Commit | Line | Data |
---|---|---|
a0d0e21e | 1 | =head1 NAME |
d74e8afc | 2 | X<regular expression> X<regex> X<regexp> |
a0d0e21e LW |
3 | |
4 | perlre - Perl regular expressions | |
5 | ||
6 | =head1 DESCRIPTION | |
7 | ||
5d458dd8 | 8 | This page describes the syntax of regular expressions in Perl. |
91e0c79e | 9 | |
cc46d5f2 | 10 | If you haven't used regular expressions before, a quick-start |
91e0c79e MJD |
11 | introduction is available in L<perlrequick>, and a longer tutorial |
12 | introduction is available in L<perlretut>. | |
13 | ||
14 | For reference on how regular expressions are used in matching | |
15 | operations, plus various examples of the same, see discussions of | |
16 | C<m//>, C<s///>, C<qr//> and C<??> in L<perlop/"Regexp Quote-Like | |
17 | Operators">. | |
cb1a09d0 | 18 | |
0d017f4d WL |
19 | |
20 | =head2 Modifiers | |
21 | ||
19799a22 | 22 | Matching operations can have various modifiers. Modifiers |
5a964f20 | 23 | that relate to the interpretation of the regular expression inside |
19799a22 | 24 | are listed below. Modifiers that alter the way a regular expression |
5d458dd8 | 25 | is used by Perl are detailed in L<perlop/"Regexp Quote-Like Operators"> and |
1e66bd83 | 26 | L<perlop/"Gory details of parsing quoted constructs">. |
a0d0e21e | 27 | |
55497cff | 28 | =over 4 |
29 | ||
54310121 | 30 | =item m |
d74e8afc | 31 | X</m> X<regex, multiline> X<regexp, multiline> X<regular expression, multiline> |
55497cff | 32 | |
33 | Treat string as multiple lines. That is, change "^" and "$" from matching | |
14218588 | 34 | the start or end of the string to matching the start or end of any |
7f761169 | 35 | line anywhere within the string. |
55497cff | 36 | |
54310121 | 37 | =item s |
d74e8afc ITB |
38 | X</s> X<regex, single-line> X<regexp, single-line> |
39 | X<regular expression, single-line> | |
55497cff | 40 | |
41 | Treat string as single line. That is, change "." to match any character | |
19799a22 | 42 | whatsoever, even a newline, which normally it would not match. |
55497cff | 43 | |
34d67d80 | 44 | Used together, as C</ms>, they let the "." match any character whatsoever, |
fb55449c | 45 | while still allowing "^" and "$" to match, respectively, just after |
19799a22 | 46 | and just before newlines within the string. |
7b8d334a | 47 | |
87e95b7f YO |
48 | =item i |
49 | X</i> X<regex, case-insensitive> X<regexp, case-insensitive> | |
50 | X<regular expression, case-insensitive> | |
51 | ||
52 | Do case-insensitive pattern matching. | |
53 | ||
54 | If C<use locale> is in effect, the case map is taken from the current | |
55 | locale. See L<perllocale>. | |
56 | ||
54310121 | 57 | =item x |
d74e8afc | 58 | X</x> |
55497cff | 59 | |
60 | Extend your pattern's legibility by permitting whitespace and comments. | |
61 | ||
87e95b7f YO |
62 | =item p |
63 | X</p> X<regex, preserve> X<regexp, preserve> | |
64 | ||
632a1772 | 65 | Preserve the string matched such that ${^PREMATCH}, ${^MATCH}, and |
87e95b7f YO |
66 | ${^POSTMATCH} are available for use after matching. |
67 | ||
e2e6bec7 DN |
68 | =item g and c |
69 | X</g> X</c> | |
70 | ||
71 | Global matching, and keep the Current position after failed matching. | |
72 | Unlike i, m, s and x, these two flags affect the way the regex is used | |
73 | rather than the regex itself. See | |
74 | L<perlretut/"Using regular expressions in Perl"> for further explanation | |
75 | of the g and c modifiers. | |
76 | ||
55497cff | 77 | =back |
a0d0e21e LW |
78 | |
79 | These are usually written as "the C</x> modifier", even though the delimiter | |
ab7bb42d LB |
80 | in question might not really be a slash. The modifiers C</imsx> |
81 | may also be embedded within the regular expression itself using | |
da392a17 KW |
82 | the C<(?...)> construct. Also are new (in 5.14) character set semantics |
83 | modifiers B<C<<"a">>, B<C<"d">>, B<C<"l">> and B<C<"u">>, which, in 5.14 | |
84 | only, must be used embedded in the regular expression, and not after the | |
85 | trailing delimiter. All this is discussed below in | |
86 | L</Extended Patterns>. | |
87 | X</a> X</d> X</l> X</u> | |
a0d0e21e | 88 | |
4633a7c4 | 89 | The C</x> modifier itself needs a little more explanation. It tells |
7b059540 | 90 | the regular expression parser to ignore most whitespace that is neither |
55497cff | 91 | backslashed nor within a character class. You can use this to break up |
4633a7c4 | 92 | your regular expression into (slightly) more readable parts. The C<#> |
54310121 | 93 | character is also treated as a metacharacter introducing a comment, |
55497cff | 94 | just as in ordinary Perl code. This also means that if you want real |
14218588 | 95 | whitespace or C<#> characters in the pattern (outside a character |
f9a3ff1a | 96 | class, where they are unaffected by C</x>), then you'll either have to |
7b059540 KW |
97 | escape them (using backslashes or C<\Q...\E>) or encode them using octal, |
98 | hex, or C<\N{}> escapes. Taken together, these features go a long way towards | |
8933a740 RGS |
99 | making Perl's regular expressions more readable. Note that you have to |
100 | be careful not to include the pattern delimiter in the comment--perl has | |
101 | no way of knowing you did not intend to close the pattern early. See | |
102 | the C-comment deletion code in L<perlop>. Also note that anything inside | |
7651b971 KW |
103 | a C<\Q...\E> stays unaffected by C</x>. And note that C</x> doesn't affect |
104 | whether space interpretation within a single multi-character construct. For | |
105 | example in C<\x{...}>, regardless of the C</x> modifier, there can be no | |
9bb1f947 | 106 | spaces. Same for a L<quantifier|/Quantifiers> such as C<{3}> or |
f9e949fd KW |
107 | C<{5,}>. Similarly, C<(?:...)> can't have a space between the C<?> and C<:>, |
108 | but can between the C<(> and C<?>. Within any delimiters for such a | |
109 | construct, allowed spaces are not affected by C</x>, and depend on the | |
110 | construct. For example, C<\x{...}> can't have spaces because hexadecimal | |
111 | numbers don't have spaces in them. But, Unicode properties can have spaces, so | |
112 | in C<\p{...}> there can be spaces that follow the Unicode rules, for which see | |
9bb1f947 | 113 | L<perluniprops/Properties accessible through \p{} and \P{}>. |
d74e8afc | 114 | X</x> |
a0d0e21e LW |
115 | |
116 | =head2 Regular Expressions | |
117 | ||
04838cea RGS |
118 | =head3 Metacharacters |
119 | ||
384f06ae | 120 | The patterns used in Perl pattern matching evolved from those supplied in |
14218588 | 121 | the Version 8 regex routines. (The routines are derived |
19799a22 GS |
122 | (distantly) from Henry Spencer's freely redistributable reimplementation |
123 | of the V8 routines.) See L<Version 8 Regular Expressions> for | |
124 | details. | |
a0d0e21e LW |
125 | |
126 | In particular the following metacharacters have their standard I<egrep>-ish | |
127 | meanings: | |
d74e8afc ITB |
128 | X<metacharacter> |
129 | X<\> X<^> X<.> X<$> X<|> X<(> X<()> X<[> X<[]> | |
130 | ||
a0d0e21e | 131 | |
f793d64a KW |
132 | \ Quote the next metacharacter |
133 | ^ Match the beginning of the line | |
134 | . Match any character (except newline) | |
135 | $ Match the end of the line (or before newline at the end) | |
136 | | Alternation | |
137 | () Grouping | |
138 | [] Bracketed Character class | |
a0d0e21e | 139 | |
14218588 GS |
140 | By default, the "^" character is guaranteed to match only the |
141 | beginning of the string, the "$" character only the end (or before the | |
142 | newline at the end), and Perl does certain optimizations with the | |
a0d0e21e LW |
143 | assumption that the string contains only one line. Embedded newlines |
144 | will not be matched by "^" or "$". You may, however, wish to treat a | |
4a6725af | 145 | string as a multi-line buffer, such that the "^" will match after any |
0d520e8e YO |
146 | newline within the string (except if the newline is the last character in |
147 | the string), and "$" will match before any newline. At the | |
a0d0e21e LW |
148 | cost of a little more overhead, you can do this by using the /m modifier |
149 | on the pattern match operator. (Older programs did this by setting C<$*>, | |
f02c194e | 150 | but this practice has been removed in perl 5.9.) |
d74e8afc | 151 | X<^> X<$> X</m> |
a0d0e21e | 152 | |
14218588 | 153 | To simplify multi-line substitutions, the "." character never matches a |
55497cff | 154 | newline unless you use the C</s> modifier, which in effect tells Perl to pretend |
f02c194e | 155 | the string is a single line--even if it isn't. |
d74e8afc | 156 | X<.> X</s> |
a0d0e21e | 157 | |
04838cea RGS |
158 | =head3 Quantifiers |
159 | ||
a0d0e21e | 160 | The following standard quantifiers are recognized: |
d74e8afc | 161 | X<metacharacter> X<quantifier> X<*> X<+> X<?> X<{n}> X<{n,}> X<{n,m}> |
a0d0e21e | 162 | |
f793d64a KW |
163 | * Match 0 or more times |
164 | + Match 1 or more times | |
165 | ? Match 1 or 0 times | |
166 | {n} Match exactly n times | |
167 | {n,} Match at least n times | |
168 | {n,m} Match at least n but not more than m times | |
a0d0e21e LW |
169 | |
170 | (If a curly bracket occurs in any other context, it is treated | |
b975c076 | 171 | as a regular character. In particular, the lower bound |
527e91da BB |
172 | is not optional.) The "*" quantifier is equivalent to C<{0,}>, the "+" |
173 | quantifier to C<{1,}>, and the "?" quantifier to C<{0,1}>. n and m are limited | |
d0b16107 | 174 | to non-negative integral values less than a preset limit defined when perl is built. |
9c79236d GS |
175 | This is usually 32766 on the most common platforms. The actual limit can |
176 | be seen in the error message generated by code such as this: | |
177 | ||
820475bd | 178 | $_ **= $_ , / {$_} / for 2 .. 42; |
a0d0e21e | 179 | |
54310121 | 180 | By default, a quantified subpattern is "greedy", that is, it will match as |
181 | many times as possible (given a particular starting location) while still | |
182 | allowing the rest of the pattern to match. If you want it to match the | |
183 | minimum number of times possible, follow the quantifier with a "?". Note | |
184 | that the meanings don't change, just the "greediness": | |
0d017f4d | 185 | X<metacharacter> X<greedy> X<greediness> |
d74e8afc | 186 | X<?> X<*?> X<+?> X<??> X<{n}?> X<{n,}?> X<{n,m}?> |
a0d0e21e | 187 | |
f793d64a KW |
188 | *? Match 0 or more times, not greedily |
189 | +? Match 1 or more times, not greedily | |
190 | ?? Match 0 or 1 time, not greedily | |
191 | {n}? Match exactly n times, not greedily | |
192 | {n,}? Match at least n times, not greedily | |
193 | {n,m}? Match at least n but not more than m times, not greedily | |
a0d0e21e | 194 | |
b9b4dddf YO |
195 | By default, when a quantified subpattern does not allow the rest of the |
196 | overall pattern to match, Perl will backtrack. However, this behaviour is | |
0d017f4d | 197 | sometimes undesirable. Thus Perl provides the "possessive" quantifier form |
b9b4dddf YO |
198 | as well. |
199 | ||
f793d64a KW |
200 | *+ Match 0 or more times and give nothing back |
201 | ++ Match 1 or more times and give nothing back | |
202 | ?+ Match 0 or 1 time and give nothing back | |
203 | {n}+ Match exactly n times and give nothing back (redundant) | |
204 | {n,}+ Match at least n times and give nothing back | |
205 | {n,m}+ Match at least n but not more than m times and give nothing back | |
b9b4dddf YO |
206 | |
207 | For instance, | |
208 | ||
209 | 'aaaa' =~ /a++a/ | |
210 | ||
211 | will never match, as the C<a++> will gobble up all the C<a>'s in the | |
212 | string and won't leave any for the remaining part of the pattern. This | |
213 | feature can be extremely useful to give perl hints about where it | |
214 | shouldn't backtrack. For instance, the typical "match a double-quoted | |
215 | string" problem can be most efficiently performed when written as: | |
216 | ||
217 | /"(?:[^"\\]++|\\.)*+"/ | |
218 | ||
0d017f4d | 219 | as we know that if the final quote does not match, backtracking will not |
b9b4dddf YO |
220 | help. See the independent subexpression C<< (?>...) >> for more details; |
221 | possessive quantifiers are just syntactic sugar for that construct. For | |
222 | instance the above example could also be written as follows: | |
223 | ||
224 | /"(?>(?:(?>[^"\\]+)|\\.)*)"/ | |
225 | ||
04838cea RGS |
226 | =head3 Escape sequences |
227 | ||
5f05dabc | 228 | Because patterns are processed as double quoted strings, the following |
a0d0e21e LW |
229 | also work: |
230 | ||
f793d64a KW |
231 | \t tab (HT, TAB) |
232 | \n newline (LF, NL) | |
233 | \r return (CR) | |
234 | \f form feed (FF) | |
235 | \a alarm (bell) (BEL) | |
236 | \e escape (think troff) (ESC) | |
f793d64a | 237 | \cK control char (example: VT) |
dc0d9c48 | 238 | \x{}, \x00 character whose ordinal is the given hexadecimal number |
fb121860 | 239 | \N{name} named Unicode character or character sequence |
f793d64a | 240 | \N{U+263D} Unicode character (example: FIRST QUARTER MOON) |
f0a2b745 | 241 | \o{}, \000 character whose ordinal is the given octal number |
f793d64a KW |
242 | \l lowercase next char (think vi) |
243 | \u uppercase next char (think vi) | |
244 | \L lowercase till \E (think vi) | |
245 | \U uppercase till \E (think vi) | |
246 | \Q quote (disable) pattern metacharacters till \E | |
247 | \E end either case modification or quoted section, think vi | |
a0d0e21e | 248 | |
9bb1f947 | 249 | Details are in L<perlop/Quote and Quote-like Operators>. |
1d2dff63 | 250 | |
e1d1eefb | 251 | =head3 Character Classes and other Special Escapes |
04838cea | 252 | |
a0d0e21e | 253 | In addition, Perl defines the following: |
d0b16107 | 254 | X<\g> X<\k> X<\K> X<backreference> |
a0d0e21e | 255 | |
f793d64a KW |
256 | Sequence Note Description |
257 | [...] [1] Match a character according to the rules of the | |
258 | bracketed character class defined by the "...". | |
259 | Example: [a-z] matches "a" or "b" or "c" ... or "z" | |
260 | [[:...:]] [2] Match a character according to the rules of the POSIX | |
261 | character class "..." within the outer bracketed | |
262 | character class. Example: [[:upper:]] matches any | |
263 | uppercase character. | |
d35dd6c6 KW |
264 | \w [3] Match a "word" character (alphanumeric plus "_", plus |
265 | other connector punctuation chars plus Unicode | |
266 | marks | |
f793d64a KW |
267 | \W [3] Match a non-"word" character |
268 | \s [3] Match a whitespace character | |
269 | \S [3] Match a non-whitespace character | |
270 | \d [3] Match a decimal digit character | |
271 | \D [3] Match a non-digit character | |
272 | \pP [3] Match P, named property. Use \p{Prop} for longer names | |
273 | \PP [3] Match non-P | |
274 | \X [4] Match Unicode "eXtended grapheme cluster" | |
275 | \C Match a single C-language char (octet) even if that is | |
276 | part of a larger UTF-8 character. Thus it breaks up | |
277 | characters into their UTF-8 bytes, so you may end up | |
278 | with malformed pieces of UTF-8. Unsupported in | |
279 | lookbehind. | |
c27a5cfe | 280 | \1 [5] Backreference to a specific capture group or buffer. |
f793d64a KW |
281 | '1' may actually be any positive integer. |
282 | \g1 [5] Backreference to a specific or previous group, | |
283 | \g{-1} [5] The number may be negative indicating a relative | |
c27a5cfe | 284 | previous group and may optionally be wrapped in |
f793d64a KW |
285 | curly brackets for safer parsing. |
286 | \g{name} [5] Named backreference | |
287 | \k<name> [5] Named backreference | |
288 | \K [6] Keep the stuff left of the \K, don't include it in $& | |
289 | \N [7] Any character but \n (experimental). Not affected by | |
290 | /s modifier | |
291 | \v [3] Vertical whitespace | |
292 | \V [3] Not vertical whitespace | |
293 | \h [3] Horizontal whitespace | |
294 | \H [3] Not horizontal whitespace | |
295 | \R [4] Linebreak | |
e1d1eefb | 296 | |
9bb1f947 KW |
297 | =over 4 |
298 | ||
299 | =item [1] | |
300 | ||
301 | See L<perlrecharclass/Bracketed Character Classes> for details. | |
df225385 | 302 | |
9bb1f947 | 303 | =item [2] |
b8c5462f | 304 | |
9bb1f947 | 305 | See L<perlrecharclass/POSIX Character Classes> for details. |
b8c5462f | 306 | |
9bb1f947 | 307 | =item [3] |
5496314a | 308 | |
9bb1f947 | 309 | See L<perlrecharclass/Backslash sequences> for details. |
5496314a | 310 | |
9bb1f947 | 311 | =item [4] |
5496314a | 312 | |
9bb1f947 | 313 | See L<perlrebackslash/Misc> for details. |
d0b16107 | 314 | |
9bb1f947 | 315 | =item [5] |
b8c5462f | 316 | |
c27a5cfe | 317 | See L</Capture groups> below for details. |
93733859 | 318 | |
9bb1f947 | 319 | =item [6] |
b8c5462f | 320 | |
9bb1f947 KW |
321 | See L</Extended Patterns> below for details. |
322 | ||
323 | =item [7] | |
324 | ||
325 | Note that C<\N> has two meanings. When of the form C<\N{NAME}>, it matches the | |
fb121860 KW |
326 | character or character sequence whose name is C<NAME>; and similarly |
327 | when of the form C<\N{U+I<hex>}>, it matches the character whose Unicode | |
328 | code point is I<hex>. Otherwise it matches any character but C<\n>. | |
9bb1f947 KW |
329 | |
330 | =back | |
d0b16107 | 331 | |
04838cea RGS |
332 | =head3 Assertions |
333 | ||
a0d0e21e | 334 | Perl defines the following zero-width assertions: |
d74e8afc ITB |
335 | X<zero-width assertion> X<assertion> X<regex, zero-width assertion> |
336 | X<regexp, zero-width assertion> | |
337 | X<regular expression, zero-width assertion> | |
338 | X<\b> X<\B> X<\A> X<\Z> X<\z> X<\G> | |
a0d0e21e | 339 | |
9bb1f947 KW |
340 | \b Match a word boundary |
341 | \B Match except at a word boundary | |
342 | \A Match only at beginning of string | |
343 | \Z Match only at end of string, or before newline at the end | |
344 | \z Match only at end of string | |
345 | \G Match only at pos() (e.g. at the end-of-match position | |
9da458fc | 346 | of prior m//g) |
a0d0e21e | 347 | |
14218588 | 348 | A word boundary (C<\b>) is a spot between two characters |
19799a22 GS |
349 | that has a C<\w> on one side of it and a C<\W> on the other side |
350 | of it (in either order), counting the imaginary characters off the | |
351 | beginning and end of the string as matching a C<\W>. (Within | |
352 | character classes C<\b> represents backspace rather than a word | |
353 | boundary, just as it normally does in any double-quoted string.) | |
354 | The C<\A> and C<\Z> are just like "^" and "$", except that they | |
355 | won't match multiple times when the C</m> modifier is used, while | |
356 | "^" and "$" will match at every internal line boundary. To match | |
357 | the actual end of the string and not ignore an optional trailing | |
358 | newline, use C<\z>. | |
d74e8afc | 359 | X<\b> X<\A> X<\Z> X<\z> X</m> |
19799a22 GS |
360 | |
361 | The C<\G> assertion can be used to chain global matches (using | |
362 | C<m//g>), as described in L<perlop/"Regexp Quote-Like Operators">. | |
363 | It is also useful when writing C<lex>-like scanners, when you have | |
364 | several patterns that you want to match against consequent substrings | |
365 | of your string, see the previous reference. The actual location | |
366 | where C<\G> will match can also be influenced by using C<pos()> as | |
58e23c8d YO |
367 | an lvalue: see L<perlfunc/pos>. Note that the rule for zero-length |
368 | matches is modified somewhat, in that contents to the left of C<\G> is | |
369 | not counted when determining the length of the match. Thus the following | |
370 | will not match forever: | |
d74e8afc | 371 | X<\G> |
c47ff5f1 | 372 | |
e761bb84 CO |
373 | my $string = 'ABC'; |
374 | pos($string) = 1; | |
375 | while ($string =~ /(.\G)/g) { | |
376 | print $1; | |
377 | } | |
58e23c8d YO |
378 | |
379 | It will print 'A' and then terminate, as it considers the match to | |
380 | be zero-width, and thus will not match at the same position twice in a | |
381 | row. | |
382 | ||
383 | It is worth noting that C<\G> improperly used can result in an infinite | |
384 | loop. Take care when using patterns that include C<\G> in an alternation. | |
385 | ||
c27a5cfe | 386 | =head3 Capture groups |
04838cea | 387 | |
c27a5cfe KW |
388 | The bracketing construct C<( ... )> creates capture groups (also referred to as |
389 | capture buffers). To refer to the current contents of a group later on, within | |
d8b950dc KW |
390 | the same pattern, use C<\g1> (or C<\g{1}>) for the first, C<\g2> (or C<\g{2}>) |
391 | for the second, and so on. | |
392 | This is called a I<backreference>. | |
d74e8afc | 393 | X<regex, capture buffer> X<regexp, capture buffer> |
c27a5cfe | 394 | X<regex, capture group> X<regexp, capture group> |
d74e8afc | 395 | X<regular expression, capture buffer> X<backreference> |
c27a5cfe | 396 | X<regular expression, capture group> X<backreference> |
1f1031fe | 397 | X<\g{1}> X<\g{-1}> X<\g{name}> X<relative backreference> X<named backreference> |
d8b950dc KW |
398 | X<named capture buffer> X<regular expression, named capture buffer> |
399 | X<named capture group> X<regular expression, named capture group> | |
400 | X<%+> X<$+{name}> X<< \k<name> >> | |
401 | There is no limit to the number of captured substrings that you may use. | |
402 | Groups are numbered with the leftmost open parenthesis being number 1, etc. If | |
403 | a group did not match, the associated backreference won't match either. (This | |
404 | can happen if the group is optional, or in a different branch of an | |
405 | alternation.) | |
406 | You can omit the C<"g">, and write C<"\1">, etc, but there are some issues with | |
407 | this form, described below. | |
408 | ||
409 | You can also refer to capture groups relatively, by using a negative number, so | |
410 | that C<\g-1> and C<\g{-1}> both refer to the immediately preceding capture | |
411 | group, and C<\g-2> and C<\g{-2}> both refer to the group before it. For | |
412 | example: | |
5624f11d YO |
413 | |
414 | / | |
c27a5cfe KW |
415 | (Y) # group 1 |
416 | ( # group 2 | |
417 | (X) # group 3 | |
418 | \g{-1} # backref to group 3 | |
419 | \g{-3} # backref to group 1 | |
5624f11d YO |
420 | ) |
421 | /x | |
422 | ||
d8b950dc KW |
423 | would match the same as C</(Y) ( (X) \g3 \g1 )/x>. This allows you to |
424 | interpolate regexes into larger regexes and not have to worry about the | |
425 | capture groups being renumbered. | |
426 | ||
427 | You can dispense with numbers altogether and create named capture groups. | |
428 | The notation is C<(?E<lt>I<name>E<gt>...)> to declare and C<\g{I<name>}> to | |
429 | reference. (To be compatible with .Net regular expressions, C<\g{I<name>}> may | |
430 | also be written as C<\k{I<name>}>, C<\kE<lt>I<name>E<gt>> or C<\k'I<name>'>.) | |
431 | I<name> must not begin with a number, nor contain hyphens. | |
432 | When different groups within the same pattern have the same name, any reference | |
433 | to that name assumes the leftmost defined group. Named groups count in | |
434 | absolute and relative numbering, and so can also be referred to by those | |
435 | numbers. | |
436 | (It's possible to do things with named capture groups that would otherwise | |
437 | require C<(??{})>.) | |
438 | ||
439 | Capture group contents are dynamically scoped and available to you outside the | |
440 | pattern until the end of the enclosing block or until the next successful | |
441 | match, whichever comes first. (See L<perlsyn/"Compound Statements">.) | |
442 | You can refer to them by absolute number (using C<"$1"> instead of C<"\g1">, | |
443 | etc); or by name via the C<%+> hash, using C<"$+{I<name>}">. | |
444 | ||
445 | Braces are required in referring to named capture groups, but are optional for | |
446 | absolute or relative numbered ones. Braces are safer when creating a regex by | |
447 | concatenating smaller strings. For example if you have C<qr/$a$b/>, and C<$a> | |
448 | contained C<"\g1">, and C<$b> contained C<"37">, you would get C</\g137/> which | |
449 | is probably not what you intended. | |
450 | ||
451 | The C<\g> and C<\k> notations were introduced in Perl 5.10.0. Prior to that | |
452 | there were no named nor relative numbered capture groups. Absolute numbered | |
453 | groups were referred to using C<\1>, C<\2>, etc, and this notation is still | |
454 | accepted (and likely always will be). But it leads to some ambiguities if | |
455 | there are more than 9 capture groups, as C<\10> could mean either the tenth | |
456 | capture group, or the character whose ordinal in octal is 010 (a backspace in | |
457 | ASCII). Perl resolves this ambiguity by interpreting C<\10> as a backreference | |
458 | only if at least 10 left parentheses have opened before it. Likewise C<\11> is | |
459 | a backreference only if at least 11 left parentheses have opened before it. | |
e1f120a9 KW |
460 | And so on. C<\1> through C<\9> are always interpreted as backreferences. |
461 | There are several examples below that illustrate these perils. You can avoid | |
462 | the ambiguity by always using C<\g{}> or C<\g> if you mean capturing groups; | |
463 | and for octal constants always using C<\o{}>, or for C<\077> and below, using 3 | |
464 | digits padded with leading zeros, since a leading zero implies an octal | |
465 | constant. | |
d8b950dc KW |
466 | |
467 | The C<\I<digit>> notation also works in certain circumstances outside | |
468 | the pattern. See L</Warning on \1 Instead of $1> below for details.) | |
81714fb9 | 469 | |
14218588 | 470 | Examples: |
a0d0e21e LW |
471 | |
472 | s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words | |
473 | ||
d8b950dc | 474 | /(.)\g1/ # find first doubled char |
81714fb9 YO |
475 | and print "'$1' is the first doubled character\n"; |
476 | ||
477 | /(?<char>.)\k<char>/ # ... a different way | |
478 | and print "'$+{char}' is the first doubled character\n"; | |
479 | ||
d8b950dc | 480 | /(?'char'.)\g1/ # ... mix and match |
81714fb9 | 481 | and print "'$1' is the first doubled character\n"; |
c47ff5f1 | 482 | |
14218588 | 483 | if (/Time: (..):(..):(..)/) { # parse out values |
f793d64a KW |
484 | $hours = $1; |
485 | $minutes = $2; | |
486 | $seconds = $3; | |
a0d0e21e | 487 | } |
c47ff5f1 | 488 | |
9d860678 KW |
489 | /(.)(.)(.)(.)(.)(.)(.)(.)(.)\g10/ # \g10 is a backreference |
490 | /(.)(.)(.)(.)(.)(.)(.)(.)(.)\10/ # \10 is octal | |
491 | /((.)(.)(.)(.)(.)(.)(.)(.)(.))\10/ # \10 is a backreference | |
492 | /((.)(.)(.)(.)(.)(.)(.)(.)(.))\010/ # \010 is octal | |
493 | ||
494 | $a = '(.)\1'; # Creates problems when concatenated. | |
495 | $b = '(.)\g{1}'; # Avoids the problems. | |
496 | "aa" =~ /${a}/; # True | |
497 | "aa" =~ /${b}/; # True | |
498 | "aa0" =~ /${a}0/; # False! | |
499 | "aa0" =~ /${b}0/; # True | |
dc0d9c48 KW |
500 | "aa\x08" =~ /${a}0/; # True! |
501 | "aa\x08" =~ /${b}0/; # False | |
9d860678 | 502 | |
14218588 GS |
503 | Several special variables also refer back to portions of the previous |
504 | match. C<$+> returns whatever the last bracket match matched. | |
505 | C<$&> returns the entire matched string. (At one point C<$0> did | |
506 | also, but now it returns the name of the program.) C<$`> returns | |
77ea4f6d JV |
507 | everything before the matched string. C<$'> returns everything |
508 | after the matched string. And C<$^N> contains whatever was matched by | |
509 | the most-recently closed group (submatch). C<$^N> can be used in | |
510 | extended patterns (see below), for example to assign a submatch to a | |
81714fb9 | 511 | variable. |
d74e8afc | 512 | X<$+> X<$^N> X<$&> X<$`> X<$'> |
14218588 | 513 | |
d8b950dc KW |
514 | These special variables, like the C<%+> hash and the numbered match variables |
515 | (C<$1>, C<$2>, C<$3>, etc.) are dynamically scoped | |
14218588 GS |
516 | until the end of the enclosing block or until the next successful |
517 | match, whichever comes first. (See L<perlsyn/"Compound Statements">.) | |
d74e8afc ITB |
518 | X<$+> X<$^N> X<$&> X<$`> X<$'> |
519 | X<$1> X<$2> X<$3> X<$4> X<$5> X<$6> X<$7> X<$8> X<$9> | |
520 | ||
0d017f4d | 521 | B<NOTE>: Failed matches in Perl do not reset the match variables, |
5146ce24 | 522 | which makes it easier to write code that tests for a series of more |
665e98b9 JH |
523 | specific cases and remembers the best match. |
524 | ||
14218588 GS |
525 | B<WARNING>: Once Perl sees that you need one of C<$&>, C<$`>, or |
526 | C<$'> anywhere in the program, it has to provide them for every | |
527 | pattern match. This may substantially slow your program. Perl | |
d8b950dc | 528 | uses the same mechanism to produce C<$1>, C<$2>, etc, so you also pay a |
14218588 GS |
529 | price for each pattern that contains capturing parentheses. (To |
530 | avoid this cost while retaining the grouping behaviour, use the | |
531 | extended regular expression C<(?: ... )> instead.) But if you never | |
532 | use C<$&>, C<$`> or C<$'>, then patterns I<without> capturing | |
533 | parentheses will not be penalized. So avoid C<$&>, C<$'>, and C<$`> | |
534 | if you can, but if you can't (and some algorithms really appreciate | |
535 | them), once you've used them once, use them at will, because you've | |
536 | already paid the price. As of 5.005, C<$&> is not so costly as the | |
537 | other two. | |
d74e8afc | 538 | X<$&> X<$`> X<$'> |
68dc0745 | 539 | |
99d59c4d | 540 | As a workaround for this problem, Perl 5.10.0 introduces C<${^PREMATCH}>, |
cde0cee5 YO |
541 | C<${^MATCH}> and C<${^POSTMATCH}>, which are equivalent to C<$`>, C<$&> |
542 | and C<$'>, B<except> that they are only guaranteed to be defined after a | |
87e95b7f | 543 | successful match that was executed with the C</p> (preserve) modifier. |
cde0cee5 YO |
544 | The use of these variables incurs no global performance penalty, unlike |
545 | their punctuation char equivalents, however at the trade-off that you | |
546 | have to tell perl when you want to use them. | |
87e95b7f | 547 | X</p> X<p modifier> |
cde0cee5 | 548 | |
9d727203 KW |
549 | =head2 Quoting metacharacters |
550 | ||
19799a22 GS |
551 | Backslashed metacharacters in Perl are alphanumeric, such as C<\b>, |
552 | C<\w>, C<\n>. Unlike some other regular expression languages, there | |
553 | are no backslashed symbols that aren't alphanumeric. So anything | |
c47ff5f1 | 554 | that looks like \\, \(, \), \<, \>, \{, or \} is always |
19799a22 GS |
555 | interpreted as a literal character, not a metacharacter. This was |
556 | once used in a common idiom to disable or quote the special meanings | |
557 | of regular expression metacharacters in a string that you want to | |
36bbe248 | 558 | use for a pattern. Simply quote all non-"word" characters: |
a0d0e21e LW |
559 | |
560 | $pattern =~ s/(\W)/\\$1/g; | |
561 | ||
f1cbbd6e | 562 | (If C<use locale> is set, then this depends on the current locale.) |
14218588 GS |
563 | Today it is more common to use the quotemeta() function or the C<\Q> |
564 | metaquoting escape sequence to disable all metacharacters' special | |
565 | meanings like this: | |
a0d0e21e LW |
566 | |
567 | /$unquoted\Q$quoted\E$unquoted/ | |
568 | ||
9da458fc IZ |
569 | Beware that if you put literal backslashes (those not inside |
570 | interpolated variables) between C<\Q> and C<\E>, double-quotish | |
571 | backslash interpolation may lead to confusing results. If you | |
572 | I<need> to use literal backslashes within C<\Q...\E>, | |
573 | consult L<perlop/"Gory details of parsing quoted constructs">. | |
574 | ||
19799a22 GS |
575 | =head2 Extended Patterns |
576 | ||
14218588 GS |
577 | Perl also defines a consistent extension syntax for features not |
578 | found in standard tools like B<awk> and B<lex>. The syntax is a | |
579 | pair of parentheses with a question mark as the first thing within | |
580 | the parentheses. The character after the question mark indicates | |
581 | the extension. | |
19799a22 | 582 | |
14218588 GS |
583 | The stability of these extensions varies widely. Some have been |
584 | part of the core language for many years. Others are experimental | |
585 | and may change without warning or be completely removed. Check | |
586 | the documentation on an individual feature to verify its current | |
587 | status. | |
19799a22 | 588 | |
14218588 GS |
589 | A question mark was chosen for this and for the minimal-matching |
590 | construct because 1) question marks are rare in older regular | |
591 | expressions, and 2) whenever you see one, you should stop and | |
592 | "question" exactly what is going on. That's psychology... | |
a0d0e21e LW |
593 | |
594 | =over 10 | |
595 | ||
cc6b7395 | 596 | =item C<(?#text)> |
d74e8afc | 597 | X<(?#)> |
a0d0e21e | 598 | |
14218588 | 599 | A comment. The text is ignored. If the C</x> modifier enables |
19799a22 | 600 | whitespace formatting, a simple C<#> will suffice. Note that Perl closes |
259138e3 GS |
601 | the comment as soon as it sees a C<)>, so there is no way to put a literal |
602 | C<)> in the comment. | |
a0d0e21e | 603 | |
cfaf538b | 604 | =item C<(?adlupimsx-imsx)> |
fb85c044 | 605 | |
cfaf538b | 606 | =item C<(?^alupimsx)> |
fb85c044 | 607 | X<(?)> X<(?^)> |
19799a22 | 608 | |
0b6d1084 JH |
609 | One or more embedded pattern-match modifiers, to be turned on (or |
610 | turned off, if preceded by C<->) for the remainder of the pattern or | |
fb85c044 KW |
611 | the remainder of the enclosing pattern group (if any). |
612 | ||
fb85c044 | 613 | This is particularly useful for dynamic patterns, such as those read in from a |
0d017f4d WL |
614 | configuration file, taken from an argument, or specified in a table |
615 | somewhere. Consider the case where some patterns want to be case | |
616 | sensitive and some do not: The case insensitive ones merely need to | |
617 | include C<(?i)> at the front of the pattern. For example: | |
19799a22 GS |
618 | |
619 | $pattern = "foobar"; | |
5d458dd8 | 620 | if ( /$pattern/i ) { } |
19799a22 GS |
621 | |
622 | # more flexible: | |
623 | ||
624 | $pattern = "(?i)foobar"; | |
5d458dd8 | 625 | if ( /$pattern/ ) { } |
19799a22 | 626 | |
0b6d1084 | 627 | These modifiers are restored at the end of the enclosing group. For example, |
19799a22 | 628 | |
d8b950dc | 629 | ( (?i) blah ) \s+ \g1 |
19799a22 | 630 | |
0d017f4d WL |
631 | will match C<blah> in any case, some spaces, and an exact (I<including the case>!) |
632 | repetition of the previous word, assuming the C</x> modifier, and no C</i> | |
633 | modifier outside this group. | |
19799a22 | 634 | |
8eb5594e DR |
635 | These modifiers do not carry over into named subpatterns called in the |
636 | enclosing group. In other words, a pattern such as C<((?i)(&NAME))> does not | |
637 | change the case-sensitivity of the "NAME" pattern. | |
638 | ||
dc925305 KW |
639 | Any of these modifiers can be set to apply globally to all regular |
640 | expressions compiled within the scope of a C<use re>. See | |
a0bbd6ff | 641 | L<re/"'/flags' mode">. |
dc925305 | 642 | |
9de15fec KW |
643 | Starting in Perl 5.14, a C<"^"> (caret or circumflex accent) immediately |
644 | after the C<"?"> is a shorthand equivalent to C<d-imsx>. Flags (except | |
645 | C<"d">) may follow the caret to override it. | |
646 | But a minus sign is not legal with it. | |
647 | ||
cfaf538b KW |
648 | Also, starting in Perl 5.14, are modifiers C<"a">, C<"d">, C<"l">, and |
649 | C<"u">, which for 5.14 may not be used as suffix modifiers. | |
9de15fec KW |
650 | |
651 | C<"l"> means to use a locale (see L<perllocale>) when pattern matching. | |
652 | The locale used will be the one in effect at the time of execution of | |
653 | the pattern match. This may not be the same as the compilation-time | |
654 | locale, and can differ from one match to another if there is an | |
655 | intervening call of the | |
656 | L<setlocale() function|perllocale/The setlocale function>. | |
657 | This modifier is automatically set if the regular expression is compiled | |
20db7501 KW |
658 | within the scope of a C<"use locale"> pragma. Results are not |
659 | well-defined when using this and matching against a utf8-encoded string. | |
9de15fec | 660 | |
a12cf05f | 661 | C<"u"> means to use Unicode semantics when pattern matching. It is |
20db7501 KW |
662 | automatically set if the regular expression is encoded in utf8, or is |
663 | compiled within the scope of a | |
664 | L<C<"use feature 'unicode_strings">|feature> pragma (and isn't also in | |
665 | the scope of L<C<"use locale">|locale> nor L<C<"use bytes">|bytes> | |
666 | pragmas. On ASCII platforms, the code points between 128 and 255 take on their | |
667 | Latin-1 (ISO-8859-1) meanings (which are the same as Unicode's), whereas | |
668 | in strict ASCII their meanings are undefined. Thus the platform | |
669 | effectively becomes a Unicode platform. The ASCII characters remain as | |
670 | ASCII characters (since ASCII is a subset of Latin-1 and Unicode). For | |
dc925305 | 671 | example, when this option is not on, on a non-utf8 string, C<"\w"> |
20db7501 KW |
672 | matches precisely C<[A-Za-z0-9_]>. When the option is on, it matches |
673 | not just those, but all the Latin-1 word characters (such as an "n" with | |
674 | a tilde). On EBCDIC platforms, which already are equivalent to Latin-1, | |
675 | this modifier changes behavior only when the C<"/i"> modifier is also | |
676 | specified, and affects only two characters, giving them full Unicode | |
677 | semantics: the C<MICRO SIGN> will match the Greek capital and | |
678 | small letters C<MU>; otherwise not; and the C<LATIN CAPITAL LETTER SHARP | |
679 | S> will match any of C<SS>, C<Ss>, C<sS>, and C<ss>, otherwise not. | |
680 | (This last case is buggy, however.) | |
9de15fec | 681 | |
dc925305 KW |
682 | C<"a"> is the same as C<"u">, except that C<\d>, C<\s>, C<\w>, and the |
683 | Posix character classes are restricted to matching in the ASCII range | |
684 | only. That is, with this modifier, C<\d> always means precisely the | |
685 | digits C<"0"> to C<"9">; C<\s> means the five characters C<[ \f\n\r\t]>; | |
686 | C<\w> means the 63 characters C<[A-Za-z0-9_]>; and likewise, all the | |
cfaf538b KW |
687 | Posix classes such as C<[[:print:]]> match only the appropriate |
688 | ASCII-range characters. As you would expect, this modifier causes, for | |
dc925305 KW |
689 | example, C<\D> to mean the same thing as C<[^0-9]>; in fact, all |
690 | non-ASCII characters match C<\D>, C<\S>, and C<\W>. C<\b> still means | |
691 | to match at the boundary between C<\w> and C<\W>, using the C<"a"> | |
692 | definitions of them (similarly for C<\B>). Otherwise, C<"a"> behaves | |
693 | like the C<"u"> modifier, in that case-insensitive matching uses Unicode | |
694 | semantics; for example, "k" will match the Unicode C<\N{KELVIN SIGN}> | |
695 | under C</i> matching, and code points in the Latin1 range, above ASCII | |
696 | will have Unicode semantics when it comes to case-insensitive matching. | |
2f7f8cb1 KW |
697 | But writing two in "a"'s in a row will increase its effect, causing the |
698 | Kelvin sign and all other non-ASCII characters to not match any ASCII | |
699 | character under C</i> matching. | |
cfaf538b | 700 | |
9de15fec KW |
701 | C<"d"> means to use the traditional Perl pattern matching behavior. |
702 | This is dualistic (hence the name C<"d">, which also could stand for | |
e40e74fe KW |
703 | "depends"). When this is in effect, Perl matches according to the |
704 | platform's native character set rules unless there is something that | |
705 | indicates to use Unicode rules. If either the target string or the | |
706 | pattern itself is encoded in UTF-8, Unicode rules are used. Also, if | |
707 | the pattern contains Unicode-only features, such as code points above | |
708 | 255, C<\p()> Unicode properties or C<\N{}> Unicode names, Unicode rules | |
709 | will be used. It is automatically selected by default if the regular | |
710 | expression is compiled neither within the scope of a C<"use locale"> | |
711 | pragma nor a <C<"use feature 'unicode_strings"> pragma. | |
712 | This behavior causes a number of glitches, see | |
713 | L<perlunicode/The "Unicode Bug">. | |
9de15fec | 714 | |
dc925305 | 715 | Note that the C<a>, C<d>, C<l>, C<p>, and C<u> modifiers are special in |
e1d8d8ac | 716 | that they can only be enabled, not disabled, and the C<a>, C<d>, C<l>, and |
dc925305 | 717 | C<u> modifiers are mutually exclusive: specifying one de-specifies the |
bdc22dd5 | 718 | others, and a maximum of one may appear in the construct. Thus, for |
315f11aa | 719 | example, C<(?-p)>, C<(?-d:...)>, and C<(?dl:...)> will warn when |
bdc22dd5 | 720 | compiled under C<use warnings>. |
9de15fec KW |
721 | |
722 | Note also that the C<p> modifier is special in that its presence | |
723 | anywhere in a pattern has a global effect. | |
cde0cee5 | 724 | |
5a964f20 | 725 | =item C<(?:pattern)> |
d74e8afc | 726 | X<(?:)> |
a0d0e21e | 727 | |
cfaf538b | 728 | =item C<(?adluimsx-imsx:pattern)> |
ca9dfc88 | 729 | |
cfaf538b | 730 | =item C<(?^aluimsx:pattern)> |
fb85c044 KW |
731 | X<(?^:)> |
732 | ||
5a964f20 TC |
733 | This is for clustering, not capturing; it groups subexpressions like |
734 | "()", but doesn't make backreferences as "()" does. So | |
a0d0e21e | 735 | |
5a964f20 | 736 | @fields = split(/\b(?:a|b|c)\b/) |
a0d0e21e LW |
737 | |
738 | is like | |
739 | ||
5a964f20 | 740 | @fields = split(/\b(a|b|c)\b/) |
a0d0e21e | 741 | |
19799a22 GS |
742 | but doesn't spit out extra fields. It's also cheaper not to capture |
743 | characters if you don't need to. | |
a0d0e21e | 744 | |
19799a22 | 745 | Any letters between C<?> and C<:> act as flags modifiers as with |
cfaf538b | 746 | C<(?adluimsx-imsx)>. For example, |
ca9dfc88 IZ |
747 | |
748 | /(?s-i:more.*than).*million/i | |
749 | ||
14218588 | 750 | is equivalent to the more verbose |
ca9dfc88 IZ |
751 | |
752 | /(?:(?s-i)more.*than).*million/i | |
753 | ||
fb85c044 | 754 | Starting in Perl 5.14, a C<"^"> (caret or circumflex accent) immediately |
9de15fec KW |
755 | after the C<"?"> is a shorthand equivalent to C<d-imsx>. Any positive |
756 | flags (except C<"d">) may follow the caret, so | |
fb85c044 KW |
757 | |
758 | (?^x:foo) | |
759 | ||
760 | is equivalent to | |
761 | ||
762 | (?x-ims:foo) | |
763 | ||
764 | The caret tells Perl that this cluster doesn't inherit the flags of any | |
9de15fec | 765 | surrounding pattern, but to go back to the system defaults (C<d-imsx>), |
fb85c044 KW |
766 | modified by any flags specified. |
767 | ||
768 | The caret allows for simpler stringification of compiled regular | |
769 | expressions. These look like | |
770 | ||
771 | (?^:pattern) | |
772 | ||
773 | with any non-default flags appearing between the caret and the colon. | |
774 | A test that looks at such stringification thus doesn't need to have the | |
775 | system default flags hard-coded in it, just the caret. If new flags are | |
776 | added to Perl, the meaning of the caret's expansion will change to include | |
777 | the default for those flags, so the test will still work, unchanged. | |
778 | ||
779 | Specifying a negative flag after the caret is an error, as the flag is | |
780 | redundant. | |
781 | ||
782 | Mnemonic for C<(?^...)>: A fresh beginning since the usual use of a caret is | |
783 | to match at the beginning. | |
784 | ||
594d7033 YO |
785 | =item C<(?|pattern)> |
786 | X<(?|)> X<Branch reset> | |
787 | ||
788 | This is the "branch reset" pattern, which has the special property | |
c27a5cfe | 789 | that the capture groups are numbered from the same starting point |
99d59c4d | 790 | in each alternation branch. It is available starting from perl 5.10.0. |
4deaaa80 | 791 | |
c27a5cfe | 792 | Capture groups are numbered from left to right, but inside this |
693596a8 | 793 | construct the numbering is restarted for each branch. |
4deaaa80 | 794 | |
c27a5cfe | 795 | The numbering within each branch will be as normal, and any groups |
4deaaa80 PJ |
796 | following this construct will be numbered as though the construct |
797 | contained only one branch, that being the one with the most capture | |
c27a5cfe | 798 | groups in it. |
4deaaa80 PJ |
799 | |
800 | This construct will be useful when you want to capture one of a | |
801 | number of alternative matches. | |
802 | ||
803 | Consider the following pattern. The numbers underneath show in | |
c27a5cfe | 804 | which group the captured content will be stored. |
594d7033 YO |
805 | |
806 | ||
807 | # before ---------------branch-reset----------- after | |
808 | / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x | |
809 | # 1 2 2 3 2 3 4 | |
810 | ||
ab106183 A |
811 | Be careful when using the branch reset pattern in combination with |
812 | named captures. Named captures are implemented as being aliases to | |
c27a5cfe | 813 | numbered groups holding the captures, and that interferes with the |
ab106183 A |
814 | implementation of the branch reset pattern. If you are using named |
815 | captures in a branch reset pattern, it's best to use the same names, | |
816 | in the same order, in each of the alternations: | |
817 | ||
818 | /(?| (?<a> x ) (?<b> y ) | |
819 | | (?<a> z ) (?<b> w )) /x | |
820 | ||
821 | Not doing so may lead to surprises: | |
822 | ||
823 | "12" =~ /(?| (?<a> \d+ ) | (?<b> \D+))/x; | |
824 | say $+ {a}; # Prints '12' | |
825 | say $+ {b}; # *Also* prints '12'. | |
826 | ||
c27a5cfe KW |
827 | The problem here is that both the group named C<< a >> and the group |
828 | named C<< b >> are aliases for the group belonging to C<< $1 >>. | |
90a18110 | 829 | |
ee9b8eae YO |
830 | =item Look-Around Assertions |
831 | X<look-around assertion> X<lookaround assertion> X<look-around> X<lookaround> | |
832 | ||
833 | Look-around assertions are zero width patterns which match a specific | |
834 | pattern without including it in C<$&>. Positive assertions match when | |
835 | their subpattern matches, negative assertions match when their subpattern | |
836 | fails. Look-behind matches text up to the current match position, | |
837 | look-ahead matches text following the current match position. | |
838 | ||
839 | =over 4 | |
840 | ||
5a964f20 | 841 | =item C<(?=pattern)> |
d74e8afc | 842 | X<(?=)> X<look-ahead, positive> X<lookahead, positive> |
a0d0e21e | 843 | |
19799a22 | 844 | A zero-width positive look-ahead assertion. For example, C</\w+(?=\t)/> |
a0d0e21e LW |
845 | matches a word followed by a tab, without including the tab in C<$&>. |
846 | ||
5a964f20 | 847 | =item C<(?!pattern)> |
d74e8afc | 848 | X<(?!)> X<look-ahead, negative> X<lookahead, negative> |
a0d0e21e | 849 | |
19799a22 | 850 | A zero-width negative look-ahead assertion. For example C</foo(?!bar)/> |
a0d0e21e | 851 | matches any occurrence of "foo" that isn't followed by "bar". Note |
19799a22 GS |
852 | however that look-ahead and look-behind are NOT the same thing. You cannot |
853 | use this for look-behind. | |
7b8d334a | 854 | |
5a964f20 | 855 | If you are looking for a "bar" that isn't preceded by a "foo", C</(?!foo)bar/> |
7b8d334a GS |
856 | will not do what you want. That's because the C<(?!foo)> is just saying that |
857 | the next thing cannot be "foo"--and it's not, it's a "bar", so "foobar" will | |
858 | match. You would have to do something like C</(?!foo)...bar/> for that. We | |
859 | say "like" because there's the case of your "bar" not having three characters | |
860 | before it. You could cover that this way: C</(?:(?!foo)...|^.{0,2})bar/>. | |
861 | Sometimes it's still easier just to say: | |
a0d0e21e | 862 | |
a3cb178b | 863 | if (/bar/ && $` !~ /foo$/) |
a0d0e21e | 864 | |
19799a22 | 865 | For look-behind see below. |
c277df42 | 866 | |
ee9b8eae YO |
867 | =item C<(?<=pattern)> C<\K> |
868 | X<(?<=)> X<look-behind, positive> X<lookbehind, positive> X<\K> | |
c277df42 | 869 | |
c47ff5f1 | 870 | A zero-width positive look-behind assertion. For example, C</(?<=\t)\w+/> |
19799a22 GS |
871 | matches a word that follows a tab, without including the tab in C<$&>. |
872 | Works only for fixed-width look-behind. | |
c277df42 | 873 | |
ee9b8eae YO |
874 | There is a special form of this construct, called C<\K>, which causes the |
875 | regex engine to "keep" everything it had matched prior to the C<\K> and | |
876 | not include it in C<$&>. This effectively provides variable length | |
877 | look-behind. The use of C<\K> inside of another look-around assertion | |
878 | is allowed, but the behaviour is currently not well defined. | |
879 | ||
c62285ac | 880 | For various reasons C<\K> may be significantly more efficient than the |
ee9b8eae YO |
881 | equivalent C<< (?<=...) >> construct, and it is especially useful in |
882 | situations where you want to efficiently remove something following | |
883 | something else in a string. For instance | |
884 | ||
885 | s/(foo)bar/$1/g; | |
886 | ||
887 | can be rewritten as the much more efficient | |
888 | ||
889 | s/foo\Kbar//g; | |
890 | ||
5a964f20 | 891 | =item C<(?<!pattern)> |
d74e8afc | 892 | X<(?<!)> X<look-behind, negative> X<lookbehind, negative> |
c277df42 | 893 | |
19799a22 GS |
894 | A zero-width negative look-behind assertion. For example C</(?<!bar)foo/> |
895 | matches any occurrence of "foo" that does not follow "bar". Works | |
896 | only for fixed-width look-behind. | |
c277df42 | 897 | |
ee9b8eae YO |
898 | =back |
899 | ||
81714fb9 YO |
900 | =item C<(?'NAME'pattern)> |
901 | ||
902 | =item C<< (?<NAME>pattern) >> | |
903 | X<< (?<NAME>) >> X<(?'NAME')> X<named capture> X<capture> | |
904 | ||
c27a5cfe | 905 | A named capture group. Identical in every respect to normal capturing |
90a18110 | 906 | parentheses C<()> but for the additional fact that C<%+> or C<%-> may be |
1b2010d5 | 907 | used after a successful match to refer to a named group. See L<perlvar> |
90a18110 | 908 | for more details on the C<%+> and C<%-> hashes. |
81714fb9 | 909 | |
c27a5cfe KW |
910 | If multiple distinct capture groups have the same name then the |
911 | $+{NAME} will refer to the leftmost defined group in the match. | |
81714fb9 | 912 | |
0d017f4d | 913 | The forms C<(?'NAME'pattern)> and C<< (?<NAME>pattern) >> are equivalent. |
81714fb9 YO |
914 | |
915 | B<NOTE:> While the notation of this construct is the same as the similar | |
c27a5cfe | 916 | function in .NET regexes, the behavior is not. In Perl the groups are |
81714fb9 YO |
917 | numbered sequentially regardless of being named or not. Thus in the |
918 | pattern | |
919 | ||
920 | /(x)(?<foo>y)(z)/ | |
921 | ||
922 | $+{foo} will be the same as $2, and $3 will contain 'z' instead of | |
923 | the opposite which is what a .NET regex hacker might expect. | |
924 | ||
1f1031fe YO |
925 | Currently NAME is restricted to simple identifiers only. |
926 | In other words, it must match C</^[_A-Za-z][_A-Za-z0-9]*\z/> or | |
927 | its Unicode extension (see L<utf8>), | |
928 | though it isn't extended by the locale (see L<perllocale>). | |
81714fb9 | 929 | |
1f1031fe | 930 | B<NOTE:> In order to make things easier for programmers with experience |
ae5648b3 | 931 | with the Python or PCRE regex engines, the pattern C<< (?PE<lt>NAMEE<gt>pattern) >> |
0d017f4d | 932 | may be used instead of C<< (?<NAME>pattern) >>; however this form does not |
64c5a566 | 933 | support the use of single quotes as a delimiter for the name. |
81714fb9 | 934 | |
1f1031fe YO |
935 | =item C<< \k<NAME> >> |
936 | ||
937 | =item C<< \k'NAME' >> | |
81714fb9 YO |
938 | |
939 | Named backreference. Similar to numeric backreferences, except that | |
940 | the group is designated by name and not number. If multiple groups | |
941 | have the same name then it refers to the leftmost defined group in | |
942 | the current match. | |
943 | ||
0d017f4d | 944 | It is an error to refer to a name not defined by a C<< (?<NAME>) >> |
81714fb9 YO |
945 | earlier in the pattern. |
946 | ||
947 | Both forms are equivalent. | |
948 | ||
1f1031fe | 949 | B<NOTE:> In order to make things easier for programmers with experience |
0d017f4d | 950 | with the Python or PCRE regex engines, the pattern C<< (?P=NAME) >> |
64c5a566 | 951 | may be used instead of C<< \k<NAME> >>. |
1f1031fe | 952 | |
cc6b7395 | 953 | =item C<(?{ code })> |
d74e8afc | 954 | X<(?{})> X<regex, code in> X<regexp, code in> X<regular expression, code in> |
c277df42 | 955 | |
19799a22 | 956 | B<WARNING>: This extended regular expression feature is considered |
b9b4dddf YO |
957 | experimental, and may be changed without notice. Code executed that |
958 | has side effects may not perform identically from version to version | |
959 | due to the effect of future optimisations in the regex engine. | |
c277df42 | 960 | |
cc46d5f2 | 961 | This zero-width assertion evaluates any embedded Perl code. It |
19799a22 GS |
962 | always succeeds, and its C<code> is not interpolated. Currently, |
963 | the rules to determine where the C<code> ends are somewhat convoluted. | |
964 | ||
77ea4f6d JV |
965 | This feature can be used together with the special variable C<$^N> to |
966 | capture the results of submatches in variables without having to keep | |
967 | track of the number of nested parentheses. For example: | |
968 | ||
969 | $_ = "The brown fox jumps over the lazy dog"; | |
970 | /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i; | |
971 | print "color = $color, animal = $animal\n"; | |
972 | ||
754091cb RGS |
973 | Inside the C<(?{...})> block, C<$_> refers to the string the regular |
974 | expression is matching against. You can also use C<pos()> to know what is | |
fa11829f | 975 | the current position of matching within this string. |
754091cb | 976 | |
19799a22 GS |
977 | The C<code> is properly scoped in the following sense: If the assertion |
978 | is backtracked (compare L<"Backtracking">), all changes introduced after | |
979 | C<local>ization are undone, so that | |
b9ac3b5b GS |
980 | |
981 | $_ = 'a' x 8; | |
5d458dd8 | 982 | m< |
f793d64a | 983 | (?{ $cnt = 0 }) # Initialize $cnt. |
b9ac3b5b | 984 | ( |
5d458dd8 | 985 | a |
b9ac3b5b | 986 | (?{ |
f793d64a | 987 | local $cnt = $cnt + 1; # Update $cnt, backtracking-safe. |
b9ac3b5b | 988 | }) |
5d458dd8 | 989 | )* |
b9ac3b5b | 990 | aaaa |
f793d64a KW |
991 | (?{ $res = $cnt }) # On success copy to |
992 | # non-localized location. | |
b9ac3b5b GS |
993 | >x; |
994 | ||
0d017f4d | 995 | will set C<$res = 4>. Note that after the match, C<$cnt> returns to the globally |
14218588 | 996 | introduced value, because the scopes that restrict C<local> operators |
b9ac3b5b GS |
997 | are unwound. |
998 | ||
19799a22 GS |
999 | This assertion may be used as a C<(?(condition)yes-pattern|no-pattern)> |
1000 | switch. If I<not> used in this way, the result of evaluation of | |
1001 | C<code> is put into the special variable C<$^R>. This happens | |
1002 | immediately, so C<$^R> can be used from other C<(?{ code })> assertions | |
1003 | inside the same regular expression. | |
b9ac3b5b | 1004 | |
19799a22 GS |
1005 | The assignment to C<$^R> above is properly localized, so the old |
1006 | value of C<$^R> is restored if the assertion is backtracked; compare | |
1007 | L<"Backtracking">. | |
b9ac3b5b | 1008 | |
19799a22 GS |
1009 | For reasons of security, this construct is forbidden if the regular |
1010 | expression involves run-time interpolation of variables, unless the | |
1011 | perilous C<use re 'eval'> pragma has been used (see L<re>), or the | |
1012 | variables contain results of C<qr//> operator (see | |
9bb1f947 | 1013 | L<perlop/"qr/STRINGE<sol>msixpo">). |
871b0233 | 1014 | |
0d017f4d | 1015 | This restriction is due to the wide-spread and remarkably convenient |
19799a22 | 1016 | custom of using run-time determined strings as patterns. For example: |
871b0233 IZ |
1017 | |
1018 | $re = <>; | |
1019 | chomp $re; | |
1020 | $string =~ /$re/; | |
1021 | ||
14218588 GS |
1022 | Before Perl knew how to execute interpolated code within a pattern, |
1023 | this operation was completely safe from a security point of view, | |
1024 | although it could raise an exception from an illegal pattern. If | |
1025 | you turn on the C<use re 'eval'>, though, it is no longer secure, | |
1026 | so you should only do so if you are also using taint checking. | |
1027 | Better yet, use the carefully constrained evaluation within a Safe | |
cc46d5f2 | 1028 | compartment. See L<perlsec> for details about both these mechanisms. |
871b0233 | 1029 | |
e95d7314 GG |
1030 | B<WARNING>: Use of lexical (C<my>) variables in these blocks is |
1031 | broken. The result is unpredictable and will make perl unstable. The | |
1032 | workaround is to use global (C<our>) variables. | |
1033 | ||
8525cfae FC |
1034 | B<WARNING>: In perl 5.12.x and earlier, the regex engine |
1035 | was not re-entrant, so interpolated code could not | |
1036 | safely invoke the regex engine either directly with | |
e95d7314 | 1037 | C<m//> or C<s///>), or indirectly with functions such as |
8525cfae | 1038 | C<split>. Invoking the regex engine in these blocks would make perl |
e95d7314 | 1039 | unstable. |
8988a1bb | 1040 | |
14455d6c | 1041 | =item C<(??{ code })> |
d74e8afc ITB |
1042 | X<(??{})> |
1043 | X<regex, postponed> X<regexp, postponed> X<regular expression, postponed> | |
0f5d15d6 | 1044 | |
19799a22 | 1045 | B<WARNING>: This extended regular expression feature is considered |
b9b4dddf YO |
1046 | experimental, and may be changed without notice. Code executed that |
1047 | has side effects may not perform identically from version to version | |
1048 | due to the effect of future optimisations in the regex engine. | |
0f5d15d6 | 1049 | |
19799a22 GS |
1050 | This is a "postponed" regular subexpression. The C<code> is evaluated |
1051 | at run time, at the moment this subexpression may match. The result | |
1052 | of evaluation is considered as a regular expression and matched as | |
61528107 | 1053 | if it were inserted instead of this construct. Note that this means |
c27a5cfe | 1054 | that the contents of capture groups defined inside an eval'ed pattern |
6bda09f9 | 1055 | are not available outside of the pattern, and vice versa, there is no |
c27a5cfe | 1056 | way for the inner pattern to refer to a capture group defined outside. |
6bda09f9 YO |
1057 | Thus, |
1058 | ||
1059 | ('a' x 100)=~/(??{'(.)' x 100})/ | |
1060 | ||
81714fb9 | 1061 | B<will> match, it will B<not> set $1. |
0f5d15d6 | 1062 | |
428594d9 | 1063 | The C<code> is not interpolated. As before, the rules to determine |
19799a22 GS |
1064 | where the C<code> ends are currently somewhat convoluted. |
1065 | ||
1066 | The following pattern matches a parenthesized group: | |
0f5d15d6 IZ |
1067 | |
1068 | $re = qr{ | |
f793d64a KW |
1069 | \( |
1070 | (?: | |
1071 | (?> [^()]+ ) # Non-parens without backtracking | |
1072 | | | |
1073 | (??{ $re }) # Group with matching parens | |
1074 | )* | |
1075 | \) | |
1076 | }x; | |
0f5d15d6 | 1077 | |
6bda09f9 YO |
1078 | See also C<(?PARNO)> for a different, more efficient way to accomplish |
1079 | the same task. | |
1080 | ||
0b370c0a A |
1081 | For reasons of security, this construct is forbidden if the regular |
1082 | expression involves run-time interpolation of variables, unless the | |
1083 | perilous C<use re 'eval'> pragma has been used (see L<re>), or the | |
1084 | variables contain results of C<qr//> operator (see | |
9bb1f947 | 1085 | L<perlop/"qrE<sol>STRINGE<sol>msixpo">). |
0b370c0a | 1086 | |
8525cfae FC |
1087 | In perl 5.12.x and earlier, because the regex engine was not re-entrant, |
1088 | delayed code could not safely invoke the regex engine either directly with | |
1089 | C<m//> or C<s///>), or indirectly with functions such as C<split>. | |
8988a1bb | 1090 | |
5d458dd8 YO |
1091 | Recursing deeper than 50 times without consuming any input string will |
1092 | result in a fatal error. The maximum depth is compiled into perl, so | |
6bda09f9 YO |
1093 | changing it requires a custom build. |
1094 | ||
542fa716 YO |
1095 | =item C<(?PARNO)> C<(?-PARNO)> C<(?+PARNO)> C<(?R)> C<(?0)> |
1096 | X<(?PARNO)> X<(?1)> X<(?R)> X<(?0)> X<(?-1)> X<(?+1)> X<(?-PARNO)> X<(?+PARNO)> | |
6bda09f9 | 1097 | X<regex, recursive> X<regexp, recursive> X<regular expression, recursive> |
542fa716 | 1098 | X<regex, relative recursion> |
6bda09f9 | 1099 | |
81714fb9 | 1100 | Similar to C<(??{ code })> except it does not involve compiling any code, |
c27a5cfe KW |
1101 | instead it treats the contents of a capture group as an independent |
1102 | pattern that must match at the current position. Capture groups | |
81714fb9 | 1103 | contained by the pattern will have the value as determined by the |
6bda09f9 YO |
1104 | outermost recursion. |
1105 | ||
894be9b7 | 1106 | PARNO is a sequence of digits (not starting with 0) whose value reflects |
c27a5cfe | 1107 | the paren-number of the capture group to recurse to. C<(?R)> recurses to |
894be9b7 | 1108 | the beginning of the whole pattern. C<(?0)> is an alternate syntax for |
542fa716 | 1109 | C<(?R)>. If PARNO is preceded by a plus or minus sign then it is assumed |
c27a5cfe | 1110 | to be relative, with negative numbers indicating preceding capture groups |
542fa716 | 1111 | and positive ones following. Thus C<(?-1)> refers to the most recently |
c27a5cfe | 1112 | declared group, and C<(?+1)> indicates the next group to be declared. |
c74340f9 | 1113 | Note that the counting for relative recursion differs from that of |
c27a5cfe | 1114 | relative backreferences, in that with recursion unclosed groups B<are> |
c74340f9 | 1115 | included. |
6bda09f9 | 1116 | |
81714fb9 | 1117 | The following pattern matches a function foo() which may contain |
f145b7e9 | 1118 | balanced parentheses as the argument. |
6bda09f9 YO |
1119 | |
1120 | $re = qr{ ( # paren group 1 (full function) | |
81714fb9 | 1121 | foo |
6bda09f9 YO |
1122 | ( # paren group 2 (parens) |
1123 | \( | |
1124 | ( # paren group 3 (contents of parens) | |
1125 | (?: | |
1126 | (?> [^()]+ ) # Non-parens without backtracking | |
1127 | | | |
1128 | (?2) # Recurse to start of paren group 2 | |
1129 | )* | |
1130 | ) | |
1131 | \) | |
1132 | ) | |
1133 | ) | |
1134 | }x; | |
1135 | ||
1136 | If the pattern was used as follows | |
1137 | ||
1138 | 'foo(bar(baz)+baz(bop))'=~/$re/ | |
1139 | and print "\$1 = $1\n", | |
1140 | "\$2 = $2\n", | |
1141 | "\$3 = $3\n"; | |
1142 | ||
1143 | the output produced should be the following: | |
1144 | ||
1145 | $1 = foo(bar(baz)+baz(bop)) | |
1146 | $2 = (bar(baz)+baz(bop)) | |
81714fb9 | 1147 | $3 = bar(baz)+baz(bop) |
6bda09f9 | 1148 | |
c27a5cfe | 1149 | If there is no corresponding capture group defined, then it is a |
61528107 | 1150 | fatal error. Recursing deeper than 50 times without consuming any input |
81714fb9 | 1151 | string will also result in a fatal error. The maximum depth is compiled |
6bda09f9 YO |
1152 | into perl, so changing it requires a custom build. |
1153 | ||
542fa716 YO |
1154 | The following shows how using negative indexing can make it |
1155 | easier to embed recursive patterns inside of a C<qr//> construct | |
1156 | for later use: | |
1157 | ||
1158 | my $parens = qr/(\((?:[^()]++|(?-1))*+\))/; | |
1159 | if (/foo $parens \s+ + \s+ bar $parens/x) { | |
1160 | # do something here... | |
1161 | } | |
1162 | ||
81714fb9 | 1163 | B<Note> that this pattern does not behave the same way as the equivalent |
0d017f4d | 1164 | PCRE or Python construct of the same form. In Perl you can backtrack into |
6bda09f9 | 1165 | a recursed group, in PCRE and Python the recursed into group is treated |
542fa716 YO |
1166 | as atomic. Also, modifiers are resolved at compile time, so constructs |
1167 | like (?i:(?1)) or (?:(?i)(?1)) do not affect how the sub-pattern will | |
1168 | be processed. | |
6bda09f9 | 1169 | |
894be9b7 YO |
1170 | =item C<(?&NAME)> |
1171 | X<(?&NAME)> | |
1172 | ||
0d017f4d WL |
1173 | Recurse to a named subpattern. Identical to C<(?PARNO)> except that the |
1174 | parenthesis to recurse to is determined by name. If multiple parentheses have | |
894be9b7 YO |
1175 | the same name, then it recurses to the leftmost. |
1176 | ||
1177 | It is an error to refer to a name that is not declared somewhere in the | |
1178 | pattern. | |
1179 | ||
1f1031fe YO |
1180 | B<NOTE:> In order to make things easier for programmers with experience |
1181 | with the Python or PCRE regex engines the pattern C<< (?P>NAME) >> | |
64c5a566 | 1182 | may be used instead of C<< (?&NAME) >>. |
1f1031fe | 1183 | |
e2e6a0f1 YO |
1184 | =item C<(?(condition)yes-pattern|no-pattern)> |
1185 | X<(?()> | |
286f584a | 1186 | |
e2e6a0f1 | 1187 | =item C<(?(condition)yes-pattern)> |
286f584a | 1188 | |
e2e6a0f1 YO |
1189 | Conditional expression. C<(condition)> should be either an integer in |
1190 | parentheses (which is valid if the corresponding pair of parentheses | |
1191 | matched), a look-ahead/look-behind/evaluate zero-width assertion, a | |
c27a5cfe | 1192 | name in angle brackets or single quotes (which is valid if a group |
e2e6a0f1 YO |
1193 | with the given name matched), or the special symbol (R) (true when |
1194 | evaluated inside of recursion or eval). Additionally the R may be | |
1195 | followed by a number, (which will be true when evaluated when recursing | |
1196 | inside of the appropriate group), or by C<&NAME>, in which case it will | |
1197 | be true only when evaluated during recursion in the named group. | |
1198 | ||
1199 | Here's a summary of the possible predicates: | |
1200 | ||
1201 | =over 4 | |
1202 | ||
1203 | =item (1) (2) ... | |
1204 | ||
c27a5cfe | 1205 | Checks if the numbered capturing group has matched something. |
e2e6a0f1 YO |
1206 | |
1207 | =item (<NAME>) ('NAME') | |
1208 | ||
c27a5cfe | 1209 | Checks if a group with the given name has matched something. |
e2e6a0f1 | 1210 | |
f01cd190 FC |
1211 | =item (?=...) (?!...) (?<=...) (?<!...) |
1212 | ||
1213 | Checks whether the pattern matches (or does not match, for the '!' | |
1214 | variants). | |
1215 | ||
e2e6a0f1 YO |
1216 | =item (?{ CODE }) |
1217 | ||
f01cd190 | 1218 | Treats the return value of the code block as the condition. |
e2e6a0f1 YO |
1219 | |
1220 | =item (R) | |
1221 | ||
1222 | Checks if the expression has been evaluated inside of recursion. | |
1223 | ||
1224 | =item (R1) (R2) ... | |
1225 | ||
1226 | Checks if the expression has been evaluated while executing directly | |
1227 | inside of the n-th capture group. This check is the regex equivalent of | |
1228 | ||
1229 | if ((caller(0))[3] eq 'subname') { ... } | |
1230 | ||
1231 | In other words, it does not check the full recursion stack. | |
1232 | ||
1233 | =item (R&NAME) | |
1234 | ||
1235 | Similar to C<(R1)>, this predicate checks to see if we're executing | |
1236 | directly inside of the leftmost group with a given name (this is the same | |
1237 | logic used by C<(?&NAME)> to disambiguate). It does not check the full | |
1238 | stack, but only the name of the innermost active recursion. | |
1239 | ||
1240 | =item (DEFINE) | |
1241 | ||
1242 | In this case, the yes-pattern is never directly executed, and no | |
1243 | no-pattern is allowed. Similar in spirit to C<(?{0})> but more efficient. | |
1244 | See below for details. | |
1245 | ||
1246 | =back | |
1247 | ||
1248 | For example: | |
1249 | ||
1250 | m{ ( \( )? | |
1251 | [^()]+ | |
1252 | (?(1) \) ) | |
1253 | }x | |
1254 | ||
1255 | matches a chunk of non-parentheses, possibly included in parentheses | |
1256 | themselves. | |
1257 | ||
1258 | A special form is the C<(DEFINE)> predicate, which never executes directly | |
1259 | its yes-pattern, and does not allow a no-pattern. This allows to define | |
1260 | subpatterns which will be executed only by using the recursion mechanism. | |
1261 | This way, you can define a set of regular expression rules that can be | |
1262 | bundled into any pattern you choose. | |
1263 | ||
1264 | It is recommended that for this usage you put the DEFINE block at the | |
1265 | end of the pattern, and that you name any subpatterns defined within it. | |
1266 | ||
1267 | Also, it's worth noting that patterns defined this way probably will | |
1268 | not be as efficient, as the optimiser is not very clever about | |
1269 | handling them. | |
1270 | ||
1271 | An example of how this might be used is as follows: | |
1272 | ||
2bf803e2 | 1273 | /(?<NAME>(?&NAME_PAT))(?<ADDR>(?&ADDRESS_PAT)) |
e2e6a0f1 | 1274 | (?(DEFINE) |
2bf803e2 YO |
1275 | (?<NAME_PAT>....) |
1276 | (?<ADRESS_PAT>....) | |
e2e6a0f1 YO |
1277 | )/x |
1278 | ||
c27a5cfe KW |
1279 | Note that capture groups matched inside of recursion are not accessible |
1280 | after the recursion returns, so the extra layer of capturing groups is | |
e2e6a0f1 YO |
1281 | necessary. Thus C<$+{NAME_PAT}> would not be defined even though |
1282 | C<$+{NAME}> would be. | |
286f584a | 1283 | |
c47ff5f1 | 1284 | =item C<< (?>pattern) >> |
6bda09f9 | 1285 | X<backtrack> X<backtracking> X<atomic> X<possessive> |
5a964f20 | 1286 | |
19799a22 GS |
1287 | An "independent" subexpression, one which matches the substring |
1288 | that a I<standalone> C<pattern> would match if anchored at the given | |
9da458fc | 1289 | position, and it matches I<nothing other than this substring>. This |
19799a22 GS |
1290 | construct is useful for optimizations of what would otherwise be |
1291 | "eternal" matches, because it will not backtrack (see L<"Backtracking">). | |
9da458fc IZ |
1292 | It may also be useful in places where the "grab all you can, and do not |
1293 | give anything back" semantic is desirable. | |
19799a22 | 1294 | |
c47ff5f1 | 1295 | For example: C<< ^(?>a*)ab >> will never match, since C<< (?>a*) >> |
19799a22 GS |
1296 | (anchored at the beginning of string, as above) will match I<all> |
1297 | characters C<a> at the beginning of string, leaving no C<a> for | |
1298 | C<ab> to match. In contrast, C<a*ab> will match the same as C<a+b>, | |
1299 | since the match of the subgroup C<a*> is influenced by the following | |
1300 | group C<ab> (see L<"Backtracking">). In particular, C<a*> inside | |
1301 | C<a*ab> will match fewer characters than a standalone C<a*>, since | |
1302 | this makes the tail match. | |
1303 | ||
c47ff5f1 | 1304 | An effect similar to C<< (?>pattern) >> may be achieved by writing |
d8b950dc KW |
1305 | C<(?=(pattern))\g1>. This matches the same substring as a standalone |
1306 | C<a+>, and the following C<\g1> eats the matched string; it therefore | |
c47ff5f1 | 1307 | makes a zero-length assertion into an analogue of C<< (?>...) >>. |
19799a22 GS |
1308 | (The difference between these two constructs is that the second one |
1309 | uses a capturing group, thus shifting ordinals of backreferences | |
1310 | in the rest of a regular expression.) | |
1311 | ||
1312 | Consider this pattern: | |
c277df42 | 1313 | |
871b0233 | 1314 | m{ \( |
e2e6a0f1 | 1315 | ( |
f793d64a | 1316 | [^()]+ # x+ |
e2e6a0f1 | 1317 | | |
871b0233 IZ |
1318 | \( [^()]* \) |
1319 | )+ | |
e2e6a0f1 | 1320 | \) |
871b0233 | 1321 | }x |
5a964f20 | 1322 | |
19799a22 GS |
1323 | That will efficiently match a nonempty group with matching parentheses |
1324 | two levels deep or less. However, if there is no such group, it | |
1325 | will take virtually forever on a long string. That's because there | |
1326 | are so many different ways to split a long string into several | |
1327 | substrings. This is what C<(.+)+> is doing, and C<(.+)+> is similar | |
1328 | to a subpattern of the above pattern. Consider how the pattern | |
1329 | above detects no-match on C<((()aaaaaaaaaaaaaaaaaa> in several | |
1330 | seconds, but that each extra letter doubles this time. This | |
1331 | exponential performance will make it appear that your program has | |
14218588 | 1332 | hung. However, a tiny change to this pattern |
5a964f20 | 1333 | |
e2e6a0f1 YO |
1334 | m{ \( |
1335 | ( | |
f793d64a | 1336 | (?> [^()]+ ) # change x+ above to (?> x+ ) |
e2e6a0f1 | 1337 | | |
871b0233 IZ |
1338 | \( [^()]* \) |
1339 | )+ | |
e2e6a0f1 | 1340 | \) |
871b0233 | 1341 | }x |
c277df42 | 1342 | |
c47ff5f1 | 1343 | which uses C<< (?>...) >> matches exactly when the one above does (verifying |
5a964f20 TC |
1344 | this yourself would be a productive exercise), but finishes in a fourth |
1345 | the time when used on a similar string with 1000000 C<a>s. Be aware, | |
1346 | however, that this pattern currently triggers a warning message under | |
9f1b1f2d | 1347 | the C<use warnings> pragma or B<-w> switch saying it |
6bab786b | 1348 | C<"matches null string many times in regex">. |
c277df42 | 1349 | |
c47ff5f1 | 1350 | On simple groups, such as the pattern C<< (?> [^()]+ ) >>, a comparable |
19799a22 | 1351 | effect may be achieved by negative look-ahead, as in C<[^()]+ (?! [^()] )>. |
c277df42 IZ |
1352 | This was only 4 times slower on a string with 1000000 C<a>s. |
1353 | ||
9da458fc IZ |
1354 | The "grab all you can, and do not give anything back" semantic is desirable |
1355 | in many situations where on the first sight a simple C<()*> looks like | |
1356 | the correct solution. Suppose we parse text with comments being delimited | |
1357 | by C<#> followed by some optional (horizontal) whitespace. Contrary to | |
4375e838 | 1358 | its appearance, C<#[ \t]*> I<is not> the correct subexpression to match |
9da458fc IZ |
1359 | the comment delimiter, because it may "give up" some whitespace if |
1360 | the remainder of the pattern can be made to match that way. The correct | |
1361 | answer is either one of these: | |
1362 | ||
1363 | (?>#[ \t]*) | |
1364 | #[ \t]*(?![ \t]) | |
1365 | ||
1366 | For example, to grab non-empty comments into $1, one should use either | |
1367 | one of these: | |
1368 | ||
1369 | / (?> \# [ \t]* ) ( .+ ) /x; | |
1370 | / \# [ \t]* ( [^ \t] .* ) /x; | |
1371 | ||
1372 | Which one you pick depends on which of these expressions better reflects | |
1373 | the above specification of comments. | |
1374 | ||
6bda09f9 YO |
1375 | In some literature this construct is called "atomic matching" or |
1376 | "possessive matching". | |
1377 | ||
b9b4dddf YO |
1378 | Possessive quantifiers are equivalent to putting the item they are applied |
1379 | to inside of one of these constructs. The following equivalences apply: | |
1380 | ||
1381 | Quantifier Form Bracketing Form | |
1382 | --------------- --------------- | |
1383 | PAT*+ (?>PAT*) | |
1384 | PAT++ (?>PAT+) | |
1385 | PAT?+ (?>PAT?) | |
1386 | PAT{min,max}+ (?>PAT{min,max}) | |
1387 | ||
e2e6a0f1 YO |
1388 | =back |
1389 | ||
1390 | =head2 Special Backtracking Control Verbs | |
1391 | ||
1392 | B<WARNING:> These patterns are experimental and subject to change or | |
0d017f4d | 1393 | removal in a future version of Perl. Their usage in production code should |
e2e6a0f1 YO |
1394 | be noted to avoid problems during upgrades. |
1395 | ||
1396 | These special patterns are generally of the form C<(*VERB:ARG)>. Unless | |
1397 | otherwise stated the ARG argument is optional; in some cases, it is | |
1398 | forbidden. | |
1399 | ||
1400 | Any pattern containing a special backtracking verb that allows an argument | |
e1020413 | 1401 | has the special behaviour that when executed it sets the current package's |
5d458dd8 YO |
1402 | C<$REGERROR> and C<$REGMARK> variables. When doing so the following |
1403 | rules apply: | |
e2e6a0f1 | 1404 | |
5d458dd8 YO |
1405 | On failure, the C<$REGERROR> variable will be set to the ARG value of the |
1406 | verb pattern, if the verb was involved in the failure of the match. If the | |
1407 | ARG part of the pattern was omitted, then C<$REGERROR> will be set to the | |
1408 | name of the last C<(*MARK:NAME)> pattern executed, or to TRUE if there was | |
1409 | none. Also, the C<$REGMARK> variable will be set to FALSE. | |
e2e6a0f1 | 1410 | |
5d458dd8 YO |
1411 | On a successful match, the C<$REGERROR> variable will be set to FALSE, and |
1412 | the C<$REGMARK> variable will be set to the name of the last | |
1413 | C<(*MARK:NAME)> pattern executed. See the explanation for the | |
1414 | C<(*MARK:NAME)> verb below for more details. | |
e2e6a0f1 | 1415 | |
5d458dd8 YO |
1416 | B<NOTE:> C<$REGERROR> and C<$REGMARK> are not magic variables like C<$1> |
1417 | and most other regex related variables. They are not local to a scope, nor | |
1418 | readonly, but instead are volatile package variables similar to C<$AUTOLOAD>. | |
1419 | Use C<local> to localize changes to them to a specific scope if necessary. | |
e2e6a0f1 YO |
1420 | |
1421 | If a pattern does not contain a special backtracking verb that allows an | |
5d458dd8 | 1422 | argument, then C<$REGERROR> and C<$REGMARK> are not touched at all. |
e2e6a0f1 YO |
1423 | |
1424 | =over 4 | |
1425 | ||
1426 | =item Verbs that take an argument | |
1427 | ||
1428 | =over 4 | |
1429 | ||
5d458dd8 | 1430 | =item C<(*PRUNE)> C<(*PRUNE:NAME)> |
f7819f85 | 1431 | X<(*PRUNE)> X<(*PRUNE:NAME)> |
54612592 | 1432 | |
5d458dd8 YO |
1433 | This zero-width pattern prunes the backtracking tree at the current point |
1434 | when backtracked into on failure. Consider the pattern C<A (*PRUNE) B>, | |
1435 | where A and B are complex patterns. Until the C<(*PRUNE)> verb is reached, | |
1436 | A may backtrack as necessary to match. Once it is reached, matching | |
1437 | continues in B, which may also backtrack as necessary; however, should B | |
1438 | not match, then no further backtracking will take place, and the pattern | |
1439 | will fail outright at the current starting position. | |
54612592 YO |
1440 | |
1441 | The following example counts all the possible matching strings in a | |
1442 | pattern (without actually matching any of them). | |
1443 | ||
e2e6a0f1 | 1444 | 'aaab' =~ /a+b?(?{print "$&\n"; $count++})(*FAIL)/; |
54612592 YO |
1445 | print "Count=$count\n"; |
1446 | ||
1447 | which produces: | |
1448 | ||
1449 | aaab | |
1450 | aaa | |
1451 | aa | |
1452 | a | |
1453 | aab | |
1454 | aa | |
1455 | a | |
1456 | ab | |
1457 | a | |
1458 | Count=9 | |
1459 | ||
5d458dd8 | 1460 | If we add a C<(*PRUNE)> before the count like the following |
54612592 | 1461 | |
5d458dd8 | 1462 | 'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/; |
54612592 YO |
1463 | print "Count=$count\n"; |
1464 | ||
1465 | we prevent backtracking and find the count of the longest matching | |
353c6505 | 1466 | at each matching starting point like so: |
54612592 YO |
1467 | |
1468 | aaab | |
1469 | aab | |
1470 | ab | |
1471 | Count=3 | |
1472 | ||
5d458dd8 | 1473 | Any number of C<(*PRUNE)> assertions may be used in a pattern. |
54612592 | 1474 | |
5d458dd8 YO |
1475 | See also C<< (?>pattern) >> and possessive quantifiers for other ways to |
1476 | control backtracking. In some cases, the use of C<(*PRUNE)> can be | |
1477 | replaced with a C<< (?>pattern) >> with no functional difference; however, | |
1478 | C<(*PRUNE)> can be used to handle cases that cannot be expressed using a | |
1479 | C<< (?>pattern) >> alone. | |
54612592 | 1480 | |
e2e6a0f1 | 1481 | |
5d458dd8 YO |
1482 | =item C<(*SKIP)> C<(*SKIP:NAME)> |
1483 | X<(*SKIP)> | |
e2e6a0f1 | 1484 | |
5d458dd8 | 1485 | This zero-width pattern is similar to C<(*PRUNE)>, except that on |
e2e6a0f1 | 1486 | failure it also signifies that whatever text that was matched leading up |
5d458dd8 YO |
1487 | to the C<(*SKIP)> pattern being executed cannot be part of I<any> match |
1488 | of this pattern. This effectively means that the regex engine "skips" forward | |
1489 | to this position on failure and tries to match again, (assuming that | |
1490 | there is sufficient room to match). | |
1491 | ||
1492 | The name of the C<(*SKIP:NAME)> pattern has special significance. If a | |
1493 | C<(*MARK:NAME)> was encountered while matching, then it is that position | |
1494 | which is used as the "skip point". If no C<(*MARK)> of that name was | |
1495 | encountered, then the C<(*SKIP)> operator has no effect. When used | |
1496 | without a name the "skip point" is where the match point was when | |
1497 | executing the (*SKIP) pattern. | |
1498 | ||
1499 | Compare the following to the examples in C<(*PRUNE)>, note the string | |
24b23f37 YO |
1500 | is twice as long: |
1501 | ||
5d458dd8 | 1502 | 'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/; |
24b23f37 YO |
1503 | print "Count=$count\n"; |
1504 | ||
1505 | outputs | |
1506 | ||
1507 | aaab | |
1508 | aaab | |
1509 | Count=2 | |
1510 | ||
5d458dd8 | 1511 | Once the 'aaab' at the start of the string has matched, and the C<(*SKIP)> |
353c6505 | 1512 | executed, the next starting point will be where the cursor was when the |
5d458dd8 YO |
1513 | C<(*SKIP)> was executed. |
1514 | ||
5d458dd8 YO |
1515 | =item C<(*MARK:NAME)> C<(*:NAME)> |
1516 | X<(*MARK)> C<(*MARK:NAME)> C<(*:NAME)> | |
1517 | ||
1518 | This zero-width pattern can be used to mark the point reached in a string | |
1519 | when a certain part of the pattern has been successfully matched. This | |
1520 | mark may be given a name. A later C<(*SKIP)> pattern will then skip | |
1521 | forward to that point if backtracked into on failure. Any number of | |
b4222fa9 | 1522 | C<(*MARK)> patterns are allowed, and the NAME portion may be duplicated. |
5d458dd8 YO |
1523 | |
1524 | In addition to interacting with the C<(*SKIP)> pattern, C<(*MARK:NAME)> | |
1525 | can be used to "label" a pattern branch, so that after matching, the | |
1526 | program can determine which branches of the pattern were involved in the | |
1527 | match. | |
1528 | ||
1529 | When a match is successful, the C<$REGMARK> variable will be set to the | |
1530 | name of the most recently executed C<(*MARK:NAME)> that was involved | |
1531 | in the match. | |
1532 | ||
1533 | This can be used to determine which branch of a pattern was matched | |
c27a5cfe | 1534 | without using a separate capture group for each branch, which in turn |
5d458dd8 YO |
1535 | can result in a performance improvement, as perl cannot optimize |
1536 | C</(?:(x)|(y)|(z))/> as efficiently as something like | |
1537 | C</(?:x(*MARK:x)|y(*MARK:y)|z(*MARK:z))/>. | |
1538 | ||
1539 | When a match has failed, and unless another verb has been involved in | |
1540 | failing the match and has provided its own name to use, the C<$REGERROR> | |
1541 | variable will be set to the name of the most recently executed | |
1542 | C<(*MARK:NAME)>. | |
1543 | ||
1544 | See C<(*SKIP)> for more details. | |
1545 | ||
b62d2d15 YO |
1546 | As a shortcut C<(*MARK:NAME)> can be written C<(*:NAME)>. |
1547 | ||
5d458dd8 YO |
1548 | =item C<(*THEN)> C<(*THEN:NAME)> |
1549 | ||
241e7389 | 1550 | This is similar to the "cut group" operator C<::> from Perl 6. Like |
5d458dd8 YO |
1551 | C<(*PRUNE)>, this verb always matches, and when backtracked into on |
1552 | failure, it causes the regex engine to try the next alternation in the | |
1553 | innermost enclosing group (capturing or otherwise). | |
1554 | ||
1555 | Its name comes from the observation that this operation combined with the | |
1556 | alternation operator (C<|>) can be used to create what is essentially a | |
1557 | pattern-based if/then/else block: | |
1558 | ||
1559 | ( COND (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) | |
1560 | ||
1561 | Note that if this operator is used and NOT inside of an alternation then | |
1562 | it acts exactly like the C<(*PRUNE)> operator. | |
1563 | ||
1564 | / A (*PRUNE) B / | |
1565 | ||
1566 | is the same as | |
1567 | ||
1568 | / A (*THEN) B / | |
1569 | ||
1570 | but | |
1571 | ||
1572 | / ( A (*THEN) B | C (*THEN) D ) / | |
1573 | ||
1574 | is not the same as | |
1575 | ||
1576 | / ( A (*PRUNE) B | C (*PRUNE) D ) / | |
1577 | ||
1578 | as after matching the A but failing on the B the C<(*THEN)> verb will | |
1579 | backtrack and try C; but the C<(*PRUNE)> verb will simply fail. | |
24b23f37 | 1580 | |
e2e6a0f1 YO |
1581 | =item C<(*COMMIT)> |
1582 | X<(*COMMIT)> | |
24b23f37 | 1583 | |
241e7389 | 1584 | This is the Perl 6 "commit pattern" C<< <commit> >> or C<:::>. It's a |
5d458dd8 YO |
1585 | zero-width pattern similar to C<(*SKIP)>, except that when backtracked |
1586 | into on failure it causes the match to fail outright. No further attempts | |
1587 | to find a valid match by advancing the start pointer will occur again. | |
1588 | For example, | |
24b23f37 | 1589 | |
e2e6a0f1 | 1590 | 'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/; |
24b23f37 YO |
1591 | print "Count=$count\n"; |
1592 | ||
1593 | outputs | |
1594 | ||
1595 | aaab | |
1596 | Count=1 | |
1597 | ||
e2e6a0f1 YO |
1598 | In other words, once the C<(*COMMIT)> has been entered, and if the pattern |
1599 | does not match, the regex engine will not try any further matching on the | |
1600 | rest of the string. | |
c277df42 | 1601 | |
e2e6a0f1 | 1602 | =back |
9af228c6 | 1603 | |
e2e6a0f1 | 1604 | =item Verbs without an argument |
9af228c6 YO |
1605 | |
1606 | =over 4 | |
1607 | ||
e2e6a0f1 YO |
1608 | =item C<(*FAIL)> C<(*F)> |
1609 | X<(*FAIL)> X<(*F)> | |
9af228c6 | 1610 | |
e2e6a0f1 YO |
1611 | This pattern matches nothing and always fails. It can be used to force the |
1612 | engine to backtrack. It is equivalent to C<(?!)>, but easier to read. In | |
1613 | fact, C<(?!)> gets optimised into C<(*FAIL)> internally. | |
9af228c6 | 1614 | |
e2e6a0f1 | 1615 | It is probably useful only when combined with C<(?{})> or C<(??{})>. |
9af228c6 | 1616 | |
e2e6a0f1 YO |
1617 | =item C<(*ACCEPT)> |
1618 | X<(*ACCEPT)> | |
9af228c6 | 1619 | |
e2e6a0f1 YO |
1620 | B<WARNING:> This feature is highly experimental. It is not recommended |
1621 | for production code. | |
9af228c6 | 1622 | |
e2e6a0f1 YO |
1623 | This pattern matches nothing and causes the end of successful matching at |
1624 | the point at which the C<(*ACCEPT)> pattern was encountered, regardless of | |
1625 | whether there is actually more to match in the string. When inside of a | |
0d017f4d | 1626 | nested pattern, such as recursion, or in a subpattern dynamically generated |
e2e6a0f1 | 1627 | via C<(??{})>, only the innermost pattern is ended immediately. |
9af228c6 | 1628 | |
c27a5cfe | 1629 | If the C<(*ACCEPT)> is inside of capturing groups then the groups are |
e2e6a0f1 YO |
1630 | marked as ended at the point at which the C<(*ACCEPT)> was encountered. |
1631 | For instance: | |
9af228c6 | 1632 | |
e2e6a0f1 | 1633 | 'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x; |
9af228c6 | 1634 | |
e2e6a0f1 | 1635 | will match, and C<$1> will be C<AB> and C<$2> will be C<B>, C<$3> will not |
0d017f4d | 1636 | be set. If another branch in the inner parentheses were matched, such as in the |
e2e6a0f1 | 1637 | string 'ACDE', then the C<D> and C<E> would have to be matched as well. |
9af228c6 YO |
1638 | |
1639 | =back | |
c277df42 | 1640 | |
a0d0e21e LW |
1641 | =back |
1642 | ||
c07a80fd | 1643 | =head2 Backtracking |
d74e8afc | 1644 | X<backtrack> X<backtracking> |
c07a80fd | 1645 | |
35a734be IZ |
1646 | NOTE: This section presents an abstract approximation of regular |
1647 | expression behavior. For a more rigorous (and complicated) view of | |
1648 | the rules involved in selecting a match among possible alternatives, | |
0d017f4d | 1649 | see L<Combining RE Pieces>. |
35a734be | 1650 | |
c277df42 | 1651 | A fundamental feature of regular expression matching involves the |
5a964f20 | 1652 | notion called I<backtracking>, which is currently used (when needed) |
0d017f4d | 1653 | by all regular non-possessive expression quantifiers, namely C<*>, C<*?>, C<+>, |
9da458fc IZ |
1654 | C<+?>, C<{n,m}>, and C<{n,m}?>. Backtracking is often optimized |
1655 | internally, but the general principle outlined here is valid. | |
c07a80fd | 1656 | |
1657 | For a regular expression to match, the I<entire> regular expression must | |
1658 | match, not just part of it. So if the beginning of a pattern containing a | |
1659 | quantifier succeeds in a way that causes later parts in the pattern to | |
1660 | fail, the matching engine backs up and recalculates the beginning | |
1661 | part--that's why it's called backtracking. | |
1662 | ||
1663 | Here is an example of backtracking: Let's say you want to find the | |
1664 | word following "foo" in the string "Food is on the foo table.": | |
1665 | ||
1666 | $_ = "Food is on the foo table."; | |
1667 | if ( /\b(foo)\s+(\w+)/i ) { | |
f793d64a | 1668 | print "$2 follows $1.\n"; |
c07a80fd | 1669 | } |
1670 | ||
1671 | When the match runs, the first part of the regular expression (C<\b(foo)>) | |
1672 | finds a possible match right at the beginning of the string, and loads up | |
1673 | $1 with "Foo". However, as soon as the matching engine sees that there's | |
1674 | no whitespace following the "Foo" that it had saved in $1, it realizes its | |
68dc0745 | 1675 | mistake and starts over again one character after where it had the |
c07a80fd | 1676 | tentative match. This time it goes all the way until the next occurrence |
1677 | of "foo". The complete regular expression matches this time, and you get | |
1678 | the expected output of "table follows foo." | |
1679 | ||
1680 | Sometimes minimal matching can help a lot. Imagine you'd like to match | |
1681 | everything between "foo" and "bar". Initially, you write something | |
1682 | like this: | |
1683 | ||
1684 | $_ = "The food is under the bar in the barn."; | |
1685 | if ( /foo(.*)bar/ ) { | |
f793d64a | 1686 | print "got <$1>\n"; |
c07a80fd | 1687 | } |
1688 | ||
1689 | Which perhaps unexpectedly yields: | |
1690 | ||
1691 | got <d is under the bar in the > | |
1692 | ||
1693 | That's because C<.*> was greedy, so you get everything between the | |
14218588 | 1694 | I<first> "foo" and the I<last> "bar". Here it's more effective |
c07a80fd | 1695 | to use minimal matching to make sure you get the text between a "foo" |
1696 | and the first "bar" thereafter. | |
1697 | ||
1698 | if ( /foo(.*?)bar/ ) { print "got <$1>\n" } | |
1699 | got <d is under the > | |
1700 | ||
0d017f4d | 1701 | Here's another example. Let's say you'd like to match a number at the end |
b6e13d97 | 1702 | of a string, and you also want to keep the preceding part of the match. |
c07a80fd | 1703 | So you write this: |
1704 | ||
1705 | $_ = "I have 2 numbers: 53147"; | |
f793d64a KW |
1706 | if ( /(.*)(\d*)/ ) { # Wrong! |
1707 | print "Beginning is <$1>, number is <$2>.\n"; | |
c07a80fd | 1708 | } |
1709 | ||
1710 | That won't work at all, because C<.*> was greedy and gobbled up the | |
1711 | whole string. As C<\d*> can match on an empty string the complete | |
1712 | regular expression matched successfully. | |
1713 | ||
8e1088bc | 1714 | Beginning is <I have 2 numbers: 53147>, number is <>. |
c07a80fd | 1715 | |
1716 | Here are some variants, most of which don't work: | |
1717 | ||
1718 | $_ = "I have 2 numbers: 53147"; | |
1719 | @pats = qw{ | |
f793d64a KW |
1720 | (.*)(\d*) |
1721 | (.*)(\d+) | |
1722 | (.*?)(\d*) | |
1723 | (.*?)(\d+) | |
1724 | (.*)(\d+)$ | |
1725 | (.*?)(\d+)$ | |
1726 | (.*)\b(\d+)$ | |
1727 | (.*\D)(\d+)$ | |
c07a80fd | 1728 | }; |
1729 | ||
1730 | for $pat (@pats) { | |
f793d64a KW |
1731 | printf "%-12s ", $pat; |
1732 | if ( /$pat/ ) { | |
1733 | print "<$1> <$2>\n"; | |
1734 | } else { | |
1735 | print "FAIL\n"; | |
1736 | } | |
c07a80fd | 1737 | } |
1738 | ||
1739 | That will print out: | |
1740 | ||
1741 | (.*)(\d*) <I have 2 numbers: 53147> <> | |
1742 | (.*)(\d+) <I have 2 numbers: 5314> <7> | |
1743 | (.*?)(\d*) <> <> | |
1744 | (.*?)(\d+) <I have > <2> | |
1745 | (.*)(\d+)$ <I have 2 numbers: 5314> <7> | |
1746 | (.*?)(\d+)$ <I have 2 numbers: > <53147> | |
1747 | (.*)\b(\d+)$ <I have 2 numbers: > <53147> | |
1748 | (.*\D)(\d+)$ <I have 2 numbers: > <53147> | |
1749 | ||
1750 | As you see, this can be a bit tricky. It's important to realize that a | |
1751 | regular expression is merely a set of assertions that gives a definition | |
1752 | of success. There may be 0, 1, or several different ways that the | |
1753 | definition might succeed against a particular string. And if there are | |
5a964f20 TC |
1754 | multiple ways it might succeed, you need to understand backtracking to |
1755 | know which variety of success you will achieve. | |
c07a80fd | 1756 | |
19799a22 | 1757 | When using look-ahead assertions and negations, this can all get even |
8b19b778 | 1758 | trickier. Imagine you'd like to find a sequence of non-digits not |
c07a80fd | 1759 | followed by "123". You might try to write that as |
1760 | ||
871b0233 | 1761 | $_ = "ABC123"; |
f793d64a KW |
1762 | if ( /^\D*(?!123)/ ) { # Wrong! |
1763 | print "Yup, no 123 in $_\n"; | |
871b0233 | 1764 | } |
c07a80fd | 1765 | |
1766 | But that isn't going to match; at least, not the way you're hoping. It | |
1767 | claims that there is no 123 in the string. Here's a clearer picture of | |
9b9391b2 | 1768 | why that pattern matches, contrary to popular expectations: |
c07a80fd | 1769 | |
4358a253 SS |
1770 | $x = 'ABC123'; |
1771 | $y = 'ABC445'; | |
c07a80fd | 1772 | |
4358a253 SS |
1773 | print "1: got $1\n" if $x =~ /^(ABC)(?!123)/; |
1774 | print "2: got $1\n" if $y =~ /^(ABC)(?!123)/; | |
c07a80fd | 1775 | |
4358a253 SS |
1776 | print "3: got $1\n" if $x =~ /^(\D*)(?!123)/; |
1777 | print "4: got $1\n" if $y =~ /^(\D*)(?!123)/; | |
c07a80fd | 1778 | |
1779 | This prints | |
1780 | ||
1781 | 2: got ABC | |
1782 | 3: got AB | |
1783 | 4: got ABC | |
1784 | ||
5f05dabc | 1785 | You might have expected test 3 to fail because it seems to a more |
c07a80fd | 1786 | general purpose version of test 1. The important difference between |
1787 | them is that test 3 contains a quantifier (C<\D*>) and so can use | |
1788 | backtracking, whereas test 1 will not. What's happening is | |
1789 | that you've asked "Is it true that at the start of $x, following 0 or more | |
5f05dabc | 1790 | non-digits, you have something that's not 123?" If the pattern matcher had |
c07a80fd | 1791 | let C<\D*> expand to "ABC", this would have caused the whole pattern to |
54310121 | 1792 | fail. |
14218588 | 1793 | |
c07a80fd | 1794 | The search engine will initially match C<\D*> with "ABC". Then it will |
14218588 | 1795 | try to match C<(?!123> with "123", which fails. But because |
c07a80fd | 1796 | a quantifier (C<\D*>) has been used in the regular expression, the |
1797 | search engine can backtrack and retry the match differently | |
54310121 | 1798 | in the hope of matching the complete regular expression. |
c07a80fd | 1799 | |
5a964f20 TC |
1800 | The pattern really, I<really> wants to succeed, so it uses the |
1801 | standard pattern back-off-and-retry and lets C<\D*> expand to just "AB" this | |
c07a80fd | 1802 | time. Now there's indeed something following "AB" that is not |
14218588 | 1803 | "123". It's "C123", which suffices. |
c07a80fd | 1804 | |
14218588 GS |
1805 | We can deal with this by using both an assertion and a negation. |
1806 | We'll say that the first part in $1 must be followed both by a digit | |
1807 | and by something that's not "123". Remember that the look-aheads | |
1808 | are zero-width expressions--they only look, but don't consume any | |
1809 | of the string in their match. So rewriting this way produces what | |
c07a80fd | 1810 | you'd expect; that is, case 5 will fail, but case 6 succeeds: |
1811 | ||
4358a253 SS |
1812 | print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/; |
1813 | print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/; | |
c07a80fd | 1814 | |
1815 | 6: got ABC | |
1816 | ||
5a964f20 | 1817 | In other words, the two zero-width assertions next to each other work as though |
19799a22 | 1818 | they're ANDed together, just as you'd use any built-in assertions: C</^$/> |
c07a80fd | 1819 | matches only if you're at the beginning of the line AND the end of the |
1820 | line simultaneously. The deeper underlying truth is that juxtaposition in | |
1821 | regular expressions always means AND, except when you write an explicit OR | |
1822 | using the vertical bar. C</ab/> means match "a" AND (then) match "b", | |
1823 | although the attempted matches are made at different positions because "a" | |
1824 | is not a zero-width assertion, but a one-width assertion. | |
1825 | ||
0d017f4d | 1826 | B<WARNING>: Particularly complicated regular expressions can take |
14218588 | 1827 | exponential time to solve because of the immense number of possible |
0d017f4d | 1828 | ways they can use backtracking to try for a match. For example, without |
9da458fc IZ |
1829 | internal optimizations done by the regular expression engine, this will |
1830 | take a painfully long time to run: | |
c07a80fd | 1831 | |
e1901655 IZ |
1832 | 'aaaaaaaaaaaa' =~ /((a{0,5}){0,5})*[c]/ |
1833 | ||
1834 | And if you used C<*>'s in the internal groups instead of limiting them | |
1835 | to 0 through 5 matches, then it would take forever--or until you ran | |
1836 | out of stack space. Moreover, these internal optimizations are not | |
1837 | always applicable. For example, if you put C<{0,5}> instead of C<*> | |
1838 | on the external group, no current optimization is applicable, and the | |
1839 | match takes a long time to finish. | |
c07a80fd | 1840 | |
9da458fc IZ |
1841 | A powerful tool for optimizing such beasts is what is known as an |
1842 | "independent group", | |
c47ff5f1 | 1843 | which does not backtrack (see L<C<< (?>pattern) >>>). Note also that |
9da458fc | 1844 | zero-length look-ahead/look-behind assertions will not backtrack to make |
5d458dd8 | 1845 | the tail match, since they are in "logical" context: only |
14218588 | 1846 | whether they match is considered relevant. For an example |
9da458fc | 1847 | where side-effects of look-ahead I<might> have influenced the |
c47ff5f1 | 1848 | following match, see L<C<< (?>pattern) >>>. |
c277df42 | 1849 | |
a0d0e21e | 1850 | =head2 Version 8 Regular Expressions |
d74e8afc | 1851 | X<regular expression, version 8> X<regex, version 8> X<regexp, version 8> |
a0d0e21e | 1852 | |
5a964f20 | 1853 | In case you're not familiar with the "regular" Version 8 regex |
a0d0e21e LW |
1854 | routines, here are the pattern-matching rules not described above. |
1855 | ||
54310121 | 1856 | Any single character matches itself, unless it is a I<metacharacter> |
a0d0e21e | 1857 | with a special meaning described here or above. You can cause |
5a964f20 | 1858 | characters that normally function as metacharacters to be interpreted |
5f05dabc | 1859 | literally by prefixing them with a "\" (e.g., "\." matches a ".", not any |
0d017f4d WL |
1860 | character; "\\" matches a "\"). This escape mechanism is also required |
1861 | for the character used as the pattern delimiter. | |
1862 | ||
1863 | A series of characters matches that series of characters in the target | |
1864 | string, so the pattern C<blurfl> would match "blurfl" in the target | |
1865 | string. | |
a0d0e21e LW |
1866 | |
1867 | You can specify a character class, by enclosing a list of characters | |
5d458dd8 | 1868 | in C<[]>, which will match any character from the list. If the |
a0d0e21e | 1869 | first character after the "[" is "^", the class matches any character not |
14218588 | 1870 | in the list. Within a list, the "-" character specifies a |
5a964f20 | 1871 | range, so that C<a-z> represents all characters between "a" and "z", |
8a4f6ac2 GS |
1872 | inclusive. If you want either "-" or "]" itself to be a member of a |
1873 | class, put it at the start of the list (possibly after a "^"), or | |
1874 | escape it with a backslash. "-" is also taken literally when it is | |
1875 | at the end of the list, just before the closing "]". (The | |
84850974 DD |
1876 | following all specify the same class of three characters: C<[-az]>, |
1877 | C<[az-]>, and C<[a\-z]>. All are different from C<[a-z]>, which | |
5d458dd8 YO |
1878 | specifies a class containing twenty-six characters, even on EBCDIC-based |
1879 | character sets.) Also, if you try to use the character | |
1880 | classes C<\w>, C<\W>, C<\s>, C<\S>, C<\d>, or C<\D> as endpoints of | |
1881 | a range, the "-" is understood literally. | |
a0d0e21e | 1882 | |
8ada0baa JH |
1883 | Note also that the whole range idea is rather unportable between |
1884 | character sets--and even within character sets they may cause results | |
1885 | you probably didn't expect. A sound principle is to use only ranges | |
0d017f4d | 1886 | that begin from and end at either alphabetics of equal case ([a-e], |
8ada0baa JH |
1887 | [A-E]), or digits ([0-9]). Anything else is unsafe. If in doubt, |
1888 | spell out the character sets in full. | |
1889 | ||
54310121 | 1890 | Characters may be specified using a metacharacter syntax much like that |
a0d0e21e LW |
1891 | used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return, |
1892 | "\f" a form feed, etc. More generally, \I<nnn>, where I<nnn> is a string | |
dc0d9c48 | 1893 | of three octal digits, matches the character whose coded character set value |
5d458dd8 | 1894 | is I<nnn>. Similarly, \xI<nn>, where I<nn> are hexadecimal digits, |
dc0d9c48 | 1895 | matches the character whose ordinal is I<nn>. The expression \cI<x> |
5d458dd8 | 1896 | matches the character control-I<x>. Finally, the "." metacharacter |
fb55449c | 1897 | matches any character except "\n" (unless you use C</s>). |
a0d0e21e LW |
1898 | |
1899 | You can specify a series of alternatives for a pattern using "|" to | |
1900 | separate them, so that C<fee|fie|foe> will match any of "fee", "fie", | |
5a964f20 | 1901 | or "foe" in the target string (as would C<f(e|i|o)e>). The |
a0d0e21e LW |
1902 | first alternative includes everything from the last pattern delimiter |
1903 | ("(", "[", or the beginning of the pattern) up to the first "|", and | |
1904 | the last alternative contains everything from the last "|" to the next | |
14218588 GS |
1905 | pattern delimiter. That's why it's common practice to include |
1906 | alternatives in parentheses: to minimize confusion about where they | |
a3cb178b GS |
1907 | start and end. |
1908 | ||
5a964f20 | 1909 | Alternatives are tried from left to right, so the first |
a3cb178b GS |
1910 | alternative found for which the entire expression matches, is the one that |
1911 | is chosen. This means that alternatives are not necessarily greedy. For | |
628afcb5 | 1912 | example: when matching C<foo|foot> against "barefoot", only the "foo" |
a3cb178b GS |
1913 | part will match, as that is the first alternative tried, and it successfully |
1914 | matches the target string. (This might not seem important, but it is | |
1915 | important when you are capturing matched text using parentheses.) | |
1916 | ||
5a964f20 | 1917 | Also remember that "|" is interpreted as a literal within square brackets, |
a3cb178b | 1918 | so if you write C<[fee|fie|foe]> you're really only matching C<[feio|]>. |
a0d0e21e | 1919 | |
14218588 GS |
1920 | Within a pattern, you may designate subpatterns for later reference |
1921 | by enclosing them in parentheses, and you may refer back to the | |
1922 | I<n>th subpattern later in the pattern using the metacharacter | |
1923 | \I<n>. Subpatterns are numbered based on the left to right order | |
1924 | of their opening parenthesis. A backreference matches whatever | |
1925 | actually matched the subpattern in the string being examined, not | |
d8b950dc | 1926 | the rules for that subpattern. Therefore, C<(0|0x)\d*\s\g1\d*> will |
14218588 GS |
1927 | match "0x1234 0x4321", but not "0x1234 01234", because subpattern |
1928 | 1 matched "0x", even though the rule C<0|0x> could potentially match | |
1929 | the leading 0 in the second number. | |
cb1a09d0 | 1930 | |
0d017f4d | 1931 | =head2 Warning on \1 Instead of $1 |
cb1a09d0 | 1932 | |
5a964f20 | 1933 | Some people get too used to writing things like: |
cb1a09d0 AD |
1934 | |
1935 | $pattern =~ s/(\W)/\\\1/g; | |
1936 | ||
3ff1c45a KW |
1937 | This is grandfathered (for \1 to \9) for the RHS of a substitute to avoid |
1938 | shocking the | |
cb1a09d0 | 1939 | B<sed> addicts, but it's a dirty habit to get into. That's because in |
d1be9408 | 1940 | PerlThink, the righthand side of an C<s///> is a double-quoted string. C<\1> in |
cb1a09d0 AD |
1941 | the usual double-quoted string means a control-A. The customary Unix |
1942 | meaning of C<\1> is kludged in for C<s///>. However, if you get into the habit | |
1943 | of doing that, you get yourself into trouble if you then add an C</e> | |
1944 | modifier. | |
1945 | ||
f793d64a | 1946 | s/(\d+)/ \1 + 1 /eg; # causes warning under -w |
cb1a09d0 AD |
1947 | |
1948 | Or if you try to do | |
1949 | ||
1950 | s/(\d+)/\1000/; | |
1951 | ||
1952 | You can't disambiguate that by saying C<\{1}000>, whereas you can fix it with | |
14218588 | 1953 | C<${1}000>. The operation of interpolation should not be confused |
cb1a09d0 AD |
1954 | with the operation of matching a backreference. Certainly they mean two |
1955 | different things on the I<left> side of the C<s///>. | |
9fa51da4 | 1956 | |
0d017f4d | 1957 | =head2 Repeated Patterns Matching a Zero-length Substring |
c84d73f1 | 1958 | |
19799a22 | 1959 | B<WARNING>: Difficult material (and prose) ahead. This section needs a rewrite. |
c84d73f1 IZ |
1960 | |
1961 | Regular expressions provide a terse and powerful programming language. As | |
1962 | with most other power tools, power comes together with the ability | |
1963 | to wreak havoc. | |
1964 | ||
1965 | A common abuse of this power stems from the ability to make infinite | |
628afcb5 | 1966 | loops using regular expressions, with something as innocuous as: |
c84d73f1 IZ |
1967 | |
1968 | 'foo' =~ m{ ( o? )* }x; | |
1969 | ||
0d017f4d | 1970 | The C<o?> matches at the beginning of C<'foo'>, and since the position |
c84d73f1 | 1971 | in the string is not moved by the match, C<o?> would match again and again |
527e91da | 1972 | because of the C<*> quantifier. Another common way to create a similar cycle |
c84d73f1 IZ |
1973 | is with the looping modifier C<//g>: |
1974 | ||
1975 | @matches = ( 'foo' =~ m{ o? }xg ); | |
1976 | ||
1977 | or | |
1978 | ||
1979 | print "match: <$&>\n" while 'foo' =~ m{ o? }xg; | |
1980 | ||
1981 | or the loop implied by split(). | |
1982 | ||
1983 | However, long experience has shown that many programming tasks may | |
14218588 GS |
1984 | be significantly simplified by using repeated subexpressions that |
1985 | may match zero-length substrings. Here's a simple example being: | |
c84d73f1 | 1986 | |
f793d64a | 1987 | @chars = split //, $string; # // is not magic in split |
c84d73f1 IZ |
1988 | ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// / |
1989 | ||
9da458fc | 1990 | Thus Perl allows such constructs, by I<forcefully breaking |
c84d73f1 | 1991 | the infinite loop>. The rules for this are different for lower-level |
527e91da | 1992 | loops given by the greedy quantifiers C<*+{}>, and for higher-level |
c84d73f1 IZ |
1993 | ones like the C</g> modifier or split() operator. |
1994 | ||
19799a22 GS |
1995 | The lower-level loops are I<interrupted> (that is, the loop is |
1996 | broken) when Perl detects that a repeated expression matched a | |
1997 | zero-length substring. Thus | |
c84d73f1 IZ |
1998 | |
1999 | m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x; | |
2000 | ||
5d458dd8 | 2001 | is made equivalent to |
c84d73f1 | 2002 | |
5d458dd8 YO |
2003 | m{ (?: NON_ZERO_LENGTH )* |
2004 | | | |
2005 | (?: ZERO_LENGTH )? | |
c84d73f1 IZ |
2006 | }x; |
2007 | ||
2008 | The higher level-loops preserve an additional state between iterations: | |
5d458dd8 | 2009 | whether the last match was zero-length. To break the loop, the following |
c84d73f1 | 2010 | match after a zero-length match is prohibited to have a length of zero. |
5d458dd8 | 2011 | This prohibition interacts with backtracking (see L<"Backtracking">), |
c84d73f1 IZ |
2012 | and so the I<second best> match is chosen if the I<best> match is of |
2013 | zero length. | |
2014 | ||
19799a22 | 2015 | For example: |
c84d73f1 IZ |
2016 | |
2017 | $_ = 'bar'; | |
2018 | s/\w??/<$&>/g; | |
2019 | ||
20fb949f | 2020 | results in C<< <><b><><a><><r><> >>. At each position of the string the best |
5d458dd8 | 2021 | match given by non-greedy C<??> is the zero-length match, and the I<second |
c84d73f1 IZ |
2022 | best> match is what is matched by C<\w>. Thus zero-length matches |
2023 | alternate with one-character-long matches. | |
2024 | ||
5d458dd8 | 2025 | Similarly, for repeated C<m/()/g> the second-best match is the match at the |
c84d73f1 IZ |
2026 | position one notch further in the string. |
2027 | ||
19799a22 | 2028 | The additional state of being I<matched with zero-length> is associated with |
c84d73f1 | 2029 | the matched string, and is reset by each assignment to pos(). |
9da458fc IZ |
2030 | Zero-length matches at the end of the previous match are ignored |
2031 | during C<split>. | |
c84d73f1 | 2032 | |
0d017f4d | 2033 | =head2 Combining RE Pieces |
35a734be IZ |
2034 | |
2035 | Each of the elementary pieces of regular expressions which were described | |
2036 | before (such as C<ab> or C<\Z>) could match at most one substring | |
2037 | at the given position of the input string. However, in a typical regular | |
2038 | expression these elementary pieces are combined into more complicated | |
2039 | patterns using combining operators C<ST>, C<S|T>, C<S*> etc | |
2040 | (in these examples C<S> and C<T> are regular subexpressions). | |
2041 | ||
2042 | Such combinations can include alternatives, leading to a problem of choice: | |
2043 | if we match a regular expression C<a|ab> against C<"abc">, will it match | |
2044 | substring C<"a"> or C<"ab">? One way to describe which substring is | |
2045 | actually matched is the concept of backtracking (see L<"Backtracking">). | |
2046 | However, this description is too low-level and makes you think | |
2047 | in terms of a particular implementation. | |
2048 | ||
2049 | Another description starts with notions of "better"/"worse". All the | |
2050 | substrings which may be matched by the given regular expression can be | |
2051 | sorted from the "best" match to the "worst" match, and it is the "best" | |
2052 | match which is chosen. This substitutes the question of "what is chosen?" | |
2053 | by the question of "which matches are better, and which are worse?". | |
2054 | ||
2055 | Again, for elementary pieces there is no such question, since at most | |
2056 | one match at a given position is possible. This section describes the | |
2057 | notion of better/worse for combining operators. In the description | |
2058 | below C<S> and C<T> are regular subexpressions. | |
2059 | ||
13a2d996 | 2060 | =over 4 |
35a734be IZ |
2061 | |
2062 | =item C<ST> | |
2063 | ||
2064 | Consider two possible matches, C<AB> and C<A'B'>, C<A> and C<A'> are | |
2065 | substrings which can be matched by C<S>, C<B> and C<B'> are substrings | |
5d458dd8 | 2066 | which can be matched by C<T>. |
35a734be IZ |
2067 | |
2068 | If C<A> is better match for C<S> than C<A'>, C<AB> is a better | |
2069 | match than C<A'B'>. | |
2070 | ||
2071 | If C<A> and C<A'> coincide: C<AB> is a better match than C<AB'> if | |
2072 | C<B> is better match for C<T> than C<B'>. | |
2073 | ||
2074 | =item C<S|T> | |
2075 | ||
2076 | When C<S> can match, it is a better match than when only C<T> can match. | |
2077 | ||
2078 | Ordering of two matches for C<S> is the same as for C<S>. Similar for | |
2079 | two matches for C<T>. | |
2080 | ||
2081 | =item C<S{REPEAT_COUNT}> | |
2082 | ||
2083 | Matches as C<SSS...S> (repeated as many times as necessary). | |
2084 | ||
2085 | =item C<S{min,max}> | |
2086 | ||
2087 | Matches as C<S{max}|S{max-1}|...|S{min+1}|S{min}>. | |
2088 | ||
2089 | =item C<S{min,max}?> | |
2090 | ||
2091 | Matches as C<S{min}|S{min+1}|...|S{max-1}|S{max}>. | |
2092 | ||
2093 | =item C<S?>, C<S*>, C<S+> | |
2094 | ||
2095 | Same as C<S{0,1}>, C<S{0,BIG_NUMBER}>, C<S{1,BIG_NUMBER}> respectively. | |
2096 | ||
2097 | =item C<S??>, C<S*?>, C<S+?> | |
2098 | ||
2099 | Same as C<S{0,1}?>, C<S{0,BIG_NUMBER}?>, C<S{1,BIG_NUMBER}?> respectively. | |
2100 | ||
c47ff5f1 | 2101 | =item C<< (?>S) >> |
35a734be IZ |
2102 | |
2103 | Matches the best match for C<S> and only that. | |
2104 | ||
2105 | =item C<(?=S)>, C<(?<=S)> | |
2106 | ||
2107 | Only the best match for C<S> is considered. (This is important only if | |
2108 | C<S> has capturing parentheses, and backreferences are used somewhere | |
2109 | else in the whole regular expression.) | |
2110 | ||
2111 | =item C<(?!S)>, C<(?<!S)> | |
2112 | ||
2113 | For this grouping operator there is no need to describe the ordering, since | |
2114 | only whether or not C<S> can match is important. | |
2115 | ||
6bda09f9 | 2116 | =item C<(??{ EXPR })>, C<(?PARNO)> |
35a734be IZ |
2117 | |
2118 | The ordering is the same as for the regular expression which is | |
c27a5cfe | 2119 | the result of EXPR, or the pattern contained by capture group PARNO. |
35a734be IZ |
2120 | |
2121 | =item C<(?(condition)yes-pattern|no-pattern)> | |
2122 | ||
2123 | Recall that which of C<yes-pattern> or C<no-pattern> actually matches is | |
2124 | already determined. The ordering of the matches is the same as for the | |
2125 | chosen subexpression. | |
2126 | ||
2127 | =back | |
2128 | ||
2129 | The above recipes describe the ordering of matches I<at a given position>. | |
2130 | One more rule is needed to understand how a match is determined for the | |
2131 | whole regular expression: a match at an earlier position is always better | |
2132 | than a match at a later position. | |
2133 | ||
0d017f4d | 2134 | =head2 Creating Custom RE Engines |
c84d73f1 IZ |
2135 | |
2136 | Overloaded constants (see L<overload>) provide a simple way to extend | |
2137 | the functionality of the RE engine. | |
2138 | ||
2139 | Suppose that we want to enable a new RE escape-sequence C<\Y|> which | |
0d017f4d | 2140 | matches at a boundary between whitespace characters and non-whitespace |
c84d73f1 IZ |
2141 | characters. Note that C<(?=\S)(?<!\S)|(?!\S)(?<=\S)> matches exactly |
2142 | at these positions, so we want to have each C<\Y|> in the place of the | |
2143 | more complicated version. We can create a module C<customre> to do | |
2144 | this: | |
2145 | ||
2146 | package customre; | |
2147 | use overload; | |
2148 | ||
2149 | sub import { | |
2150 | shift; | |
2151 | die "No argument to customre::import allowed" if @_; | |
2152 | overload::constant 'qr' => \&convert; | |
2153 | } | |
2154 | ||
2155 | sub invalid { die "/$_[0]/: invalid escape '\\$_[1]'"} | |
2156 | ||
580a9fe1 RGS |
2157 | # We must also take care of not escaping the legitimate \\Y| |
2158 | # sequence, hence the presence of '\\' in the conversion rules. | |
5d458dd8 | 2159 | my %rules = ( '\\' => '\\\\', |
f793d64a | 2160 | 'Y|' => qr/(?=\S)(?<!\S)|(?!\S)(?<=\S)/ ); |
c84d73f1 IZ |
2161 | sub convert { |
2162 | my $re = shift; | |
5d458dd8 | 2163 | $re =~ s{ |
c84d73f1 IZ |
2164 | \\ ( \\ | Y . ) |
2165 | } | |
5d458dd8 | 2166 | { $rules{$1} or invalid($re,$1) }sgex; |
c84d73f1 IZ |
2167 | return $re; |
2168 | } | |
2169 | ||
2170 | Now C<use customre> enables the new escape in constant regular | |
2171 | expressions, i.e., those without any runtime variable interpolations. | |
2172 | As documented in L<overload>, this conversion will work only over | |
2173 | literal parts of regular expressions. For C<\Y|$re\Y|> the variable | |
2174 | part of this regular expression needs to be converted explicitly | |
2175 | (but only if the special meaning of C<\Y|> should be enabled inside $re): | |
2176 | ||
2177 | use customre; | |
2178 | $re = <>; | |
2179 | chomp $re; | |
2180 | $re = customre::convert $re; | |
2181 | /\Y|$re\Y|/; | |
2182 | ||
1f1031fe YO |
2183 | =head1 PCRE/Python Support |
2184 | ||
99d59c4d | 2185 | As of Perl 5.10.0, Perl supports several Python/PCRE specific extensions |
1f1031fe | 2186 | to the regex syntax. While Perl programmers are encouraged to use the |
99d59c4d | 2187 | Perl specific syntax, the following are also accepted: |
1f1031fe YO |
2188 | |
2189 | =over 4 | |
2190 | ||
ae5648b3 | 2191 | =item C<< (?PE<lt>NAMEE<gt>pattern) >> |
1f1031fe | 2192 | |
c27a5cfe | 2193 | Define a named capture group. Equivalent to C<< (?<NAME>pattern) >>. |
1f1031fe YO |
2194 | |
2195 | =item C<< (?P=NAME) >> | |
2196 | ||
c27a5cfe | 2197 | Backreference to a named capture group. Equivalent to C<< \g{NAME} >>. |
1f1031fe YO |
2198 | |
2199 | =item C<< (?P>NAME) >> | |
2200 | ||
c27a5cfe | 2201 | Subroutine call to a named capture group. Equivalent to C<< (?&NAME) >>. |
1f1031fe | 2202 | |
ee9b8eae | 2203 | =back |
1f1031fe | 2204 | |
19799a22 GS |
2205 | =head1 BUGS |
2206 | ||
78288b8e KW |
2207 | There are numerous problems with case insensitive matching of characters |
2208 | outside the ASCII range, especially with those whose folds are multiple | |
2209 | characters, such as ligatures like C<LATIN SMALL LIGATURE FF>. | |
2210 | ||
f253210b KW |
2211 | In a bracketed character class with case insensitive matching, ranges only work |
2212 | for ASCII characters. For example, | |
2213 | C<m/[\N{CYRILLIC CAPITAL LETTER A}-\N{CYRILLIC CAPITAL LETTER YA}]/i> | |
2214 | doesn't match all the Russian upper and lower case letters. | |
2215 | ||
88c9975e KW |
2216 | Many regular expression constructs don't work on EBCDIC platforms. |
2217 | ||
9da458fc IZ |
2218 | This document varies from difficult to understand to completely |
2219 | and utterly opaque. The wandering prose riddled with jargon is | |
2220 | hard to fathom in several places. | |
2221 | ||
2222 | This document needs a rewrite that separates the tutorial content | |
2223 | from the reference content. | |
19799a22 GS |
2224 | |
2225 | =head1 SEE ALSO | |
9fa51da4 | 2226 | |
91e0c79e MJD |
2227 | L<perlrequick>. |
2228 | ||
2229 | L<perlretut>. | |
2230 | ||
9b599b2a GS |
2231 | L<perlop/"Regexp Quote-Like Operators">. |
2232 | ||
1e66bd83 PP |
2233 | L<perlop/"Gory details of parsing quoted constructs">. |
2234 | ||
14218588 GS |
2235 | L<perlfaq6>. |
2236 | ||
9b599b2a GS |
2237 | L<perlfunc/pos>. |
2238 | ||
2239 | L<perllocale>. | |
2240 | ||
fb55449c JH |
2241 | L<perlebcdic>. |
2242 | ||
14218588 GS |
2243 | I<Mastering Regular Expressions> by Jeffrey Friedl, published |
2244 | by O'Reilly and Associates. |