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Re: \Q doesn't work in interpolated regular expressions
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1=head1 NAME
2
3perlre - Perl regular expressions
4
5=head1 DESCRIPTION
6
cb1a09d0 7This page describes the syntax of regular expressions in Perl. For a
5f05dabc 8description of how to I<use> regular expressions in matching
75e14d17 9operations, plus various examples of the same, see discussion
1e66bd83 10of C<m//>, C<s///>, C<qr//> and C<??> in L<perlop/"Regexp Quote-Like Operators">.
cb1a09d0 11
68dc0745 12The matching operations can have various modifiers. The modifiers
5a964f20 13that relate to the interpretation of the regular expression inside
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14are listed below. For the modifiers that alter the way a regular expression
15is used by Perl, see L<perlop/"Regexp Quote-Like Operators"> and
16L<perlop/"Gory details of parsing quoted constructs">.
a0d0e21e 17
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18=over 4
19
20=item i
21
22Do case-insensitive pattern matching.
23
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24If C<use locale> is in effect, the case map is taken from the current
25locale. See L<perllocale>.
26
54310121 27=item m
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28
29Treat string as multiple lines. That is, change "^" and "$" from matching
5f05dabc 30at only the very start or end of the string to the start or end of any
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31line anywhere within the string,
32
54310121 33=item s
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34
35Treat string as single line. That is, change "." to match any character
36whatsoever, even a newline, which it normally would not match.
37
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38The C</s> and C</m> modifiers both override the C<$*> setting. That is, no matter
39what C<$*> contains, C</s> without C</m> will force "^" to match only at the
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40beginning of the string and "$" to match only at the end (or just before a
41newline at the end) of the string. Together, as /ms, they let the "." match
42any character whatsoever, while yet allowing "^" and "$" to match,
43respectively, just after and just before newlines within the string.
44
54310121 45=item x
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46
47Extend your pattern's legibility by permitting whitespace and comments.
48
49=back
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50
51These are usually written as "the C</x> modifier", even though the delimiter
52in question might not actually be a slash. In fact, any of these
53modifiers may also be embedded within the regular expression itself using
54the new C<(?...)> construct. See below.
55
4633a7c4 56The C</x> modifier itself needs a little more explanation. It tells
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57the regular expression parser to ignore whitespace that is neither
58backslashed nor within a character class. You can use this to break up
4633a7c4 59your regular expression into (slightly) more readable parts. The C<#>
54310121 60character is also treated as a metacharacter introducing a comment,
55497cff 61just as in ordinary Perl code. This also means that if you want real
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62whitespace or C<#> characters in the pattern (outside of a character
63class, where they are unaffected by C</x>), that you'll either have to
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64escape them or encode them using octal or hex escapes. Taken together,
65these features go a long way towards making Perl's regular expressions
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66more readable. Note that you have to be careful not to include the
67pattern delimiter in the comment--perl has no way of knowing you did
5a964f20 68not intend to close the pattern early. See the C-comment deletion code
0c815be9 69in L<perlop>.
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70
71=head2 Regular Expressions
72
73The patterns used in pattern matching are regular expressions such as
5a964f20 74those supplied in the Version 8 regex routines. (In fact, the
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75routines are derived (distantly) from Henry Spencer's freely
76redistributable reimplementation of the V8 routines.)
77See L<Version 8 Regular Expressions> for details.
78
79In particular the following metacharacters have their standard I<egrep>-ish
80meanings:
81
54310121 82 \ Quote the next metacharacter
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83 ^ Match the beginning of the line
84 . Match any character (except newline)
c07a80fd 85 $ Match the end of the line (or before newline at the end)
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86 | Alternation
87 () Grouping
88 [] Character class
89
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90By default, the "^" character is guaranteed to match at only the
91beginning of the string, the "$" character at only the end (or before the
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92newline at the end) and Perl does certain optimizations with the
93assumption that the string contains only one line. Embedded newlines
94will not be matched by "^" or "$". You may, however, wish to treat a
4a6725af 95string as a multi-line buffer, such that the "^" will match after any
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96newline within the string, and "$" will match before any newline. At the
97cost of a little more overhead, you can do this by using the /m modifier
98on the pattern match operator. (Older programs did this by setting C<$*>,
5f05dabc 99but this practice is now deprecated.)
a0d0e21e 100
4a6725af 101To facilitate multi-line substitutions, the "." character never matches a
55497cff 102newline unless you use the C</s> modifier, which in effect tells Perl to pretend
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103the string is a single line--even if it isn't. The C</s> modifier also
104overrides the setting of C<$*>, in case you have some (badly behaved) older
105code that sets it in another module.
106
107The following standard quantifiers are recognized:
108
109 * Match 0 or more times
110 + Match 1 or more times
111 ? Match 1 or 0 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
115
116(If a curly bracket occurs in any other context, it is treated
117as a regular character.) The "*" modifier is equivalent to C<{0,}>, the "+"
25f94b33 118modifier to C<{1,}>, and the "?" modifier to C<{0,1}>. n and m are limited
c07a80fd 119to integral values less than 65536.
a0d0e21e 120
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121By default, a quantified subpattern is "greedy", that is, it will match as
122many times as possible (given a particular starting location) while still
123allowing the rest of the pattern to match. If you want it to match the
124minimum number of times possible, follow the quantifier with a "?". Note
125that the meanings don't change, just the "greediness":
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126
127 *? Match 0 or more times
128 +? Match 1 or more times
129 ?? Match 0 or 1 time
130 {n}? Match exactly n times
131 {n,}? Match at least n times
132 {n,m}? Match at least n but not more than m times
133
5f05dabc 134Because patterns are processed as double quoted strings, the following
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135also work:
136
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137 \t tab (HT, TAB)
138 \n newline (LF, NL)
139 \r return (CR)
140 \f form feed (FF)
141 \a alarm (bell) (BEL)
142 \e escape (think troff) (ESC)
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143 \033 octal char (think of a PDP-11)
144 \x1B hex char
a0d0e21e 145 \c[ control char
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146 \l lowercase next char (think vi)
147 \u uppercase next char (think vi)
148 \L lowercase till \E (think vi)
149 \U uppercase till \E (think vi)
150 \E end case modification (think vi)
5a964f20 151 \Q quote (disable) pattern metacharacters till \E
a0d0e21e 152
a034a98d 153If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
7b8d334a 154and C<\U> is taken from the current locale. See L<perllocale>.
a034a98d 155
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156You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
157An unescaped C<$> or C<@> interpolates the corresponding variable,
158while escaping will cause the literal string C<\$> to be matched.
159You'll need to write something like C<m/\Quser\E\@\Qhost/>.
160
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161In addition, Perl defines the following:
162
163 \w Match a "word" character (alphanumeric plus "_")
164 \W Match a non-word character
165 \s Match a whitespace character
166 \S Match a non-whitespace character
167 \d Match a digit character
168 \D Match a non-digit character
169
5a964f20 170A C<\w> matches a single alphanumeric character, not a whole
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171word. To match a word you'd need to say C<\w+>. If C<use locale> is in
172effect, the list of alphabetic characters generated by C<\w> is taken
173from the current locale. See L<perllocale>. You may use C<\w>, C<\W>,
174C<\s>, C<\S>, C<\d>, and C<\D> within character classes (though not as
175either end of a range).
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176
177Perl defines the following zero-width assertions:
178
179 \b Match a word boundary
180 \B Match a non-(word boundary)
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181 \A Match only at beginning of string
182 \Z Match only at end of string, or before newline at the end
183 \z Match only at end of string
a99df21c 184 \G Match only where previous m//g left off (works only with /g)
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185
186A word boundary (C<\b>) is defined as a spot between two characters that
68dc0745 187has a C<\w> on one side of it and a C<\W> on the other side of it (in
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188either order), counting the imaginary characters off the beginning and
189end of the string as matching a C<\W>. (Within character classes C<\b>
190represents backspace rather than a word boundary.) The C<\A> and C<\Z> are
5a964f20 191just like "^" and "$", except that they won't match multiple times when the
a0d0e21e 192C</m> modifier is used, while "^" and "$" will match at every internal line
c07a80fd 193boundary. To match the actual end of the string, not ignoring newline,
b85d18e9 194you can use C<\z>. The C<\G> assertion can be used to chain global
a99df21c 195matches (using C<m//g>), as described in
e7ea3e70 196L<perlop/"Regexp Quote-Like Operators">.
a99df21c 197
e7ea3e70 198It is also useful when writing C<lex>-like scanners, when you have several
5a964f20 199patterns that you want to match against consequent substrings of your
e7ea3e70 200string, see the previous reference.
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201The actual location where C<\G> will match can also be influenced
202by using C<pos()> as an lvalue. See L<perlfunc/pos>.
a0d0e21e 203
0f36ee90 204When the bracketing construct C<( ... )> is used, \E<lt>digitE<gt> matches the
cb1a09d0 205digit'th substring. Outside of the pattern, always use "$" instead of "\"
0f36ee90 206in front of the digit. (While the \E<lt>digitE<gt> notation can on rare occasion work
cb1a09d0 207outside the current pattern, this should not be relied upon. See the
0f36ee90 208WARNING below.) The scope of $E<lt>digitE<gt> (and C<$`>, C<$&>, and C<$'>)
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209extends to the end of the enclosing BLOCK or eval string, or to the next
210successful pattern match, whichever comes first. If you want to use
5f05dabc 211parentheses to delimit a subpattern (e.g., a set of alternatives) without
84dc3c4d 212saving it as a subpattern, follow the ( with a ?:.
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213
214You may have as many parentheses as you wish. If you have more
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215than 9 substrings, the variables $10, $11, ... refer to the
216corresponding substring. Within the pattern, \10, \11, etc. refer back
5f05dabc 217to substrings if there have been at least that many left parentheses before
c07a80fd 218the backreference. Otherwise (for backward compatibility) \10 is the
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219same as \010, a backspace, and \11 the same as \011, a tab. And so
220on. (\1 through \9 are always backreferences.)
221
222C<$+> returns whatever the last bracket match matched. C<$&> returns the
0f36ee90 223entire matched string. (C<$0> used to return the same thing, but not any
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224more.) C<$`> returns everything before the matched string. C<$'> returns
225everything after the matched string. Examples:
226
227 s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words
228
229 if (/Time: (..):(..):(..)/) {
230 $hours = $1;
231 $minutes = $2;
232 $seconds = $3;
233 }
234
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235Once perl sees that you need one of C<$&>, C<$`> or C<$'> anywhere in
236the program, it has to provide them on each and every pattern match.
237This can slow your program down. The same mechanism that handles
238these provides for the use of $1, $2, etc., so you pay the same price
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239for each pattern that contains capturing parentheses. But if you never
240use $&, etc., in your script, then patterns I<without> capturing
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241parentheses won't be penalized. So avoid $&, $', and $` if you can,
242but if you can't (and some algorithms really appreciate them), once
243you've used them once, use them at will, because you've already paid
5a964f20 244the price. As of 5.005, $& is not so costly as the other two.
68dc0745 245
5a964f20 246Backslashed metacharacters in Perl are
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247alphanumeric, such as C<\b>, C<\w>, C<\n>. Unlike some other regular
248expression languages, there are no backslashed symbols that aren't
249alphanumeric. So anything that looks like \\, \(, \), \E<lt>, \E<gt>,
250\{, or \} is always interpreted as a literal character, not a
251metacharacter. This was once used in a common idiom to disable or
252quote the special meanings of regular expression metacharacters in a
5a964f20 253string that you want to use for a pattern. Simply quote all
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254non-alphanumeric characters:
255
256 $pattern =~ s/(\W)/\\$1/g;
257
201ecf35 258Now it is much more common to see either the quotemeta() function or
7b8d334a 259the C<\Q> escape sequence used to disable all metacharacters' special
201ecf35 260meanings like this:
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261
262 /$unquoted\Q$quoted\E$unquoted/
263
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264Perl defines a consistent extension syntax for regular expressions.
265The syntax is a pair of parentheses with a question mark as the first
266thing within the parentheses (this was a syntax error in older
267versions of Perl). The character after the question mark gives the
268function of the extension. Several extensions are already supported:
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269
270=over 10
271
cc6b7395 272=item C<(?#text)>
a0d0e21e 273
cb1a09d0 274A comment. The text is ignored. If the C</x> switch is used to enable
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275whitespace formatting, a simple C<#> will suffice. Note that perl closes
276the comment as soon as it sees a C<)>, so there is no way to put a literal
277C<)> in the comment.
a0d0e21e 278
5a964f20 279=item C<(?:pattern)>
a0d0e21e 280
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281=item C<(?imsx-imsx:pattern)>
282
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283This is for clustering, not capturing; it groups subexpressions like
284"()", but doesn't make backreferences as "()" does. So
a0d0e21e 285
5a964f20 286 @fields = split(/\b(?:a|b|c)\b/)
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287
288is like
289
5a964f20 290 @fields = split(/\b(a|b|c)\b/)
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291
292but doesn't spit out extra fields.
293
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294The letters between C<?> and C<:> act as flags modifiers, see
295L<C<(?imsx-imsx)>>. In particular,
296
297 /(?s-i:more.*than).*million/i
298
299is equivalent to more verbose
300
301 /(?:(?s-i)more.*than).*million/i
302
5a964f20 303=item C<(?=pattern)>
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304
305A zero-width positive lookahead assertion. For example, C</\w+(?=\t)/>
306matches a word followed by a tab, without including the tab in C<$&>.
307
5a964f20 308=item C<(?!pattern)>
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309
310A zero-width negative lookahead assertion. For example C</foo(?!bar)/>
311matches any occurrence of "foo" that isn't followed by "bar". Note
312however that lookahead and lookbehind are NOT the same thing. You cannot
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313use this for lookbehind.
314
5a964f20 315If you are looking for a "bar" that isn't preceded by a "foo", C</(?!foo)bar/>
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316will not do what you want. That's because the C<(?!foo)> is just saying that
317the next thing cannot be "foo"--and it's not, it's a "bar", so "foobar" will
318match. You would have to do something like C</(?!foo)...bar/> for that. We
319say "like" because there's the case of your "bar" not having three characters
320before it. You could cover that this way: C</(?:(?!foo)...|^.{0,2})bar/>.
321Sometimes it's still easier just to say:
a0d0e21e 322
a3cb178b 323 if (/bar/ && $` !~ /foo$/)
a0d0e21e 324
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325For lookbehind see below.
326
5a964f20 327=item C<(?E<lt>=pattern)>
c277df42 328
5a964f20 329A zero-width positive lookbehind assertion. For example, C</(?E<lt>=\t)\w+/>
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330matches a word following a tab, without including the tab in C<$&>.
331Works only for fixed-width lookbehind.
332
5a964f20 333=item C<(?<!pattern)>
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334
335A zero-width negative lookbehind assertion. For example C</(?<!bar)foo/>
336matches any occurrence of "foo" that isn't following "bar".
337Works only for fixed-width lookbehind.
338
cc6b7395 339=item C<(?{ code })>
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340
341Experimental "evaluate any Perl code" zero-width assertion. Always
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342succeeds. C<code> is not interpolated. Currently the rules to
343determine where the C<code> ends are somewhat convoluted.
c277df42 344
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345Owing to the risks to security, this is only available when the
346C<use re 'eval'> pragma is used, and then only for patterns that don't
347have any variables that must be interpolated at run time.
348
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349The C<code> is properly scoped in the following sense: if the assertion
350is backtracked (compare L<"Backtracking">), all the changes introduced after
351C<local>isation are undone, so
352
353 $_ = 'a' x 8;
354 m<
355 (?{ $cnt = 0 }) # Initialize $cnt.
356 (
357 a
358 (?{
359 local $cnt = $cnt + 1; # Update $cnt, backtracking-safe.
360 })
361 )*
362 aaaa
363 (?{ $res = $cnt }) # On success copy to non-localized
364 # location.
365 >x;
366
367will set C<$res = 4>. Note that after the match $cnt returns to the globally
368introduced value 0, since the scopes which restrict C<local> statements
369are unwound.
370
371This assertion may be used as L<C<(?(condition)yes-pattern|no-pattern)>>
372switch. If I<not> used in this way, the result of evaluation of C<code>
373is put into variable $^R. This happens immediately, so $^R can be used from
374other C<(?{ code })> assertions inside the same regular expression.
375
376The above assignment to $^R is properly localized, thus the old value of $^R
377is restored if the assertion is backtracked (compare L<"Backtracking">).
378
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379=item C<(?E<gt>pattern)>
380
381An "independent" subexpression. Matches the substring that a
382I<standalone> C<pattern> would match if anchored at the given position,
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383B<and only this substring>.
384
385Say, C<^(?E<gt>a*)ab> will never match, since C<(?E<gt>a*)> (anchored
5a964f20 386at the beginning of string, as above) will match I<all> characters
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387C<a> at the beginning of string, leaving no C<a> for C<ab> to match.
388In contrast, C<a*ab> will match the same as C<a+b>, since the match of
389the subgroup C<a*> is influenced by the following group C<ab> (see
390L<"Backtracking">). In particular, C<a*> inside C<a*ab> will match
391less characters that a standalone C<a*>, since this makes the tail match.
392
5a964f20 393An effect similar to C<(?E<gt>pattern)> may be achieved by
c277df42 394
5a964f20 395 (?=(pattern))\1
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396
397since the lookahead is in I<"logical"> context, thus matches the same
398substring as a standalone C<a+>. The following C<\1> eats the matched
399string, thus making a zero-length assertion into an analogue of
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400C<(?>...)>. (The difference between these two constructs is that the
401second one uses a catching group, thus shifting ordinals of
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402backreferences in the rest of a regular expression.)
403
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404This construct is useful for optimizations of "eternal"
405matches, because it will not backtrack (see L<"Backtracking">).
c277df42 406
5a964f20 407 m{ \( (
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408 [^()]+
409 |
410 \( [^()]* \)
411 )+
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412 \)
413 }x
414
415That will efficiently match a nonempty group with matching
416two-or-less-level-deep parentheses. However, if there is no such group,
417it will take virtually forever on a long string. That's because there are
418so many different ways to split a long string into several substrings.
419This is essentially what C<(.+)+> is doing, and this is a subpattern
420of the above pattern. Consider that C<((()aaaaaaaaaaaaaaaaaa> on the
421pattern above detects no-match in several seconds, but that each extra
422letter doubles this time. This exponential performance will make it
423appear that your program has hung.
424
425However, a tiny modification of this pattern
426
427 m{ \( (
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428 (?> [^()]+ )
429 |
430 \( [^()]* \)
431 )+
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432 \)
433 }x
c277df42 434
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435which uses C<(?E<gt>...)> matches exactly when the one above does (verifying
436this yourself would be a productive exercise), but finishes in a fourth
437the time when used on a similar string with 1000000 C<a>s. Be aware,
438however, that this pattern currently triggers a warning message under
439B<-w> saying it C<"matches the null string many times">):
c277df42 440
5a964f20 441On simple groups, such as the pattern C<(?> [^()]+ )>, a comparable
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442effect may be achieved by negative lookahead, as in C<[^()]+ (?! [^()] )>.
443This was only 4 times slower on a string with 1000000 C<a>s.
444
5a964f20 445=item C<(?(condition)yes-pattern|no-pattern)>
c277df42 446
5a964f20 447=item C<(?(condition)yes-pattern)>
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448
449Conditional expression. C<(condition)> should be either an integer in
450parentheses (which is valid if the corresponding pair of parentheses
451matched), or lookahead/lookbehind/evaluate zero-width assertion.
452
453Say,
454
5a964f20 455 m{ ( \( )?
c277df42 456 [^()]+
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457 (?(1) \) )
458 }x
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459
460matches a chunk of non-parentheses, possibly included in parentheses
461themselves.
a0d0e21e 462
ca9dfc88 463=item C<(?imsx-imsx)>
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464
465One or more embedded pattern-match modifiers. This is particularly
466useful for patterns that are specified in a table somewhere, some of
467which want to be case sensitive, and some of which don't. The case
5f05dabc 468insensitive ones need to include merely C<(?i)> at the front of the
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469pattern. For example:
470
471 $pattern = "foobar";
5a964f20 472 if ( /$pattern/i ) { }
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473
474 # more flexible:
475
476 $pattern = "(?i)foobar";
5a964f20 477 if ( /$pattern/ ) { }
a0d0e21e 478
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479Letters after C<-> switch modifiers off.
480
5a964f20 481These modifiers are localized inside an enclosing group (if any). Say,
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482
483 ( (?i) blah ) \s+ \1
484
485(assuming C<x> modifier, and no C<i> modifier outside of this group)
486will match a repeated (I<including the case>!) word C<blah> in any
487case.
488
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489=back
490
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491A question mark was chosen for this and for the new minimal-matching
492construct because 1) question mark is pretty rare in older regular
493expressions, and 2) whenever you see one, you should stop and "question"
494exactly what is going on. That's psychology...
a0d0e21e 495
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496=head2 Backtracking
497
c277df42 498A fundamental feature of regular expression matching involves the
5a964f20 499notion called I<backtracking>, which is currently used (when needed)
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500by all regular expression quantifiers, namely C<*>, C<*?>, C<+>,
501C<+?>, C<{n,m}>, and C<{n,m}?>.
c07a80fd
PP
502
503For a regular expression to match, the I<entire> regular expression must
504match, not just part of it. So if the beginning of a pattern containing a
505quantifier succeeds in a way that causes later parts in the pattern to
506fail, the matching engine backs up and recalculates the beginning
507part--that's why it's called backtracking.
508
509Here is an example of backtracking: Let's say you want to find the
510word following "foo" in the string "Food is on the foo table.":
511
512 $_ = "Food is on the foo table.";
513 if ( /\b(foo)\s+(\w+)/i ) {
514 print "$2 follows $1.\n";
515 }
516
517When the match runs, the first part of the regular expression (C<\b(foo)>)
518finds a possible match right at the beginning of the string, and loads up
519$1 with "Foo". However, as soon as the matching engine sees that there's
520no whitespace following the "Foo" that it had saved in $1, it realizes its
68dc0745 521mistake and starts over again one character after where it had the
c07a80fd
PP
522tentative match. This time it goes all the way until the next occurrence
523of "foo". The complete regular expression matches this time, and you get
524the expected output of "table follows foo."
525
526Sometimes minimal matching can help a lot. Imagine you'd like to match
527everything between "foo" and "bar". Initially, you write something
528like this:
529
530 $_ = "The food is under the bar in the barn.";
531 if ( /foo(.*)bar/ ) {
532 print "got <$1>\n";
533 }
534
535Which perhaps unexpectedly yields:
536
537 got <d is under the bar in the >
538
539That's because C<.*> was greedy, so you get everything between the
540I<first> "foo" and the I<last> "bar". In this case, it's more effective
541to use minimal matching to make sure you get the text between a "foo"
542and the first "bar" thereafter.
543
544 if ( /foo(.*?)bar/ ) { print "got <$1>\n" }
545 got <d is under the >
546
547Here's another example: let's say you'd like to match a number at the end
548of a string, and you also want to keep the preceding part the match.
549So you write this:
550
551 $_ = "I have 2 numbers: 53147";
552 if ( /(.*)(\d*)/ ) { # Wrong!
553 print "Beginning is <$1>, number is <$2>.\n";
554 }
555
556That won't work at all, because C<.*> was greedy and gobbled up the
557whole string. As C<\d*> can match on an empty string the complete
558regular expression matched successfully.
559
8e1088bc 560 Beginning is <I have 2 numbers: 53147>, number is <>.
c07a80fd
PP
561
562Here are some variants, most of which don't work:
563
564 $_ = "I have 2 numbers: 53147";
565 @pats = qw{
566 (.*)(\d*)
567 (.*)(\d+)
568 (.*?)(\d*)
569 (.*?)(\d+)
570 (.*)(\d+)$
571 (.*?)(\d+)$
572 (.*)\b(\d+)$
573 (.*\D)(\d+)$
574 };
575
576 for $pat (@pats) {
577 printf "%-12s ", $pat;
578 if ( /$pat/ ) {
579 print "<$1> <$2>\n";
580 } else {
581 print "FAIL\n";
582 }
583 }
584
585That will print out:
586
587 (.*)(\d*) <I have 2 numbers: 53147> <>
588 (.*)(\d+) <I have 2 numbers: 5314> <7>
589 (.*?)(\d*) <> <>
590 (.*?)(\d+) <I have > <2>
591 (.*)(\d+)$ <I have 2 numbers: 5314> <7>
592 (.*?)(\d+)$ <I have 2 numbers: > <53147>
593 (.*)\b(\d+)$ <I have 2 numbers: > <53147>
594 (.*\D)(\d+)$ <I have 2 numbers: > <53147>
595
596As you see, this can be a bit tricky. It's important to realize that a
597regular expression is merely a set of assertions that gives a definition
598of success. There may be 0, 1, or several different ways that the
599definition might succeed against a particular string. And if there are
5a964f20
TC
600multiple ways it might succeed, you need to understand backtracking to
601know which variety of success you will achieve.
c07a80fd
PP
602
603When using lookahead assertions and negations, this can all get even
54310121 604tricker. Imagine you'd like to find a sequence of non-digits not
c07a80fd
PP
605followed by "123". You might try to write that as
606
607 $_ = "ABC123";
608 if ( /^\D*(?!123)/ ) { # Wrong!
609 print "Yup, no 123 in $_\n";
610 }
611
612But that isn't going to match; at least, not the way you're hoping. It
613claims that there is no 123 in the string. Here's a clearer picture of
614why it that pattern matches, contrary to popular expectations:
615
616 $x = 'ABC123' ;
617 $y = 'ABC445' ;
618
619 print "1: got $1\n" if $x =~ /^(ABC)(?!123)/ ;
620 print "2: got $1\n" if $y =~ /^(ABC)(?!123)/ ;
621
622 print "3: got $1\n" if $x =~ /^(\D*)(?!123)/ ;
623 print "4: got $1\n" if $y =~ /^(\D*)(?!123)/ ;
624
625This prints
626
627 2: got ABC
628 3: got AB
629 4: got ABC
630
5f05dabc 631You might have expected test 3 to fail because it seems to a more
c07a80fd
PP
632general purpose version of test 1. The important difference between
633them is that test 3 contains a quantifier (C<\D*>) and so can use
634backtracking, whereas test 1 will not. What's happening is
635that you've asked "Is it true that at the start of $x, following 0 or more
5f05dabc 636non-digits, you have something that's not 123?" If the pattern matcher had
c07a80fd 637let C<\D*> expand to "ABC", this would have caused the whole pattern to
54310121 638fail.
c07a80fd 639The search engine will initially match C<\D*> with "ABC". Then it will
5a964f20 640try to match C<(?!123> with "123", which of course fails. But because
c07a80fd
PP
641a quantifier (C<\D*>) has been used in the regular expression, the
642search engine can backtrack and retry the match differently
54310121 643in the hope of matching the complete regular expression.
c07a80fd 644
5a964f20
TC
645The pattern really, I<really> wants to succeed, so it uses the
646standard pattern back-off-and-retry and lets C<\D*> expand to just "AB" this
c07a80fd
PP
647time. Now there's indeed something following "AB" that is not
648"123". It's in fact "C123", which suffices.
649
650We can deal with this by using both an assertion and a negation. We'll
651say that the first part in $1 must be followed by a digit, and in fact, it
652must also be followed by something that's not "123". Remember that the
653lookaheads are zero-width expressions--they only look, but don't consume
654any of the string in their match. So rewriting this way produces what
655you'd expect; that is, case 5 will fail, but case 6 succeeds:
656
657 print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/ ;
658 print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/ ;
659
660 6: got ABC
661
5a964f20 662In other words, the two zero-width assertions next to each other work as though
c07a80fd
PP
663they're ANDed together, just as you'd use any builtin assertions: C</^$/>
664matches only if you're at the beginning of the line AND the end of the
665line simultaneously. The deeper underlying truth is that juxtaposition in
666regular expressions always means AND, except when you write an explicit OR
667using the vertical bar. C</ab/> means match "a" AND (then) match "b",
668although the attempted matches are made at different positions because "a"
669is not a zero-width assertion, but a one-width assertion.
670
671One warning: particularly complicated regular expressions can take
672exponential time to solve due to the immense number of possible ways they
673can use backtracking to try match. For example this will take a very long
674time to run
675
676 /((a{0,5}){0,5}){0,5}/
677
678And if you used C<*>'s instead of limiting it to 0 through 5 matches, then
679it would take literally forever--or until you ran out of stack space.
680
c277df42 681A powerful tool for optimizing such beasts is "independent" groups,
5a964f20 682which do not backtrace (see L<C<(?E<gt>pattern)>>). Note also that
c277df42
IZ
683zero-length lookahead/lookbehind assertions will not backtrace to make
684the tail match, since they are in "logical" context: only the fact
685whether they match or not is considered relevant. For an example
686where side-effects of a lookahead I<might> have influenced the
5a964f20 687following match, see L<C<(?E<gt>pattern)>>.
c277df42 688
a0d0e21e
LW
689=head2 Version 8 Regular Expressions
690
5a964f20 691In case you're not familiar with the "regular" Version 8 regex
a0d0e21e
LW
692routines, here are the pattern-matching rules not described above.
693
54310121 694Any single character matches itself, unless it is a I<metacharacter>
a0d0e21e 695with a special meaning described here or above. You can cause
5a964f20 696characters that normally function as metacharacters to be interpreted
5f05dabc 697literally by prefixing them with a "\" (e.g., "\." matches a ".", not any
a0d0e21e
LW
698character; "\\" matches a "\"). A series of characters matches that
699series of characters in the target string, so the pattern C<blurfl>
700would match "blurfl" in the target string.
701
702You can specify a character class, by enclosing a list of characters
5a964f20 703in C<[]>, which will match any one character from the list. If the
a0d0e21e
LW
704first character after the "[" is "^", the class matches any character not
705in the list. Within a list, the "-" character is used to specify a
5a964f20 706range, so that C<a-z> represents all characters between "a" and "z",
84850974
DD
707inclusive. If you want "-" itself to be a member of a class, put it
708at the start or end of the list, or escape it with a backslash. (The
709following all specify the same class of three characters: C<[-az]>,
710C<[az-]>, and C<[a\-z]>. All are different from C<[a-z]>, which
711specifies a class containing twenty-six characters.)
a0d0e21e 712
54310121 713Characters may be specified using a metacharacter syntax much like that
a0d0e21e
LW
714used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return,
715"\f" a form feed, etc. More generally, \I<nnn>, where I<nnn> is a string
716of octal digits, matches the character whose ASCII value is I<nnn>.
0f36ee90 717Similarly, \xI<nn>, where I<nn> are hexadecimal digits, matches the
a0d0e21e 718character whose ASCII value is I<nn>. The expression \cI<x> matches the
54310121 719ASCII character control-I<x>. Finally, the "." metacharacter matches any
a0d0e21e
LW
720character except "\n" (unless you use C</s>).
721
722You can specify a series of alternatives for a pattern using "|" to
723separate them, so that C<fee|fie|foe> will match any of "fee", "fie",
5a964f20 724or "foe" in the target string (as would C<f(e|i|o)e>). The
a0d0e21e
LW
725first alternative includes everything from the last pattern delimiter
726("(", "[", or the beginning of the pattern) up to the first "|", and
727the last alternative contains everything from the last "|" to the next
728pattern delimiter. For this reason, it's common practice to include
729alternatives in parentheses, to minimize confusion about where they
a3cb178b
GS
730start and end.
731
5a964f20 732Alternatives are tried from left to right, so the first
a3cb178b
GS
733alternative found for which the entire expression matches, is the one that
734is chosen. This means that alternatives are not necessarily greedy. For
735example: when mathing C<foo|foot> against "barefoot", only the "foo"
736part will match, as that is the first alternative tried, and it successfully
737matches the target string. (This might not seem important, but it is
738important when you are capturing matched text using parentheses.)
739
5a964f20 740Also remember that "|" is interpreted as a literal within square brackets,
a3cb178b 741so if you write C<[fee|fie|foe]> you're really only matching C<[feio|]>.
a0d0e21e 742
54310121 743Within a pattern, you may designate subpatterns for later reference by
a0d0e21e 744enclosing them in parentheses, and you may refer back to the I<n>th
54310121
PP
745subpattern later in the pattern using the metacharacter \I<n>.
746Subpatterns are numbered based on the left to right order of their
5a964f20 747opening parenthesis. A backreference matches whatever
54310121
PP
748actually matched the subpattern in the string being examined, not the
749rules for that subpattern. Therefore, C<(0|0x)\d*\s\1\d*> will
5a964f20 750match "0x1234 0x4321", but not "0x1234 01234", because subpattern 1
748a9306 751actually matched "0x", even though the rule C<0|0x> could
a0d0e21e 752potentially match the leading 0 in the second number.
cb1a09d0
AD
753
754=head2 WARNING on \1 vs $1
755
5a964f20 756Some people get too used to writing things like:
cb1a09d0
AD
757
758 $pattern =~ s/(\W)/\\\1/g;
759
760This is grandfathered for the RHS of a substitute to avoid shocking the
761B<sed> addicts, but it's a dirty habit to get into. That's because in
5f05dabc 762PerlThink, the righthand side of a C<s///> is a double-quoted string. C<\1> in
cb1a09d0
AD
763the usual double-quoted string means a control-A. The customary Unix
764meaning of C<\1> is kludged in for C<s///>. However, if you get into the habit
765of doing that, you get yourself into trouble if you then add an C</e>
766modifier.
767
5a964f20 768 s/(\d+)/ \1 + 1 /eg; # causes warning under -w
cb1a09d0
AD
769
770Or if you try to do
771
772 s/(\d+)/\1000/;
773
774You can't disambiguate that by saying C<\{1}000>, whereas you can fix it with
775C<${1}000>. Basically, the operation of interpolation should not be confused
776with the operation of matching a backreference. Certainly they mean two
777different things on the I<left> side of the C<s///>.
9fa51da4 778
c84d73f1
IZ
779=head2 Repeated patterns matching zero-length substring
780
781WARNING: Difficult material (and prose) ahead. This section needs a rewrite.
782
783Regular expressions provide a terse and powerful programming language. As
784with most other power tools, power comes together with the ability
785to wreak havoc.
786
787A common abuse of this power stems from the ability to make infinite
788loops using regular expressions, with something as innocous as:
789
790 'foo' =~ m{ ( o? )* }x;
791
792The C<o?> can match at the beginning of C<'foo'>, and since the position
793in the string is not moved by the match, C<o?> would match again and again
794due to the C<*> modifier. Another common way to create a similar cycle
795is with the looping modifier C<//g>:
796
797 @matches = ( 'foo' =~ m{ o? }xg );
798
799or
800
801 print "match: <$&>\n" while 'foo' =~ m{ o? }xg;
802
803or the loop implied by split().
804
805However, long experience has shown that many programming tasks may
806be significantly simplified by using repeated subexpressions which
807may match zero-length substrings, with a simple example being:
808
809 @chars = split //, $string; # // is not magic in split
810 ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// /
811
812Thus Perl allows the C</()/> construct, which I<forcefully breaks
813the infinite loop>. The rules for this are different for lower-level
814loops given by the greedy modifiers C<*+{}>, and for higher-level
815ones like the C</g> modifier or split() operator.
816
817The lower-level loops are I<interrupted> when it is detected that a
818repeated expression did match a zero-length substring, thus
819
820 m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x;
821
822is made equivalent to
823
824 m{ (?: NON_ZERO_LENGTH )*
825 |
826 (?: ZERO_LENGTH )?
827 }x;
828
829The higher level-loops preserve an additional state between iterations:
830whether the last match was zero-length. To break the loop, the following
831match after a zero-length match is prohibited to have a length of zero.
832This prohibition interacts with backtracking (see L<"Backtracking">),
833and so the I<second best> match is chosen if the I<best> match is of
834zero length.
835
836Say,
837
838 $_ = 'bar';
839 s/\w??/<$&>/g;
840
841results in C<"<><b><><a><><r><>">. At each position of the string the best
842match given by non-greedy C<??> is the zero-length match, and the I<second
843best> match is what is matched by C<\w>. Thus zero-length matches
844alternate with one-character-long matches.
845
846Similarly, for repeated C<m/()/g> the second-best match is the match at the
847position one notch further in the string.
848
849The additional state of being I<matched with zero-length> is associated to
850the matched string, and is reset by each assignment to pos().
851
852=head2 Creating custom RE engines
853
854Overloaded constants (see L<overload>) provide a simple way to extend
855the functionality of the RE engine.
856
857Suppose that we want to enable a new RE escape-sequence C<\Y|> which
858matches at boundary between white-space characters and non-whitespace
859characters. Note that C<(?=\S)(?<!\S)|(?!\S)(?<=\S)> matches exactly
860at these positions, so we want to have each C<\Y|> in the place of the
861more complicated version. We can create a module C<customre> to do
862this:
863
864 package customre;
865 use overload;
866
867 sub import {
868 shift;
869 die "No argument to customre::import allowed" if @_;
870 overload::constant 'qr' => \&convert;
871 }
872
873 sub invalid { die "/$_[0]/: invalid escape '\\$_[1]'"}
874
875 my %rules = ( '\\' => '\\',
876 'Y|' => qr/(?=\S)(?<!\S)|(?!\S)(?<=\S)/ );
877 sub convert {
878 my $re = shift;
879 $re =~ s{
880 \\ ( \\ | Y . )
881 }
882 { $rules{$1} or invalid($re,$1) }sgex;
883 return $re;
884 }
885
886Now C<use customre> enables the new escape in constant regular
887expressions, i.e., those without any runtime variable interpolations.
888As documented in L<overload>, this conversion will work only over
889literal parts of regular expressions. For C<\Y|$re\Y|> the variable
890part of this regular expression needs to be converted explicitly
891(but only if the special meaning of C<\Y|> should be enabled inside $re):
892
893 use customre;
894 $re = <>;
895 chomp $re;
896 $re = customre::convert $re;
897 /\Y|$re\Y|/;
898
9fa51da4
CS
899=head2 SEE ALSO
900
9b599b2a
GS
901L<perlop/"Regexp Quote-Like Operators">.
902
1e66bd83
PP
903L<perlop/"Gory details of parsing quoted constructs">.
904
9b599b2a
GS
905L<perlfunc/pos>.
906
907L<perllocale>.
908
5a964f20 909I<Mastering Regular Expressions> (see L<perlbook>) by Jeffrey Friedl.