11)
135 \l lowercase next char (think vi)
136 \u uppercase next char (think vi)
137 \L lowercase till \E (think vi)
138 \U uppercase till \E (think vi)
139 \E end case modification (think vi)
140 \Q quote regexp metacharacters till \E
142 If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
143 and <\U> is taken from the current locale. See L<perllocale>.
145 In addition, Perl defines the following:
147 \w Match a "word" character (alphanumeric plus "_")
148 \W Match a non-word character
149 \s Match a whitespace character
150 \S Match a non-whitespace character
151 \d Match a digit character
152 \D Match a non-digit character
154 Note that C<\w> matches a single alphanumeric character, not a whole
155 word. To match a word you'd need to say C<\w+>. If C<use locale> is in
156 effect, the list of alphabetic characters generated by C<\w> is taken
157 from the current locale. See L<perllocale>. You may use C<\w>, C<\W>,
158 C<\s>, C<\S>, C<\d>, and C<\D> within character classes (though not as
159 either end of a range).
161 Perl defines the following zero-width assertions:
163 \b Match a word boundary
164 \B Match a non-(word boundary)
165 \A Match at only beginning of string
166 \Z Match at only end of string (or before newline at the end)
167 \G Match only where previous m//g left off
169 A word boundary (C<\b>) is defined as a spot between two characters that
170 has a C<\w> on one side of it and and a C<\W> on the other side of it (in
171 either order), counting the imaginary characters off the beginning and
172 end of the string as matching a C<\W>. (Within character classes C<\b>
173 represents backspace rather than a word boundary.) The C<\A> and C<\Z> are
174 just like "^" and "$" except that they won't match multiple times when the
175 C</m> modifier is used, while "^" and "$" will match at every internal line
176 boundary. To match the actual end of the string, not ignoring newline,
177 you can use C<\Z(?!\n)>.
179 When the bracketing construct C<( ... )> is used, \E<lt>digitE<gt> matches the
180 digit'th substring. Outside of the pattern, always use "$" instead of "\"
181 in front of the digit. (While the \E<lt>digitE<gt> notation can on rare occasion work
182 outside the current pattern, this should not be relied upon. See the
183 WARNING below.) The scope of $E<lt>digitE<gt> (and C<$`>, C<$&>, and C<$'>)
184 extends to the end of the enclosing BLOCK or eval string, or to the next
185 successful pattern match, whichever comes first. If you want to use
186 parentheses to delimit a subpattern (e.g., a set of alternatives) without
187 saving it as a subpattern, follow the ( with a ?:.
189 You may have as many parentheses as you wish. If you have more
190 than 9 substrings, the variables $10, $11, ... refer to the
191 corresponding substring. Within the pattern, \10, \11, etc. refer back
192 to substrings if there have been at least that many left parentheses before
193 the backreference. Otherwise (for backward compatibility) \10 is the
194 same as \010, a backspace, and \11 the same as \011, a tab. And so
195 on. (\1 through \9 are always backreferences.)
197 C<$+> returns whatever the last bracket match matched. C<$&> returns the
198 entire matched string. (C<$0> used to return the same thing, but not any
199 more.) C<$`> returns everything before the matched string. C<$'> returns
200 everything after the matched string. Examples:
202 s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words
204 if (/Time: (..):(..):(..)/) {
210 You will note that all backslashed metacharacters in Perl are
211 alphanumeric, such as C<\b>, C<\w>, C<\n>. Unlike some other regular expression
212 languages, there are no backslashed symbols that aren't alphanumeric.
213 So anything that looks like \\, \(, \), \E<lt>, \E<gt>, \{, or \} is always
214 interpreted as a literal character, not a meta-character. This makes it
215 simple to quote a string that you want to use for a pattern but that
216 you are afraid might contain metacharacters. Quote simply all the
217 non-alphanumeric characters:
219 $pattern =~ s/(\W)/\\$1/g;
221 You can also use the built-in quotemeta() function to do this.
222 An even easier way to quote metacharacters right in the match operator
225 /$unquoted\Q$quoted\E$unquoted/
227 Perl defines a consistent extension syntax for regular expressions.
228 The syntax is a pair of parentheses with a question mark as the first
229 thing within the parentheses (this was a syntax error in older
230 versions of Perl). The character after the question mark gives the
231 function of the extension. Several extensions are already supported:
237 A comment. The text is ignored. If the C</x> switch is used to enable
238 whitespace formatting, a simple C<#> will suffice.
242 This groups things like "()" but doesn't make backreferences like "()" does. So
244 split(/\b(?:a|b|c)\b/)
250 but doesn't spit out extra fields.
254 A zero-width positive lookahead assertion. For example, C</\w+(?=\t)/>
255 matches a word followed by a tab, without including the tab in C<$&>.
259 A zero-width negative lookahead assertion. For example C</foo(?!bar)/>
260 matches any occurrence of "foo" that isn't followed by "bar". Note
261 however that lookahead and lookbehind are NOT the same thing. You cannot
262 use this for lookbehind: C</(?!foo)bar/> will not find an occurrence of
263 "bar" that is preceded by something which is not "foo". That's because
264 the C<(?!foo)> is just saying that the next thing cannot be "foo"--and
265 it's not, it's a "bar", so "foobar" will match. You would have to do
266 something like C</(?!foo)...bar/> for that. We say "like" because there's
267 the case of your "bar" not having three characters before it. You could
268 cover that this way: C</(?:(?!foo)...|^..?)bar/>. Sometimes it's still
271 if (/foo/ && $` =~ /bar$/)
276 One or more embedded pattern-match modifiers. This is particularly
277 useful for patterns that are specified in a table somewhere, some of
278 which want to be case sensitive, and some of which don't. The case
279 insensitive ones need to include merely C<(?i)> at the front of the
280 pattern. For example:
287 $pattern = "(?i)foobar";
292 The specific choice of question mark for this and the new minimal
293 matching construct was because 1) question mark is pretty rare in older
294 regular expressions, and 2) whenever you see one, you should stop
295 and "question" exactly what is going on. That's psychology...
299 A fundamental feature of regular expression matching involves the notion
300 called I<backtracking>. which is used (when needed) by all regular
301 expression quantifiers, namely C<*>, C<*?>, C<+>, C<+?>, C<{n,m}>, and
304 For a regular expression to match, the I<entire> regular expression must
305 match, not just part of it. So if the beginning of a pattern containing a
306 quantifier succeeds in a way that causes later parts in the pattern to
307 fail, the matching engine backs up and recalculates the beginning
308 part--that's why it's called backtracking.
310 Here is an example of backtracking: Let's say you want to find the
311 word following "foo" in the string "Food is on the foo table.":
313 $_ = "Food is on the foo table.";
314 if ( /\b(foo)\s+(\w+)/i ) {
315 print "$2 follows $1.\n";
318 When the match runs, the first part of the regular expression (C<\b(foo)>)
319 finds a possible match right at the beginning of the string, and loads up
320 $1 with "Foo". However, as soon as the matching engine sees that there's
321 no whitespace following the "Foo" that it had saved in $1, it realizes its
322 mistake and starts over again one character after where it had had the
323 tentative match. This time it goes all the way until the next occurrence
324 of "foo". The complete regular expression matches this time, and you get
325 the expected output of "table follows foo."
327 Sometimes minimal matching can help a lot. Imagine you'd like to match
328 everything between "foo" and "bar". Initially, you write something
331 $_ = "The food is under the bar in the barn.";
332 if ( /foo(.*)bar/ ) {
336 Which perhaps unexpectedly yields:
338 got <d is under the bar in the >
340 That's because C<.*> was greedy, so you get everything between the
341 I<first> "foo" and the I<last> "bar". In this case, it's more effective
342 to use minimal matching to make sure you get the text between a "foo"
343 and the first "bar" thereafter.
345 if ( /foo(.*?)bar/ ) { print "got <$1>\n" }
346 got <d is under the >
348 Here's another example: let's say you'd like to match a number at the end
349 of a string, and you also want to keep the preceding part the match.
352 $_ = "I have 2 numbers: 53147";
353 if ( /(.*)(\d*)/ ) { # Wrong!
354 print "Beginning is <$1>, number is <$2>.\n";
357 That won't work at all, because C<.*> was greedy and gobbled up the
358 whole string. As C<\d*> can match on an empty string the complete
359 regular expression matched successfully.
361 Beginning is <I have 2 numbers: 53147>, number is <>.
363 Here are some variants, most of which don't work:
365 $_ = "I have 2 numbers: 53147";
378 printf "%-12s ", $pat;
388 (.*)(\d*) <I have 2 numbers: 53147> <>
389 (.*)(\d+) <I have 2 numbers: 5314> <7>
391 (.*?)(\d+) <I have > <2>
392 (.*)(\d+)$ <I have 2 numbers: 5314> <7>
393 (.*?)(\d+)$ <I have 2 numbers: > <53147>
394 (.*)\b(\d+)$ <I have 2 numbers: > <53147>
395 (.*\D)(\d+)$ <I have 2 numbers: > <53147>
397 As you see, this can be a bit tricky. It's important to realize that a
398 regular expression is merely a set of assertions that gives a definition
399 of success. There may be 0, 1, or several different ways that the
400 definition might succeed against a particular string. And if there are
401 multiple ways it might succeed, you need to understand backtracking to know which variety of success you will achieve.
403 When using lookahead assertions and negations, this can all get even
404 tricker. Imagine you'd like to find a sequence of non-digits not
405 followed by "123". You might try to write that as
408 if ( /^\D*(?!123)/ ) { # Wrong!
409 print "Yup, no 123 in $_\n";
412 But that isn't going to match; at least, not the way you're hoping. It
413 claims that there is no 123 in the string. Here's a clearer picture of
414 why it that pattern matches, contrary to popular expectations:
419 print "1: got $1\n" if $x =~ /^(ABC)(?!123)/ ;
420 print "2: got $1\n" if $y =~ /^(ABC)(?!123)/ ;
422 print "3: got $1\n" if $x =~ /^(\D*)(?!123)/ ;
423 print "4: got $1\n" if $y =~ /^(\D*)(?!123)/ ;
431 You might have expected test 3 to fail because it seems to a more
432 general purpose version of test 1. The important difference between
433 them is that test 3 contains a quantifier (C<\D*>) and so can use
434 backtracking, whereas test 1 will not. What's happening is
435 that you've asked "Is it true that at the start of $x, following 0 or more
436 non-digits, you have something that's not 123?" If the pattern matcher had
437 let C<\D*> expand to "ABC", this would have caused the whole pattern to
439 The search engine will initially match C<\D*> with "ABC". Then it will
440 try to match C<(?!123> with "123" which, of course, fails. But because
441 a quantifier (C<\D*>) has been used in the regular expression, the
442 search engine can backtrack and retry the match differently
443 in the hope of matching the complete regular expression.
446 the pattern really, I<really> wants to succeed, so it uses the
447 standard regexp back-off-and-retry and lets C<\D*> expand to just "AB" this
448 time. Now there's indeed something following "AB" that is not
449 "123". It's in fact "C123", which suffices.
451 We can deal with this by using both an assertion and a negation. We'll
452 say that the first part in $1 must be followed by a digit, and in fact, it
453 must also be followed by something that's not "123". Remember that the
454 lookaheads are zero-width expressions--they only look, but don't consume
455 any of the string in their match. So rewriting this way produces what
456 you'd expect; that is, case 5 will fail, but case 6 succeeds:
458 print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/ ;
459 print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/ ;
463 In other words, the two zero-width assertions next to each other work like
464 they're ANDed together, just as you'd use any builtin assertions: C</^$/>
465 matches only if you're at the beginning of the line AND the end of the
466 line simultaneously. The deeper underlying truth is that juxtaposition in
467 regular expressions always means AND, except when you write an explicit OR
468 using the vertical bar. C</ab/> means match "a" AND (then) match "b",
469 although the attempted matches are made at different positions because "a"
470 is not a zero-width assertion, but a one-width assertion.
472 One warning: particularly complicated regular expressions can take
473 exponential time to solve due to the immense number of possible ways they
474 can use backtracking to try match. For example this will take a very long
477 /((a{0,5}){0,5}){0,5}/
479 And if you used C<*>'s instead of limiting it to 0 through 5 matches, then
480 it would take literally forever--or until you ran out of stack space.
482 =head2 Version 8 Regular Expressions
484 In case you're not familiar with the "regular" Version 8 regexp
485 routines, here are the pattern-matching rules not described above.
487 Any single character matches itself, unless it is a I<meta-character>
488 with a special meaning described here or above. You can cause
489 characters which normally function as metacharacters to be interpreted
490 literally by prefixing them with a "\" (e.g., "\." matches a ".", not any
491 character; "\\" matches a "\"). A series of characters matches that
492 series of characters in the target string, so the pattern C<blurfl>
493 would match "blurfl" in the target string.
495 You can specify a character class, by enclosing a list of characters
496 in C<[]>, which will match any one of the characters in the list. If the
497 first character after the "[" is "^", the class matches any character not
498 in the list. Within a list, the "-" character is used to specify a
499 range, so that C<a-z> represents all the characters between "a" and "z",
502 Characters may be specified using a meta-character syntax much like that
503 used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return,
504 "\f" a form feed, etc. More generally, \I<nnn>, where I<nnn> is a string
505 of octal digits, matches the character whose ASCII value is I<nnn>.
506 Similarly, \xI<nn>, where I<nn> are hexadecimal digits, matches the
507 character whose ASCII value is I<nn>. The expression \cI<x> matches the
508 ASCII character control-I<x>. Finally, the "." meta-character matches any
509 character except "\n" (unless you use C</s>).
511 You can specify a series of alternatives for a pattern using "|" to
512 separate them, so that C<fee|fie|foe> will match any of "fee", "fie",
513 or "foe" in the target string (as would C<f(e|i|o)e>). Note that the
514 first alternative includes everything from the last pattern delimiter
515 ("(", "[", or the beginning of the pattern) up to the first "|", and
516 the last alternative contains everything from the last "|" to the next
517 pattern delimiter. For this reason, it's common practice to include
518 alternatives in parentheses, to minimize confusion about where they
519 start and end. Note however that "|" is interpreted as a literal with
520 square brackets, so if you write C<[fee|fie|foe]> you're really only
523 Within a pattern, you may designate sub-patterns for later reference by
524 enclosing them in parentheses, and you may refer back to the I<n>th
525 sub-pattern later in the pattern using the meta-character \I<n>.
526 Sub-patterns are numbered based on the left to right order of their
527 opening parenthesis. Note that a backreference matches whatever
528 actually matched the sub-pattern in the string being examined, not the
529 rules for that sub-pattern. Therefore, C<(0|0x)\d*\s\1\d*> will
530 match "0x1234 0x4321",but not "0x1234 01234", because sub-pattern 1
531 actually matched "0x", even though the rule C<0|0x> could
532 potentially match the leading 0 in the second number.
534 =head2 WARNING on \1 vs $1
536 Some people get too used to writing things like
538 $pattern =~ s/(\W)/\\\1/g;
540 This is grandfathered for the RHS of a substitute to avoid shocking the
541 B<sed> addicts, but it's a dirty habit to get into. That's because in
542 PerlThink, the righthand side of a C<s///> is a double-quoted string. C<\1> in
543 the usual double-quoted string means a control-A. The customary Unix
544 meaning of C<\1> is kludged in for C<s///>. However, if you get into the habit
545 of doing that, you get yourself into trouble if you then add an C</e>
554 You can't disambiguate that by saying C<\{1}000>, whereas you can fix it with
555 C<${1}000>. Basically, the operation of interpolation should not be confused
556 with the operation of matching a backreference. Certainly they mean two
557 different things on the I<left> side of the C<s///>.