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 discussion
10 of C<m//>, C<s///>, C<qr//> and C<??> in L<perlop/"Regexp Quote-Like Operators">.
12 The matching operations can have various modifiers. The modifiers
13 that relate to the interpretation of the regular expression inside
14 are listed below. For the modifiers that alter the way a regular expression
15 is used by Perl, see L<perlop/"Regexp Quote-Like Operators"> and
16 L<perlop/"Gory details of parsing quoted constructs">.
22 Do case-insensitive pattern matching.
24 If C<use locale> is in effect, the case map is taken from the current
25 locale. See L<perllocale>.
29 Treat string as multiple lines. That is, change "^" and "$" from matching
30 at only the very start or end of the string to the start or end of any
31 line anywhere within the string,
35 Treat string as single line. That is, change "." to match any character
36 whatsoever, even a newline, which it normally would not match.
38 The C</s> and C</m> modifiers both override the C<$*> setting. That is, no matter
39 what C<$*> contains, C</s> without C</m> will force "^" to match only at the
40 beginning of the string and "$" to match only at the end (or just before a
41 newline at the end) of the string. Together, as /ms, they let the "." match
42 any character whatsoever, while yet allowing "^" and "$" to match,
43 respectively, just after and just before newlines within the string.
47 Extend your pattern's legibility by permitting whitespace and comments.
51 These are usually written as "the C</x> modifier", even though the delimiter
52 in question might not actually be a slash. In fact, any of these
53 modifiers may also be embedded within the regular expression itself using
54 the new C<(?...)> construct. See below.
56 The C</x> modifier itself needs a little more explanation. It tells
57 the regular expression parser to ignore whitespace that is neither
58 backslashed nor within a character class. You can use this to break up
59 your regular expression into (slightly) more readable parts. The C<#>
60 character is also treated as a metacharacter introducing a comment,
61 just as in ordinary Perl code. This also means that if you want real
62 whitespace or C<#> characters in the pattern (outside of a character
63 class, where they are unaffected by C</x>), that you'll either have to
64 escape them or encode them using octal or hex escapes. Taken together,
65 these features go a long way towards making Perl's regular expressions
66 more readable. Note that you have to be careful not to include the
67 pattern delimiter in the comment--perl has no way of knowing you did
68 not intend to close the pattern early. See the C-comment deletion code
71 =head2 Regular Expressions
73 The patterns used in pattern matching are regular expressions such as
74 those supplied in the Version 8 regex routines. (In fact, the
75 routines are derived (distantly) from Henry Spencer's freely
76 redistributable reimplementation of the V8 routines.)
77 See L<Version 8 Regular Expressions> for details.
79 In particular the following metacharacters have their standard I<egrep>-ish
82 \ Quote the next metacharacter
83 ^ Match the beginning of the line
84 . Match any character (except newline)
85 $ Match the end of the line (or before newline at the end)
90 By default, the "^" character is guaranteed to match at only the
91 beginning of the string, the "$" character at only the end (or before the
92 newline at the end) and Perl does certain optimizations with the
93 assumption that the string contains only one line. Embedded newlines
94 will not be matched by "^" or "$". You may, however, wish to treat a
95 string as a multi-line buffer, such that the "^" will match after any
96 newline within the string, and "$" will match before any newline. At the
97 cost of a little more overhead, you can do this by using the /m modifier
98 on the pattern match operator. (Older programs did this by setting C<$*>,
99 but this practice is now deprecated.)
101 To facilitate multi-line substitutions, the "." character never matches a
102 newline unless you use the C</s> modifier, which in effect tells Perl to pretend
103 the string is a single line--even if it isn't. The C</s> modifier also
104 overrides the setting of C<$*>, in case you have some (badly behaved) older
105 code that sets it in another module.
107 The following standard quantifiers are recognized:
109 * Match 0 or more times
110 + Match 1 or more times
112 {n} Match exactly n times
113 {n,} Match at least n times
114 {n,m} Match at least n but not more than m times
116 (If a curly bracket occurs in any other context, it is treated
117 as a regular character.) The "*" modifier is equivalent to C<{0,}>, the "+"
118 modifier to C<{1,}>, and the "?" modifier to C<{0,1}>. n and m are limited
119 to integral values less than a preset limit defined when perl is built.
120 This is usually 32766 on the most common platforms. The actual limit can
121 be seen in the error message generated by code such as this:
123 $_ **= $_ , / {$_} / for 2 .. 42;
125 By default, a quantified subpattern is "greedy", that is, it will match as
126 many times as possible (given a particular starting location) while still
127 allowing the rest of the pattern to match. If you want it to match the
128 minimum number of times possible, follow the quantifier with a "?". Note
129 that the meanings don't change, just the "greediness":
131 *? Match 0 or more times
132 +? Match 1 or more times
134 {n}? Match exactly n times
135 {n,}? Match at least n times
136 {n,m}? Match at least n but not more than m times
138 Because patterns are processed as double quoted strings, the following
145 \a alarm (bell) (BEL)
146 \e escape (think troff) (ESC)
147 \033 octal char (think of a PDP-11)
149 \x{263a} wide hex char (Unicode SMILEY)
151 \l lowercase next char (think vi)
152 \u uppercase next char (think vi)
153 \L lowercase till \E (think vi)
154 \U uppercase till \E (think vi)
155 \E end case modification (think vi)
156 \Q quote (disable) pattern metacharacters till \E
158 If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
159 and C<\U> is taken from the current locale. See L<perllocale>.
161 You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
162 An unescaped C<$> or C<@> interpolates the corresponding variable,
163 while escaping will cause the literal string C<\$> to be matched.
164 You'll need to write something like C<m/\Quser\E\@\Qhost/>.
166 In addition, Perl defines the following:
168 \w Match a "word" character (alphanumeric plus "_")
169 \W Match a non-word character
170 \s Match a whitespace character
171 \S Match a non-whitespace character
172 \d Match a digit character
173 \D Match a non-digit character
174 \pP Match P, named property. Use \p{Prop} for longer names.
176 \X Match eXtended Unicode "combining character sequence",
177 equivalent to C<(?:\PM\pM*)>
178 \C Match a single C char (octet) even under utf8.
180 A C<\w> matches a single alphanumeric character, not a whole
181 word. To match a word you'd need to say C<\w+>. If C<use locale> is in
182 effect, the list of alphabetic characters generated by C<\w> is taken
183 from the current locale. See L<perllocale>. You may use C<\w>, C<\W>,
184 C<\s>, C<\S>, C<\d>, and C<\D> within character classes (though not as
185 either end of a range).
187 Perl defines the following zero-width assertions:
189 \b Match a word boundary
190 \B Match a non-(word boundary)
191 \A Match only at beginning of string
192 \Z Match only at end of string, or before newline at the end
193 \z Match only at end of string
194 \G Match only where previous m//g left off (works only with /g)
196 A word boundary (C<\b>) is defined as a spot between two characters that
197 has a C<\w> on one side of it and a C<\W> on the other side of it (in
198 either order), counting the imaginary characters off the beginning and
199 end of the string as matching a C<\W>. (Within character classes C<\b>
200 represents backspace rather than a word boundary.) The C<\A> and C<\Z> are
201 just like "^" and "$", except that they won't match multiple times when the
202 C</m> modifier is used, while "^" and "$" will match at every internal line
203 boundary. To match the actual end of the string, not ignoring newline,
204 you can use C<\z>. The C<\G> assertion can be used to chain global
205 matches (using C<m//g>), as described in
206 L<perlop/"Regexp Quote-Like Operators">.
208 It is also useful when writing C<lex>-like scanners, when you have several
209 patterns that you want to match against consequent substrings of your
210 string, see the previous reference.
211 The actual location where C<\G> will match can also be influenced
212 by using C<pos()> as an lvalue. See L<perlfunc/pos>.
214 When the bracketing construct C<( ... )> is used, \E<lt>digitE<gt> matches the
215 digit'th substring. Outside of the pattern, always use "$" instead of "\"
216 in front of the digit. (While the \E<lt>digitE<gt> notation can on rare occasion work
217 outside the current pattern, this should not be relied upon. See the
218 WARNING below.) The scope of $E<lt>digitE<gt> (and C<$`>, C<$&>, and C<$'>)
219 extends to the end of the enclosing BLOCK or eval string, or to the next
220 successful pattern match, whichever comes first. If you want to use
221 parentheses to delimit a subpattern (e.g., a set of alternatives) without
222 saving it as a subpattern, follow the ( with a ?:.
224 You may have as many parentheses as you wish. If you have more
225 than 9 substrings, the variables $10, $11, ... refer to the
226 corresponding substring. Within the pattern, \10, \11, etc. refer back
227 to substrings if there have been at least that many left parentheses before
228 the backreference. Otherwise (for backward compatibility) \10 is the
229 same as \010, a backspace, and \11 the same as \011, a tab. And so
230 on. (\1 through \9 are always backreferences.)
232 C<$+> returns whatever the last bracket match matched. C<$&> returns the
233 entire matched string. (C<$0> used to return the same thing, but not any
234 more.) C<$`> returns everything before the matched string. C<$'> returns
235 everything after the matched string. Examples:
237 s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words
239 if (/Time: (..):(..):(..)/) {
245 Once perl sees that you need one of C<$&>, C<$`> or C<$'> anywhere in
246 the program, it has to provide them on each and every pattern match.
247 This can slow your program down. The same mechanism that handles
248 these provides for the use of $1, $2, etc., so you pay the same price
249 for each pattern that contains capturing parentheses. But if you never
250 use $&, etc., in your script, then patterns I<without> capturing
251 parentheses won't be penalized. So avoid $&, $', and $` if you can,
252 but if you can't (and some algorithms really appreciate them), once
253 you've used them once, use them at will, because you've already paid
254 the price. As of 5.005, $& is not so costly as the other two.
256 Backslashed metacharacters in Perl are
257 alphanumeric, such as C<\b>, C<\w>, C<\n>. Unlike some other regular
258 expression languages, there are no backslashed symbols that aren't
259 alphanumeric. So anything that looks like \\, \(, \), \E<lt>, \E<gt>,
260 \{, or \} is always interpreted as a literal character, not a
261 metacharacter. This was once used in a common idiom to disable or
262 quote the special meanings of regular expression metacharacters in a
263 string that you want to use for a pattern. Simply quote all
264 non-alphanumeric characters:
266 $pattern =~ s/(\W)/\\$1/g;
268 Now it is much more common to see either the quotemeta() function or
269 the C<\Q> escape sequence used to disable all metacharacters' special
272 /$unquoted\Q$quoted\E$unquoted/
274 Perl defines a consistent extension syntax for regular expressions.
275 The syntax is a pair of parentheses with a question mark as the first
276 thing within the parentheses (this was a syntax error in older
277 versions of Perl). The character after the question mark gives the
278 function of the extension. Several extensions are already supported:
284 A comment. The text is ignored. If the C</x> switch is used to enable
285 whitespace formatting, a simple C<#> will suffice. Note that perl closes
286 the comment as soon as it sees a C<)>, so there is no way to put a literal
291 =item C<(?imsx-imsx:pattern)>
293 This is for clustering, not capturing; it groups subexpressions like
294 "()", but doesn't make backreferences as "()" does. So
296 @fields = split(/\b(?:a|b|c)\b/)
300 @fields = split(/\b(a|b|c)\b/)
302 but doesn't spit out extra fields.
304 The letters between C<?> and C<:> act as flags modifiers, see
305 L<C<(?imsx-imsx)>>. In particular,
307 /(?s-i:more.*than).*million/i
309 is equivalent to more verbose
311 /(?:(?s-i)more.*than).*million/i
315 A zero-width positive lookahead assertion. For example, C</\w+(?=\t)/>
316 matches a word followed by a tab, without including the tab in C<$&>.
320 A zero-width negative lookahead assertion. For example C</foo(?!bar)/>
321 matches any occurrence of "foo" that isn't followed by "bar". Note
322 however that lookahead and lookbehind are NOT the same thing. You cannot
323 use this for lookbehind.
325 If you are looking for a "bar" that isn't preceded by a "foo", C</(?!foo)bar/>
326 will not do what you want. That's because the C<(?!foo)> is just saying that
327 the next thing cannot be "foo"--and it's not, it's a "bar", so "foobar" will
328 match. You would have to do something like C</(?!foo)...bar/> for that. We
329 say "like" because there's the case of your "bar" not having three characters
330 before it. You could cover that this way: C</(?:(?!foo)...|^.{0,2})bar/>.
331 Sometimes it's still easier just to say:
333 if (/bar/ && $` !~ /foo$/)
335 For lookbehind see below.
337 =item C<(?E<lt>=pattern)>
339 A zero-width positive lookbehind assertion. For example, C</(?E<lt>=\t)\w+/>
340 matches a word following a tab, without including the tab in C<$&>.
341 Works only for fixed-width lookbehind.
343 =item C<(?<!pattern)>
345 A zero-width negative lookbehind assertion. For example C</(?<!bar)foo/>
346 matches any occurrence of "foo" that isn't following "bar".
347 Works only for fixed-width lookbehind.
351 Experimental "evaluate any Perl code" zero-width assertion. Always
352 succeeds. C<code> is not interpolated. Currently the rules to
353 determine where the C<code> ends are somewhat convoluted.
355 The C<code> is properly scoped in the following sense: if the assertion
356 is backtracked (compare L<"Backtracking">), all the changes introduced after
357 C<local>isation are undone, so
361 (?{ $cnt = 0 }) # Initialize $cnt.
365 local $cnt = $cnt + 1; # Update $cnt, backtracking-safe.
369 (?{ $res = $cnt }) # On success copy to non-localized
373 will set C<$res = 4>. Note that after the match $cnt returns to the globally
374 introduced value 0, since the scopes which restrict C<local> statements
377 This assertion may be used as L<C<(?(condition)yes-pattern|no-pattern)>>
378 switch. If I<not> used in this way, the result of evaluation of C<code>
379 is put into variable $^R. This happens immediately, so $^R can be used from
380 other C<(?{ code })> assertions inside the same regular expression.
382 The above assignment to $^R is properly localized, thus the old value of $^R
383 is restored if the assertion is backtracked (compare L<"Backtracking">).
385 Due to security concerns, this construction is not allowed if the regular
386 expression involves run-time interpolation of variables, unless
387 C<use re 'eval'> pragma is used (see L<re>), or the variables contain
388 results of qr() operator (see L<perlop/"qr/STRING/imosx">).
390 This restriction is due to the wide-spread (questionable) practice of
397 without tainting. While this code is frowned upon from security point
398 of view, when C<(?{})> was introduced, it was considered bad to add
399 I<new> security holes to existing scripts.
401 B<NOTE:> Use of the above insecure snippet without also enabling taint mode
402 is to be severely frowned upon. C<use re 'eval'> does not disable tainting
403 checks, thus to allow $re in the above snippet to contain C<(?{})>
404 I<with tainting enabled>, one needs both C<use re 'eval'> and untaint
407 =item C<(?p{ code })>
409 I<Very experimental> "postponed" regular subexpression. C<code> is evaluated
410 at runtime, at the moment this subexpression may match. The result of
411 evaluation is considered as a regular expression, and matched as if it
412 were inserted instead of this construct.
414 C<code> is not interpolated. Currently the rules to
415 determine where the C<code> ends are somewhat convoluted.
417 The following regular expression matches matching parenthesized group:
422 (?> [^()]+ ) # Non-parens without backtracking
424 (?p{ $re }) # Group with matching parens
429 =item C<(?E<gt>pattern)>
431 An "independent" subexpression. Matches the substring that a
432 I<standalone> C<pattern> would match if anchored at the given position,
433 B<and only this substring>.
435 Say, C<^(?E<gt>a*)ab> will never match, since C<(?E<gt>a*)> (anchored
436 at the beginning of string, as above) will match I<all> characters
437 C<a> at the beginning of string, leaving no C<a> for C<ab> to match.
438 In contrast, C<a*ab> will match the same as C<a+b>, since the match of
439 the subgroup C<a*> is influenced by the following group C<ab> (see
440 L<"Backtracking">). In particular, C<a*> inside C<a*ab> will match
441 fewer characters than a standalone C<a*>, since this makes the tail match.
443 An effect similar to C<(?E<gt>pattern)> may be achieved by
447 since the lookahead is in I<"logical"> context, thus matches the same
448 substring as a standalone C<a+>. The following C<\1> eats the matched
449 string, thus making a zero-length assertion into an analogue of
450 C<(?E<gt>...)>. (The difference between these two constructs is that the
451 second one uses a catching group, thus shifting ordinals of
452 backreferences in the rest of a regular expression.)
454 This construct is useful for optimizations of "eternal"
455 matches, because it will not backtrack (see L<"Backtracking">).
466 That will efficiently match a nonempty group with matching
467 two-or-less-level-deep parentheses. However, if there is no such group,
468 it will take virtually forever on a long string. That's because there are
469 so many different ways to split a long string into several substrings.
470 This is what C<(.+)+> is doing, and C<(.+)+> is similar to a subpattern
471 of the above pattern. Consider that the above pattern detects no-match
472 on C<((()aaaaaaaaaaaaaaaaaa> in several seconds, but that each extra
473 letter doubles this time. This exponential performance will make it
474 appear that your program has hung.
476 However, a tiny modification of this pattern
487 which uses C<(?E<gt>...)> matches exactly when the one above does (verifying
488 this yourself would be a productive exercise), but finishes in a fourth
489 the time when used on a similar string with 1000000 C<a>s. Be aware,
490 however, that this pattern currently triggers a warning message under
491 B<-w> saying it C<"matches the null string many times">):
493 On simple groups, such as the pattern C<(?E<gt> [^()]+ )>, a comparable
494 effect may be achieved by negative lookahead, as in C<[^()]+ (?! [^()] )>.
495 This was only 4 times slower on a string with 1000000 C<a>s.
497 =item C<(?(condition)yes-pattern|no-pattern)>
499 =item C<(?(condition)yes-pattern)>
501 Conditional expression. C<(condition)> should be either an integer in
502 parentheses (which is valid if the corresponding pair of parentheses
503 matched), or lookahead/lookbehind/evaluate zero-width assertion.
512 matches a chunk of non-parentheses, possibly included in parentheses
515 =item C<(?imsx-imsx)>
517 One or more embedded pattern-match modifiers. This is particularly
518 useful for patterns that are specified in a table somewhere, some of
519 which want to be case sensitive, and some of which don't. The case
520 insensitive ones need to include merely C<(?i)> at the front of the
521 pattern. For example:
524 if ( /$pattern/i ) { }
528 $pattern = "(?i)foobar";
529 if ( /$pattern/ ) { }
531 Letters after C<-> switch modifiers off.
533 These modifiers are localized inside an enclosing group (if any). Say,
537 (assuming C<x> modifier, and no C<i> modifier outside of this group)
538 will match a repeated (I<including the case>!) word C<blah> in any
543 A question mark was chosen for this and for the new minimal-matching
544 construct because 1) question mark is pretty rare in older regular
545 expressions, and 2) whenever you see one, you should stop and "question"
546 exactly what is going on. That's psychology...
550 A fundamental feature of regular expression matching involves the
551 notion called I<backtracking>, which is currently used (when needed)
552 by all regular expression quantifiers, namely C<*>, C<*?>, C<+>,
553 C<+?>, C<{n,m}>, and C<{n,m}?>.
555 For a regular expression to match, the I<entire> regular expression must
556 match, not just part of it. So if the beginning of a pattern containing a
557 quantifier succeeds in a way that causes later parts in the pattern to
558 fail, the matching engine backs up and recalculates the beginning
559 part--that's why it's called backtracking.
561 Here is an example of backtracking: Let's say you want to find the
562 word following "foo" in the string "Food is on the foo table.":
564 $_ = "Food is on the foo table.";
565 if ( /\b(foo)\s+(\w+)/i ) {
566 print "$2 follows $1.\n";
569 When the match runs, the first part of the regular expression (C<\b(foo)>)
570 finds a possible match right at the beginning of the string, and loads up
571 $1 with "Foo". However, as soon as the matching engine sees that there's
572 no whitespace following the "Foo" that it had saved in $1, it realizes its
573 mistake and starts over again one character after where it had the
574 tentative match. This time it goes all the way until the next occurrence
575 of "foo". The complete regular expression matches this time, and you get
576 the expected output of "table follows foo."
578 Sometimes minimal matching can help a lot. Imagine you'd like to match
579 everything between "foo" and "bar". Initially, you write something
582 $_ = "The food is under the bar in the barn.";
583 if ( /foo(.*)bar/ ) {
587 Which perhaps unexpectedly yields:
589 got <d is under the bar in the >
591 That's because C<.*> was greedy, so you get everything between the
592 I<first> "foo" and the I<last> "bar". In this case, it's more effective
593 to use minimal matching to make sure you get the text between a "foo"
594 and the first "bar" thereafter.
596 if ( /foo(.*?)bar/ ) { print "got <$1>\n" }
597 got <d is under the >
599 Here's another example: let's say you'd like to match a number at the end
600 of a string, and you also want to keep the preceding part the match.
603 $_ = "I have 2 numbers: 53147";
604 if ( /(.*)(\d*)/ ) { # Wrong!
605 print "Beginning is <$1>, number is <$2>.\n";
608 That won't work at all, because C<.*> was greedy and gobbled up the
609 whole string. As C<\d*> can match on an empty string the complete
610 regular expression matched successfully.
612 Beginning is <I have 2 numbers: 53147>, number is <>.
614 Here are some variants, most of which don't work:
616 $_ = "I have 2 numbers: 53147";
629 printf "%-12s ", $pat;
639 (.*)(\d*) <I have 2 numbers: 53147> <>
640 (.*)(\d+) <I have 2 numbers: 5314> <7>
642 (.*?)(\d+) <I have > <2>
643 (.*)(\d+)$ <I have 2 numbers: 5314> <7>
644 (.*?)(\d+)$ <I have 2 numbers: > <53147>
645 (.*)\b(\d+)$ <I have 2 numbers: > <53147>
646 (.*\D)(\d+)$ <I have 2 numbers: > <53147>
648 As you see, this can be a bit tricky. It's important to realize that a
649 regular expression is merely a set of assertions that gives a definition
650 of success. There may be 0, 1, or several different ways that the
651 definition might succeed against a particular string. And if there are
652 multiple ways it might succeed, you need to understand backtracking to
653 know which variety of success you will achieve.
655 When using lookahead assertions and negations, this can all get even
656 tricker. Imagine you'd like to find a sequence of non-digits not
657 followed by "123". You might try to write that as
660 if ( /^\D*(?!123)/ ) { # Wrong!
661 print "Yup, no 123 in $_\n";
664 But that isn't going to match; at least, not the way you're hoping. It
665 claims that there is no 123 in the string. Here's a clearer picture of
666 why it that pattern matches, contrary to popular expectations:
671 print "1: got $1\n" if $x =~ /^(ABC)(?!123)/ ;
672 print "2: got $1\n" if $y =~ /^(ABC)(?!123)/ ;
674 print "3: got $1\n" if $x =~ /^(\D*)(?!123)/ ;
675 print "4: got $1\n" if $y =~ /^(\D*)(?!123)/ ;
683 You might have expected test 3 to fail because it seems to a more
684 general purpose version of test 1. The important difference between
685 them is that test 3 contains a quantifier (C<\D*>) and so can use
686 backtracking, whereas test 1 will not. What's happening is
687 that you've asked "Is it true that at the start of $x, following 0 or more
688 non-digits, you have something that's not 123?" If the pattern matcher had
689 let C<\D*> expand to "ABC", this would have caused the whole pattern to
691 The search engine will initially match C<\D*> with "ABC". Then it will
692 try to match C<(?!123> with "123", which of course fails. But because
693 a quantifier (C<\D*>) has been used in the regular expression, the
694 search engine can backtrack and retry the match differently
695 in the hope of matching the complete regular expression.
697 The pattern really, I<really> wants to succeed, so it uses the
698 standard pattern back-off-and-retry and lets C<\D*> expand to just "AB" this
699 time. Now there's indeed something following "AB" that is not
700 "123". It's in fact "C123", which suffices.
702 We can deal with this by using both an assertion and a negation. We'll
703 say that the first part in $1 must be followed by a digit, and in fact, it
704 must also be followed by something that's not "123". Remember that the
705 lookaheads are zero-width expressions--they only look, but don't consume
706 any of the string in their match. So rewriting this way produces what
707 you'd expect; that is, case 5 will fail, but case 6 succeeds:
709 print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/ ;
710 print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/ ;
714 In other words, the two zero-width assertions next to each other work as though
715 they're ANDed together, just as you'd use any builtin assertions: C</^$/>
716 matches only if you're at the beginning of the line AND the end of the
717 line simultaneously. The deeper underlying truth is that juxtaposition in
718 regular expressions always means AND, except when you write an explicit OR
719 using the vertical bar. C</ab/> means match "a" AND (then) match "b",
720 although the attempted matches are made at different positions because "a"
721 is not a zero-width assertion, but a one-width assertion.
723 One warning: particularly complicated regular expressions can take
724 exponential time to solve due to the immense number of possible ways they
725 can use backtracking to try match. For example this will take a very long
728 /((a{0,5}){0,5}){0,5}/
730 And if you used C<*>'s instead of limiting it to 0 through 5 matches, then
731 it would take literally forever--or until you ran out of stack space.
733 A powerful tool for optimizing such beasts is "independent" groups,
734 which do not backtrace (see L<C<(?E<gt>pattern)>>). Note also that
735 zero-length lookahead/lookbehind assertions will not backtrace to make
736 the tail match, since they are in "logical" context: only the fact
737 whether they match or not is considered relevant. For an example
738 where side-effects of a lookahead I<might> have influenced the
739 following match, see L<C<(?E<gt>pattern)>>.
741 =head2 Version 8 Regular Expressions
743 In case you're not familiar with the "regular" Version 8 regex
744 routines, here are the pattern-matching rules not described above.
746 Any single character matches itself, unless it is a I<metacharacter>
747 with a special meaning described here or above. You can cause
748 characters that normally function as metacharacters to be interpreted
749 literally by prefixing them with a "\" (e.g., "\." matches a ".", not any
750 character; "\\" matches a "\"). A series of characters matches that
751 series of characters in the target string, so the pattern C<blurfl>
752 would match "blurfl" in the target string.
754 You can specify a character class, by enclosing a list of characters
755 in C<[]>, which will match any one character from the list. If the
756 first character after the "[" is "^", the class matches any character not
757 in the list. Within a list, the "-" character is used to specify a
758 range, so that C<a-z> represents all characters between "a" and "z",
759 inclusive. If you want "-" itself to be a member of a class, put it
760 at the start or end of the list, or escape it with a backslash. (The
761 following all specify the same class of three characters: C<[-az]>,
762 C<[az-]>, and C<[a\-z]>. All are different from C<[a-z]>, which
763 specifies a class containing twenty-six characters.)
765 Note also that the whole range idea is rather unportable between
766 character sets--and even within character sets they may cause results
767 you probably didn't expect. A sound principle is to use only ranges
768 that begin from and end at either alphabets of equal case ([a-e],
769 [A-E]), or digits ([0-9]). Anything else is unsafe. If in doubt,
770 spell out the character sets in full.
772 Characters may be specified using a metacharacter syntax much like that
773 used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return,
774 "\f" a form feed, etc. More generally, \I<nnn>, where I<nnn> is a string
775 of octal digits, matches the character whose ASCII value is I<nnn>.
776 Similarly, \xI<nn>, where I<nn> are hexadecimal digits, matches the
777 character whose ASCII value is I<nn>. The expression \cI<x> matches the
778 ASCII character control-I<x>. Finally, the "." metacharacter matches any
779 character except "\n" (unless you use C</s>).
781 You can specify a series of alternatives for a pattern using "|" to
782 separate them, so that C<fee|fie|foe> will match any of "fee", "fie",
783 or "foe" in the target string (as would C<f(e|i|o)e>). The
784 first alternative includes everything from the last pattern delimiter
785 ("(", "[", or the beginning of the pattern) up to the first "|", and
786 the last alternative contains everything from the last "|" to the next
787 pattern delimiter. For this reason, it's common practice to include
788 alternatives in parentheses, to minimize confusion about where they
791 Alternatives are tried from left to right, so the first
792 alternative found for which the entire expression matches, is the one that
793 is chosen. This means that alternatives are not necessarily greedy. For
794 example: when matching C<foo|foot> against "barefoot", only the "foo"
795 part will match, as that is the first alternative tried, and it successfully
796 matches the target string. (This might not seem important, but it is
797 important when you are capturing matched text using parentheses.)
799 Also remember that "|" is interpreted as a literal within square brackets,
800 so if you write C<[fee|fie|foe]> you're really only matching C<[feio|]>.
802 Within a pattern, you may designate subpatterns for later reference by
803 enclosing them in parentheses, and you may refer back to the I<n>th
804 subpattern later in the pattern using the metacharacter \I<n>.
805 Subpatterns are numbered based on the left to right order of their
806 opening parenthesis. A backreference matches whatever
807 actually matched the subpattern in the string being examined, not the
808 rules for that subpattern. Therefore, C<(0|0x)\d*\s\1\d*> will
809 match "0x1234 0x4321", but not "0x1234 01234", because subpattern 1
810 actually matched "0x", even though the rule C<0|0x> could
811 potentially match the leading 0 in the second number.
813 =head2 WARNING on \1 vs $1
815 Some people get too used to writing things like:
817 $pattern =~ s/(\W)/\\\1/g;
819 This is grandfathered for the RHS of a substitute to avoid shocking the
820 B<sed> addicts, but it's a dirty habit to get into. That's because in
821 PerlThink, the righthand side of a C<s///> is a double-quoted string. C<\1> in
822 the usual double-quoted string means a control-A. The customary Unix
823 meaning of C<\1> is kludged in for C<s///>. However, if you get into the habit
824 of doing that, you get yourself into trouble if you then add an C</e>
827 s/(\d+)/ \1 + 1 /eg; # causes warning under -w
833 You can't disambiguate that by saying C<\{1}000>, whereas you can fix it with
834 C<${1}000>. Basically, the operation of interpolation should not be confused
835 with the operation of matching a backreference. Certainly they mean two
836 different things on the I<left> side of the C<s///>.
838 =head2 Repeated patterns matching zero-length substring
840 WARNING: Difficult material (and prose) ahead. This section needs a rewrite.
842 Regular expressions provide a terse and powerful programming language. As
843 with most other power tools, power comes together with the ability
846 A common abuse of this power stems from the ability to make infinite
847 loops using regular expressions, with something as innocuous as:
849 'foo' =~ m{ ( o? )* }x;
851 The C<o?> can match at the beginning of C<'foo'>, and since the position
852 in the string is not moved by the match, C<o?> would match again and again
853 due to the C<*> modifier. Another common way to create a similar cycle
854 is with the looping modifier C<//g>:
856 @matches = ( 'foo' =~ m{ o? }xg );
860 print "match: <$&>\n" while 'foo' =~ m{ o? }xg;
862 or the loop implied by split().
864 However, long experience has shown that many programming tasks may
865 be significantly simplified by using repeated subexpressions which
866 may match zero-length substrings, with a simple example being:
868 @chars = split //, $string; # // is not magic in split
869 ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// /
871 Thus Perl allows the C</()/> construct, which I<forcefully breaks
872 the infinite loop>. The rules for this are different for lower-level
873 loops given by the greedy modifiers C<*+{}>, and for higher-level
874 ones like the C</g> modifier or split() operator.
876 The lower-level loops are I<interrupted> when it is detected that a
877 repeated expression did match a zero-length substring, thus
879 m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x;
881 is made equivalent to
883 m{ (?: NON_ZERO_LENGTH )*
888 The higher level-loops preserve an additional state between iterations:
889 whether the last match was zero-length. To break the loop, the following
890 match after a zero-length match is prohibited to have a length of zero.
891 This prohibition interacts with backtracking (see L<"Backtracking">),
892 and so the I<second best> match is chosen if the I<best> match is of
900 results in C<"<><b><><a><><r><>">. At each position of the string the best
901 match given by non-greedy C<??> is the zero-length match, and the I<second
902 best> match is what is matched by C<\w>. Thus zero-length matches
903 alternate with one-character-long matches.
905 Similarly, for repeated C<m/()/g> the second-best match is the match at the
906 position one notch further in the string.
908 The additional state of being I<matched with zero-length> is associated to
909 the matched string, and is reset by each assignment to pos().
911 =head2 Creating custom RE engines
913 Overloaded constants (see L<overload>) provide a simple way to extend
914 the functionality of the RE engine.
916 Suppose that we want to enable a new RE escape-sequence C<\Y|> which
917 matches at boundary between white-space characters and non-whitespace
918 characters. Note that C<(?=\S)(?<!\S)|(?!\S)(?<=\S)> matches exactly
919 at these positions, so we want to have each C<\Y|> in the place of the
920 more complicated version. We can create a module C<customre> to do
928 die "No argument to customre::import allowed" if @_;
929 overload::constant 'qr' => \&convert;
932 sub invalid { die "/$_[0]/: invalid escape '\\$_[1]'"}
934 my %rules = ( '\\' => '\\',
935 'Y|' => qr/(?=\S)(?<!\S)|(?!\S)(?<=\S)/ );
941 { $rules{$1} or invalid($re,$1) }sgex;
945 Now C<use customre> enables the new escape in constant regular
946 expressions, i.e., those without any runtime variable interpolations.
947 As documented in L<overload>, this conversion will work only over
948 literal parts of regular expressions. For C<\Y|$re\Y|> the variable
949 part of this regular expression needs to be converted explicitly
950 (but only if the special meaning of C<\Y|> should be enabled inside $re):
955 $re = customre::convert $re;
960 L<perlop/"Regexp Quote-Like Operators">.
962 L<perlop/"Gory details of parsing quoted constructs">.
968 I<Mastering Regular Expressions> (see L<perlbook>) by Jeffrey Friedl.