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1 | This file contains the PCRE man page that describes the regular expressions |
2 | supported by PCRE version 5.0. Note that not all of the features are relevant | |
3 | in the context of Exim. In particular, the version of PCRE that is compiled | |
4 | with Exim does not include UTF-8 support, there is no mechanism for changing | |
5 | the options with which the PCRE functions are called, and features such as | |
6 | callout are not accessible. | |
7 | ----------------------------------------------------------------------------- | |
8 | ||
9 | PCRE(3) PCRE(3) | |
10 | ||
11 | ||
12 | ||
13 | NAME | |
14 | PCRE - Perl-compatible regular expressions | |
15 | ||
16 | PCRE REGULAR EXPRESSION DETAILS | |
17 | ||
18 | The syntax and semantics of the regular expressions supported by PCRE | |
19 | are described below. Regular expressions are also described in the Perl | |
20 | documentation and in a number of books, some of which have copious | |
21 | examples. Jeffrey Friedl's "Mastering Regular Expressions", published | |
22 | by O'Reilly, covers regular expressions in great detail. This descrip- | |
23 | tion of PCRE's regular expressions is intended as reference material. | |
24 | ||
25 | The original operation of PCRE was on strings of one-byte characters. | |
26 | However, there is now also support for UTF-8 character strings. To use | |
27 | this, you must build PCRE to include UTF-8 support, and then call | |
28 | pcre_compile() with the PCRE_UTF8 option. How this affects pattern | |
29 | matching is mentioned in several places below. There is also a summary | |
30 | of UTF-8 features in the section on UTF-8 support in the main pcre | |
31 | page. | |
32 | ||
33 | A regular expression is a pattern that is matched against a subject | |
34 | string from left to right. Most characters stand for themselves in a | |
35 | pattern, and match the corresponding characters in the subject. As a | |
36 | trivial example, the pattern | |
37 | ||
38 | The quick brown fox | |
39 | ||
40 | matches a portion of a subject string that is identical to itself. The | |
41 | power of regular expressions comes from the ability to include alterna- | |
42 | tives and repetitions in the pattern. These are encoded in the pattern | |
43 | by the use of metacharacters, which do not stand for themselves but | |
44 | instead are interpreted in some special way. | |
45 | ||
46 | There are two different sets of metacharacters: those that are recog- | |
47 | nized anywhere in the pattern except within square brackets, and those | |
48 | that are recognized in square brackets. Outside square brackets, the | |
49 | metacharacters are as follows: | |
50 | ||
51 | \ general escape character with several uses | |
52 | ^ assert start of string (or line, in multiline mode) | |
53 | $ assert end of string (or line, in multiline mode) | |
54 | . match any character except newline (by default) | |
55 | [ start character class definition | |
56 | | start of alternative branch | |
57 | ( start subpattern | |
58 | ) end subpattern | |
59 | ? extends the meaning of ( | |
60 | also 0 or 1 quantifier | |
61 | also quantifier minimizer | |
62 | * 0 or more quantifier | |
63 | + 1 or more quantifier | |
64 | also "possessive quantifier" | |
65 | { start min/max quantifier | |
66 | ||
67 | Part of a pattern that is in square brackets is called a "character | |
68 | class". In a character class the only metacharacters are: | |
69 | ||
70 | \ general escape character | |
71 | ^ negate the class, but only if the first character | |
72 | - indicates character range | |
73 | [ POSIX character class (only if followed by POSIX | |
74 | syntax) | |
75 | ] terminates the character class | |
76 | ||
77 | The following sections describe the use of each of the metacharacters. | |
78 | ||
79 | ||
80 | BACKSLASH | |
81 | ||
82 | The backslash character has several uses. Firstly, if it is followed by | |
83 | a non-alphanumeric character, it takes away any special meaning that | |
84 | character may have. This use of backslash as an escape character | |
85 | applies both inside and outside character classes. | |
86 | ||
87 | For example, if you want to match a * character, you write \* in the | |
88 | pattern. This escaping action applies whether or not the following | |
89 | character would otherwise be interpreted as a metacharacter, so it is | |
90 | always safe to precede a non-alphanumeric with backslash to specify | |
91 | that it stands for itself. In particular, if you want to match a back- | |
92 | slash, you write \\. | |
93 | ||
94 | If a pattern is compiled with the PCRE_EXTENDED option, whitespace in | |
95 | the pattern (other than in a character class) and characters between a | |
96 | # outside a character class and the next newline character are ignored. | |
97 | An escaping backslash can be used to include a whitespace or # charac- | |
98 | ter as part of the pattern. | |
99 | ||
100 | If you want to remove the special meaning from a sequence of charac- | |
101 | ters, you can do so by putting them between \Q and \E. This is differ- | |
102 | ent from Perl in that $ and @ are handled as literals in \Q...\E | |
103 | sequences in PCRE, whereas in Perl, $ and @ cause variable interpola- | |
104 | tion. Note the following examples: | |
105 | ||
106 | Pattern PCRE matches Perl matches | |
107 | ||
108 | \Qabc$xyz\E abc$xyz abc followed by the | |
109 | contents of $xyz | |
110 | \Qabc\$xyz\E abc\$xyz abc\$xyz | |
111 | \Qabc\E\$\Qxyz\E abc$xyz abc$xyz | |
112 | ||
113 | The \Q...\E sequence is recognized both inside and outside character | |
114 | classes. | |
115 | ||
116 | Non-printing characters | |
117 | ||
118 | A second use of backslash provides a way of encoding non-printing char- | |
119 | acters in patterns in a visible manner. There is no restriction on the | |
120 | appearance of non-printing characters, apart from the binary zero that | |
121 | terminates a pattern, but when a pattern is being prepared by text | |
122 | editing, it is usually easier to use one of the following escape | |
123 | sequences than the binary character it represents: | |
124 | ||
125 | \a alarm, that is, the BEL character (hex 07) | |
126 | \cx "control-x", where x is any character | |
127 | \e escape (hex 1B) | |
128 | \f formfeed (hex 0C) | |
129 | \n newline (hex 0A) | |
130 | \r carriage return (hex 0D) | |
131 | \t tab (hex 09) | |
132 | \ddd character with octal code ddd, or backreference | |
133 | \xhh character with hex code hh | |
134 | \x{hhh..} character with hex code hhh... (UTF-8 mode only) | |
135 | ||
136 | The precise effect of \cx is as follows: if x is a lower case letter, | |
137 | it is converted to upper case. Then bit 6 of the character (hex 40) is | |
138 | inverted. Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c; | |
139 | becomes hex 7B. | |
140 | ||
141 | After \x, from zero to two hexadecimal digits are read (letters can be | |
142 | in upper or lower case). In UTF-8 mode, any number of hexadecimal dig- | |
143 | its may appear between \x{ and }, but the value of the character code | |
144 | must be less than 2**31 (that is, the maximum hexadecimal value is | |
145 | 7FFFFFFF). If characters other than hexadecimal digits appear between | |
146 | \x{ and }, or if there is no terminating }, this form of escape is not | |
147 | recognized. Instead, the initial \x will be interpreted as a basic hex- | |
148 | adecimal escape, with no following digits, giving a character whose | |
149 | value is zero. | |
150 | ||
151 | Characters whose value is less than 256 can be defined by either of the | |
152 | two syntaxes for \x when PCRE is in UTF-8 mode. There is no difference | |
153 | in the way they are handled. For example, \xdc is exactly the same as | |
154 | \x{dc}. | |
155 | ||
156 | After \0 up to two further octal digits are read. In both cases, if | |
157 | there are fewer than two digits, just those that are present are used. | |
158 | Thus the sequence \0\x\07 specifies two binary zeros followed by a BEL | |
159 | character (code value 7). Make sure you supply two digits after the | |
160 | initial zero if the pattern character that follows is itself an octal | |
161 | digit. | |
162 | ||
163 | The handling of a backslash followed by a digit other than 0 is compli- | |
164 | cated. Outside a character class, PCRE reads it and any following dig- | |
165 | its as a decimal number. If the number is less than 10, or if there | |
166 | have been at least that many previous capturing left parentheses in the | |
167 | expression, the entire sequence is taken as a back reference. A | |
168 | description of how this works is given later, following the discussion | |
169 | of parenthesized subpatterns. | |
170 | ||
171 | Inside a character class, or if the decimal number is greater than 9 | |
172 | and there have not been that many capturing subpatterns, PCRE re-reads | |
173 | up to three octal digits following the backslash, and generates a sin- | |
174 | gle byte from the least significant 8 bits of the value. Any subsequent | |
175 | digits stand for themselves. For example: | |
176 | ||
177 | \040 is another way of writing a space | |
178 | \40 is the same, provided there are fewer than 40 | |
179 | previous capturing subpatterns | |
180 | \7 is always a back reference | |
181 | \11 might be a back reference, or another way of | |
182 | writing a tab | |
183 | \011 is always a tab | |
184 | \0113 is a tab followed by the character "3" | |
185 | \113 might be a back reference, otherwise the | |
186 | character with octal code 113 | |
187 | \377 might be a back reference, otherwise | |
188 | the byte consisting entirely of 1 bits | |
189 | \81 is either a back reference, or a binary zero | |
190 | followed by the two characters "8" and "1" | |
191 | ||
192 | Note that octal values of 100 or greater must not be introduced by a | |
193 | leading zero, because no more than three octal digits are ever read. | |
194 | ||
195 | All the sequences that define a single byte value or a single UTF-8 | |
196 | character (in UTF-8 mode) can be used both inside and outside character | |
197 | classes. In addition, inside a character class, the sequence \b is | |
198 | interpreted as the backspace character (hex 08), and the sequence \X is | |
199 | interpreted as the character "X". Outside a character class, these | |
200 | sequences have different meanings (see below). | |
201 | ||
202 | Generic character types | |
203 | ||
204 | The third use of backslash is for specifying generic character types. | |
205 | The following are always recognized: | |
206 | ||
207 | \d any decimal digit | |
208 | \D any character that is not a decimal digit | |
209 | \s any whitespace character | |
210 | \S any character that is not a whitespace character | |
211 | \w any "word" character | |
212 | \W any "non-word" character | |
213 | ||
214 | Each pair of escape sequences partitions the complete set of characters | |
215 | into two disjoint sets. Any given character matches one, and only one, | |
216 | of each pair. | |
217 | ||
218 | These character type sequences can appear both inside and outside char- | |
219 | acter classes. They each match one character of the appropriate type. | |
220 | If the current matching point is at the end of the subject string, all | |
221 | of them fail, since there is no character to match. | |
222 | ||
223 | For compatibility with Perl, \s does not match the VT character (code | |
224 | 11). This makes it different from the the POSIX "space" class. The \s | |
225 | characters are HT (9), LF (10), FF (12), CR (13), and space (32). | |
226 | ||
227 | A "word" character is an underscore or any character less than 256 that | |
228 | is a letter or digit. The definition of letters and digits is con- | |
229 | trolled by PCRE's low-valued character tables, and may vary if locale- | |
230 | specific matching is taking place (see "Locale support" in the pcreapi | |
231 | page). For example, in the "fr_FR" (French) locale, some character | |
232 | codes greater than 128 are used for accented letters, and these are | |
233 | matched by \w. | |
234 | ||
235 | In UTF-8 mode, characters with values greater than 128 never match \d, | |
236 | \s, or \w, and always match \D, \S, and \W. This is true even when Uni- | |
237 | code character property support is available. | |
238 | ||
239 | Unicode character properties | |
240 | ||
241 | When PCRE is built with Unicode character property support, three addi- | |
242 | tional escape sequences to match generic character types are available | |
243 | when UTF-8 mode is selected. They are: | |
244 | ||
245 | \p{xx} a character with the xx property | |
246 | \P{xx} a character without the xx property | |
247 | \X an extended Unicode sequence | |
248 | ||
249 | The property names represented by xx above are limited to the Unicode | |
250 | general category properties. Each character has exactly one such prop- | |
251 | erty, specified by a two-letter abbreviation. For compatibility with | |
252 | Perl, negation can be specified by including a circumflex between the | |
253 | opening brace and the property name. For example, \p{^Lu} is the same | |
254 | as \P{Lu}. | |
255 | ||
256 | If only one letter is specified with \p or \P, it includes all the | |
257 | properties that start with that letter. In this case, in the absence of | |
258 | negation, the curly brackets in the escape sequence are optional; these | |
259 | two examples have the same effect: | |
260 | ||
261 | \p{L} | |
262 | \pL | |
263 | ||
264 | The following property codes are supported: | |
265 | ||
266 | C Other | |
267 | Cc Control | |
268 | Cf Format | |
269 | Cn Unassigned | |
270 | Co Private use | |
271 | Cs Surrogate | |
272 | ||
273 | L Letter | |
274 | Ll Lower case letter | |
275 | Lm Modifier letter | |
276 | Lo Other letter | |
277 | Lt Title case letter | |
278 | Lu Upper case letter | |
279 | ||
280 | M Mark | |
281 | Mc Spacing mark | |
282 | Me Enclosing mark | |
283 | Mn Non-spacing mark | |
284 | ||
285 | N Number | |
286 | Nd Decimal number | |
287 | Nl Letter number | |
288 | No Other number | |
289 | ||
290 | P Punctuation | |
291 | Pc Connector punctuation | |
292 | Pd Dash punctuation | |
293 | Pe Close punctuation | |
294 | Pf Final punctuation | |
295 | Pi Initial punctuation | |
296 | Po Other punctuation | |
297 | Ps Open punctuation | |
298 | ||
299 | S Symbol | |
300 | Sc Currency symbol | |
301 | Sk Modifier symbol | |
302 | Sm Mathematical symbol | |
303 | So Other symbol | |
304 | ||
305 | Z Separator | |
306 | Zl Line separator | |
307 | Zp Paragraph separator | |
308 | Zs Space separator | |
309 | ||
310 | Extended properties such as "Greek" or "InMusicalSymbols" are not sup- | |
311 | ported by PCRE. | |
312 | ||
313 | Specifying caseless matching does not affect these escape sequences. | |
314 | For example, \p{Lu} always matches only upper case letters. | |
315 | ||
316 | The \X escape matches any number of Unicode characters that form an | |
317 | extended Unicode sequence. \X is equivalent to | |
318 | ||
319 | (?>\PM\pM*) | |
320 | ||
321 | That is, it matches a character without the "mark" property, followed | |
322 | by zero or more characters with the "mark" property, and treats the | |
323 | sequence as an atomic group (see below). Characters with the "mark" | |
324 | property are typically accents that affect the preceding character. | |
325 | ||
326 | Matching characters by Unicode property is not fast, because PCRE has | |
327 | to search a structure that contains data for over fifteen thousand | |
328 | characters. That is why the traditional escape sequences such as \d and | |
329 | \w do not use Unicode properties in PCRE. | |
330 | ||
331 | Simple assertions | |
332 | ||
333 | The fourth use of backslash is for certain simple assertions. An asser- | |
334 | tion specifies a condition that has to be met at a particular point in | |
335 | a match, without consuming any characters from the subject string. The | |
336 | use of subpatterns for more complicated assertions is described below. | |
337 | The backslashed assertions are: | |
338 | ||
339 | \b matches at a word boundary | |
340 | \B matches when not at a word boundary | |
341 | \A matches at start of subject | |
342 | \Z matches at end of subject or before newline at end | |
343 | \z matches at end of subject | |
344 | \G matches at first matching position in subject | |
345 | ||
346 | These assertions may not appear in character classes (but note that \b | |
347 | has a different meaning, namely the backspace character, inside a char- | |
348 | acter class). | |
349 | ||
350 | A word boundary is a position in the subject string where the current | |
351 | character and the previous character do not both match \w or \W (i.e. | |
352 | one matches \w and the other matches \W), or the start or end of the | |
353 | string if the first or last character matches \w, respectively. | |
354 | ||
355 | The \A, \Z, and \z assertions differ from the traditional circumflex | |
356 | and dollar (described in the next section) in that they only ever match | |
357 | at the very start and end of the subject string, whatever options are | |
358 | set. Thus, they are independent of multiline mode. These three asser- | |
359 | tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which | |
360 | affect only the behaviour of the circumflex and dollar metacharacters. | |
361 | However, if the startoffset argument of pcre_exec() is non-zero, indi- | |
362 | cating that matching is to start at a point other than the beginning of | |
363 | the subject, \A can never match. The difference between \Z and \z is | |
364 | that \Z matches before a newline that is the last character of the | |
365 | string as well as at the end of the string, whereas \z matches only at | |
366 | the end. | |
367 | ||
368 | The \G assertion is true only when the current matching position is at | |
369 | the start point of the match, as specified by the startoffset argument | |
370 | of pcre_exec(). It differs from \A when the value of startoffset is | |
371 | non-zero. By calling pcre_exec() multiple times with appropriate argu- | |
372 | ments, you can mimic Perl's /g option, and it is in this kind of imple- | |
373 | mentation where \G can be useful. | |
374 | ||
375 | Note, however, that PCRE's interpretation of \G, as the start of the | |
376 | current match, is subtly different from Perl's, which defines it as the | |
377 | end of the previous match. In Perl, these can be different when the | |
378 | previously matched string was empty. Because PCRE does just one match | |
379 | at a time, it cannot reproduce this behaviour. | |
380 | ||
381 | If all the alternatives of a pattern begin with \G, the expression is | |
382 | anchored to the starting match position, and the "anchored" flag is set | |
383 | in the compiled regular expression. | |
384 | ||
385 | ||
386 | CIRCUMFLEX AND DOLLAR | |
387 | ||
388 | Outside a character class, in the default matching mode, the circumflex | |
389 | character is an assertion that is true only if the current matching | |
390 | point is at the start of the subject string. If the startoffset argu- | |
391 | ment of pcre_exec() is non-zero, circumflex can never match if the | |
392 | PCRE_MULTILINE option is unset. Inside a character class, circumflex | |
393 | has an entirely different meaning (see below). | |
394 | ||
395 | Circumflex need not be the first character of the pattern if a number | |
396 | of alternatives are involved, but it should be the first thing in each | |
397 | alternative in which it appears if the pattern is ever to match that | |
398 | branch. If all possible alternatives start with a circumflex, that is, | |
399 | if the pattern is constrained to match only at the start of the sub- | |
400 | ject, it is said to be an "anchored" pattern. (There are also other | |
401 | constructs that can cause a pattern to be anchored.) | |
402 | ||
403 | A dollar character is an assertion that is true only if the current | |
404 | matching point is at the end of the subject string, or immediately | |
405 | before a newline character that is the last character in the string (by | |
406 | default). Dollar need not be the last character of the pattern if a | |
407 | number of alternatives are involved, but it should be the last item in | |
408 | any branch in which it appears. Dollar has no special meaning in a | |
409 | character class. | |
410 | ||
411 | The meaning of dollar can be changed so that it matches only at the | |
412 | very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at | |
413 | compile time. This does not affect the \Z assertion. | |
414 | ||
415 | The meanings of the circumflex and dollar characters are changed if the | |
416 | PCRE_MULTILINE option is set. When this is the case, they match immedi- | |
417 | ately after and immediately before an internal newline character, | |
418 | respectively, in addition to matching at the start and end of the sub- | |
419 | ject string. For example, the pattern /^abc$/ matches the subject | |
420 | string "def\nabc" (where \n represents a newline character) in multi- | |
421 | line mode, but not otherwise. Consequently, patterns that are anchored | |
422 | in single line mode because all branches start with ^ are not anchored | |
423 | in multiline mode, and a match for circumflex is possible when the | |
424 | startoffset argument of pcre_exec() is non-zero. The PCRE_DOL- | |
425 | LAR_ENDONLY option is ignored if PCRE_MULTILINE is set. | |
426 | ||
427 | Note that the sequences \A, \Z, and \z can be used to match the start | |
428 | and end of the subject in both modes, and if all branches of a pattern | |
429 | start with \A it is always anchored, whether PCRE_MULTILINE is set or | |
430 | not. | |
431 | ||
432 | ||
433 | FULL STOP (PERIOD, DOT) | |
434 | ||
435 | Outside a character class, a dot in the pattern matches any one charac- | |
436 | ter in the subject, including a non-printing character, but not (by | |
437 | default) newline. In UTF-8 mode, a dot matches any UTF-8 character, | |
438 | which might be more than one byte long, except (by default) newline. If | |
439 | the PCRE_DOTALL option is set, dots match newlines as well. The han- | |
440 | dling of dot is entirely independent of the handling of circumflex and | |
441 | dollar, the only relationship being that they both involve newline | |
442 | characters. Dot has no special meaning in a character class. | |
443 | ||
444 | ||
445 | MATCHING A SINGLE BYTE | |
446 | ||
447 | Outside a character class, the escape sequence \C matches any one byte, | |
448 | both in and out of UTF-8 mode. Unlike a dot, it can match a newline. | |
449 | The feature is provided in Perl in order to match individual bytes in | |
450 | UTF-8 mode. Because it breaks up UTF-8 characters into individual | |
451 | bytes, what remains in the string may be a malformed UTF-8 string. For | |
452 | this reason, the \C escape sequence is best avoided. | |
453 | ||
454 | PCRE does not allow \C to appear in lookbehind assertions (described | |
455 | below), because in UTF-8 mode this would make it impossible to calcu- | |
456 | late the length of the lookbehind. | |
457 | ||
458 | ||
459 | SQUARE BRACKETS AND CHARACTER CLASSES | |
460 | ||
461 | An opening square bracket introduces a character class, terminated by a | |
462 | closing square bracket. A closing square bracket on its own is not spe- | |
463 | cial. If a closing square bracket is required as a member of the class, | |
464 | it should be the first data character in the class (after an initial | |
465 | circumflex, if present) or escaped with a backslash. | |
466 | ||
467 | A character class matches a single character in the subject. In UTF-8 | |
468 | mode, the character may occupy more than one byte. A matched character | |
469 | must be in the set of characters defined by the class, unless the first | |
470 | character in the class definition is a circumflex, in which case the | |
471 | subject character must not be in the set defined by the class. If a | |
472 | circumflex is actually required as a member of the class, ensure it is | |
473 | not the first character, or escape it with a backslash. | |
474 | ||
475 | For example, the character class [aeiou] matches any lower case vowel, | |
476 | while [^aeiou] matches any character that is not a lower case vowel. | |
477 | Note that a circumflex is just a convenient notation for specifying the | |
478 | characters that are in the class by enumerating those that are not. A | |
479 | class that starts with a circumflex is not an assertion: it still con- | |
480 | sumes a character from the subject string, and therefore it fails if | |
481 | the current pointer is at the end of the string. | |
482 | ||
483 | In UTF-8 mode, characters with values greater than 255 can be included | |
484 | in a class as a literal string of bytes, or by using the \x{ escaping | |
485 | mechanism. | |
486 | ||
487 | When caseless matching is set, any letters in a class represent both | |
488 | their upper case and lower case versions, so for example, a caseless | |
489 | [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not | |
490 | match "A", whereas a caseful version would. When running in UTF-8 mode, | |
491 | PCRE supports the concept of case for characters with values greater | |
492 | than 128 only when it is compiled with Unicode property support. | |
493 | ||
494 | The newline character is never treated in any special way in character | |
495 | classes, whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE | |
496 | options is. A class such as [^a] will always match a newline. | |
497 | ||
498 | The minus (hyphen) character can be used to specify a range of charac- | |
499 | ters in a character class. For example, [d-m] matches any letter | |
500 | between d and m, inclusive. If a minus character is required in a | |
501 | class, it must be escaped with a backslash or appear in a position | |
502 | where it cannot be interpreted as indicating a range, typically as the | |
503 | first or last character in the class. | |
504 | ||
505 | It is not possible to have the literal character "]" as the end charac- | |
506 | ter of a range. A pattern such as [W-]46] is interpreted as a class of | |
507 | two characters ("W" and "-") followed by a literal string "46]", so it | |
508 | would match "W46]" or "-46]". However, if the "]" is escaped with a | |
509 | backslash it is interpreted as the end of range, so [W-\]46] is inter- | |
510 | preted as a class containing a range followed by two other characters. | |
511 | The octal or hexadecimal representation of "]" can also be used to end | |
512 | a range. | |
513 | ||
514 | Ranges operate in the collating sequence of character values. They can | |
515 | also be used for characters specified numerically, for example | |
516 | [\000-\037]. In UTF-8 mode, ranges can include characters whose values | |
517 | are greater than 255, for example [\x{100}-\x{2ff}]. | |
518 | ||
519 | If a range that includes letters is used when caseless matching is set, | |
520 | it matches the letters in either case. For example, [W-c] is equivalent | |
521 | to [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if | |
522 | character tables for the "fr_FR" locale are in use, [\xc8-\xcb] matches | |
523 | accented E characters in both cases. In UTF-8 mode, PCRE supports the | |
524 | concept of case for characters with values greater than 128 only when | |
525 | it is compiled with Unicode property support. | |
526 | ||
527 | The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear | |
528 | in a character class, and add the characters that they match to the | |
529 | class. For example, [\dABCDEF] matches any hexadecimal digit. A circum- | |
530 | flex can conveniently be used with the upper case character types to | |
531 | specify a more restricted set of characters than the matching lower | |
532 | case type. For example, the class [^\W_] matches any letter or digit, | |
533 | but not underscore. | |
534 | ||
535 | The only metacharacters that are recognized in character classes are | |
536 | backslash, hyphen (only where it can be interpreted as specifying a | |
537 | range), circumflex (only at the start), opening square bracket (only | |
538 | when it can be interpreted as introducing a POSIX class name - see the | |
539 | next section), and the terminating closing square bracket. However, | |
540 | escaping other non-alphanumeric characters does no harm. | |
541 | ||
542 | ||
543 | POSIX CHARACTER CLASSES | |
544 | ||
545 | Perl supports the POSIX notation for character classes. This uses names | |
546 | enclosed by [: and :] within the enclosing square brackets. PCRE also | |
547 | supports this notation. For example, | |
548 | ||
549 | [01[:alpha:]%] | |
550 | ||
551 | matches "0", "1", any alphabetic character, or "%". The supported class | |
552 | names are | |
553 | ||
554 | alnum letters and digits | |
555 | alpha letters | |
556 | ascii character codes 0 - 127 | |
557 | blank space or tab only | |
558 | cntrl control characters | |
559 | digit decimal digits (same as \d) | |
560 | graph printing characters, excluding space | |
561 | lower lower case letters | |
562 | print printing characters, including space | |
563 | punct printing characters, excluding letters and digits | |
564 | space white space (not quite the same as \s) | |
565 | upper upper case letters | |
566 | word "word" characters (same as \w) | |
567 | xdigit hexadecimal digits | |
568 | ||
569 | The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), | |
570 | and space (32). Notice that this list includes the VT character (code | |
571 | 11). This makes "space" different to \s, which does not include VT (for | |
572 | Perl compatibility). | |
573 | ||
574 | The name "word" is a Perl extension, and "blank" is a GNU extension | |
575 | from Perl 5.8. Another Perl extension is negation, which is indicated | |
576 | by a ^ character after the colon. For example, | |
577 | ||
578 | [12[:^digit:]] | |
579 | ||
580 | matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the | |
581 | POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but | |
582 | these are not supported, and an error is given if they are encountered. | |
583 | ||
584 | In UTF-8 mode, characters with values greater than 128 do not match any | |
585 | of the POSIX character classes. | |
586 | ||
587 | ||
588 | VERTICAL BAR | |
589 | ||
590 | Vertical bar characters are used to separate alternative patterns. For | |
591 | example, the pattern | |
592 | ||
593 | gilbert|sullivan | |
594 | ||
595 | matches either "gilbert" or "sullivan". Any number of alternatives may | |
596 | appear, and an empty alternative is permitted (matching the empty | |
597 | string). The matching process tries each alternative in turn, from | |
598 | left to right, and the first one that succeeds is used. If the alterna- | |
599 | tives are within a subpattern (defined below), "succeeds" means match- | |
600 | ing the rest of the main pattern as well as the alternative in the sub- | |
601 | pattern. | |
602 | ||
603 | ||
604 | INTERNAL OPTION SETTING | |
605 | ||
606 | The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and | |
607 | PCRE_EXTENDED options can be changed from within the pattern by a | |
608 | sequence of Perl option letters enclosed between "(?" and ")". The | |
609 | option letters are | |
610 | ||
611 | i for PCRE_CASELESS | |
612 | m for PCRE_MULTILINE | |
613 | s for PCRE_DOTALL | |
614 | x for PCRE_EXTENDED | |
615 | ||
616 | For example, (?im) sets caseless, multiline matching. It is also possi- | |
617 | ble to unset these options by preceding the letter with a hyphen, and a | |
618 | combined setting and unsetting such as (?im-sx), which sets PCRE_CASE- | |
619 | LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, | |
620 | is also permitted. If a letter appears both before and after the | |
621 | hyphen, the option is unset. | |
622 | ||
623 | When an option change occurs at top level (that is, not inside subpat- | |
624 | tern parentheses), the change applies to the remainder of the pattern | |
625 | that follows. If the change is placed right at the start of a pattern, | |
626 | PCRE extracts it into the global options (and it will therefore show up | |
627 | in data extracted by the pcre_fullinfo() function). | |
628 | ||
629 | An option change within a subpattern affects only that part of the cur- | |
630 | rent pattern that follows it, so | |
631 | ||
632 | (a(?i)b)c | |
633 | ||
634 | matches abc and aBc and no other strings (assuming PCRE_CASELESS is not | |
635 | used). By this means, options can be made to have different settings | |
636 | in different parts of the pattern. Any changes made in one alternative | |
637 | do carry on into subsequent branches within the same subpattern. For | |
638 | example, | |
639 | ||
640 | (a(?i)b|c) | |
641 | ||
642 | matches "ab", "aB", "c", and "C", even though when matching "C" the | |
643 | first branch is abandoned before the option setting. This is because | |
644 | the effects of option settings happen at compile time. There would be | |
645 | some very weird behaviour otherwise. | |
646 | ||
647 | The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed | |
648 | in the same way as the Perl-compatible options by using the characters | |
649 | U and X respectively. The (?X) flag setting is special in that it must | |
650 | always occur earlier in the pattern than any of the additional features | |
651 | it turns on, even when it is at top level. It is best to put it at the | |
652 | start. | |
653 | ||
654 | ||
655 | SUBPATTERNS | |
656 | ||
657 | Subpatterns are delimited by parentheses (round brackets), which can be | |
658 | nested. Turning part of a pattern into a subpattern does two things: | |
659 | ||
660 | 1. It localizes a set of alternatives. For example, the pattern | |
661 | ||
662 | cat(aract|erpillar|) | |
663 | ||
664 | matches one of the words "cat", "cataract", or "caterpillar". Without | |
665 | the parentheses, it would match "cataract", "erpillar" or the empty | |
666 | string. | |
667 | ||
668 | 2. It sets up the subpattern as a capturing subpattern. This means | |
669 | that, when the whole pattern matches, that portion of the subject | |
670 | string that matched the subpattern is passed back to the caller via the | |
671 | ovector argument of pcre_exec(). Opening parentheses are counted from | |
672 | left to right (starting from 1) to obtain numbers for the capturing | |
673 | subpatterns. | |
674 | ||
675 | For example, if the string "the red king" is matched against the pat- | |
676 | tern | |
677 | ||
678 | the ((red|white) (king|queen)) | |
679 | ||
680 | the captured substrings are "red king", "red", and "king", and are num- | |
681 | bered 1, 2, and 3, respectively. | |
682 | ||
683 | The fact that plain parentheses fulfil two functions is not always | |
684 | helpful. There are often times when a grouping subpattern is required | |
685 | without a capturing requirement. If an opening parenthesis is followed | |
686 | by a question mark and a colon, the subpattern does not do any captur- | |
687 | ing, and is not counted when computing the number of any subsequent | |
688 | capturing subpatterns. For example, if the string "the white queen" is | |
689 | matched against the pattern | |
690 | ||
691 | the ((?:red|white) (king|queen)) | |
692 | ||
693 | the captured substrings are "white queen" and "queen", and are numbered | |
694 | 1 and 2. The maximum number of capturing subpatterns is 65535, and the | |
695 | maximum depth of nesting of all subpatterns, both capturing and non- | |
696 | capturing, is 200. | |
697 | ||
698 | As a convenient shorthand, if any option settings are required at the | |
699 | start of a non-capturing subpattern, the option letters may appear | |
700 | between the "?" and the ":". Thus the two patterns | |
701 | ||
702 | (?i:saturday|sunday) | |
703 | (?:(?i)saturday|sunday) | |
704 | ||
705 | match exactly the same set of strings. Because alternative branches are | |
706 | tried from left to right, and options are not reset until the end of | |
707 | the subpattern is reached, an option setting in one branch does affect | |
708 | subsequent branches, so the above patterns match "SUNDAY" as well as | |
709 | "Saturday". | |
710 | ||
711 | ||
712 | NAMED SUBPATTERNS | |
713 | ||
714 | Identifying capturing parentheses by number is simple, but it can be | |
715 | very hard to keep track of the numbers in complicated regular expres- | |
716 | sions. Furthermore, if an expression is modified, the numbers may | |
717 | change. To help with this difficulty, PCRE supports the naming of sub- | |
718 | patterns, something that Perl does not provide. The Python syntax | |
719 | (?P<name>...) is used. Names consist of alphanumeric characters and | |
720 | underscores, and must be unique within a pattern. | |
721 | ||
722 | Named capturing parentheses are still allocated numbers as well as | |
723 | names. The PCRE API provides function calls for extracting the name-to- | |
724 | number translation table from a compiled pattern. There is also a con- | |
725 | venience function for extracting a captured substring by name. For fur- | |
726 | ther details see the pcreapi documentation. | |
727 | ||
728 | ||
729 | REPETITION | |
730 | ||
731 | Repetition is specified by quantifiers, which can follow any of the | |
732 | following items: | |
733 | ||
734 | a literal data character | |
735 | the . metacharacter | |
736 | the \C escape sequence | |
737 | the \X escape sequence (in UTF-8 mode with Unicode properties) | |
738 | an escape such as \d that matches a single character | |
739 | a character class | |
740 | a back reference (see next section) | |
741 | a parenthesized subpattern (unless it is an assertion) | |
742 | ||
743 | The general repetition quantifier specifies a minimum and maximum num- | |
744 | ber of permitted matches, by giving the two numbers in curly brackets | |
745 | (braces), separated by a comma. The numbers must be less than 65536, | |
746 | and the first must be less than or equal to the second. For example: | |
747 | ||
748 | z{2,4} | |
749 | ||
750 | matches "zz", "zzz", or "zzzz". A closing brace on its own is not a | |
751 | special character. If the second number is omitted, but the comma is | |
752 | present, there is no upper limit; if the second number and the comma | |
753 | are both omitted, the quantifier specifies an exact number of required | |
754 | matches. Thus | |
755 | ||
756 | [aeiou]{3,} | |
757 | ||
758 | matches at least 3 successive vowels, but may match many more, while | |
759 | ||
760 | \d{8} | |
761 | ||
762 | matches exactly 8 digits. An opening curly bracket that appears in a | |
763 | position where a quantifier is not allowed, or one that does not match | |
764 | the syntax of a quantifier, is taken as a literal character. For exam- | |
765 | ple, {,6} is not a quantifier, but a literal string of four characters. | |
766 | ||
767 | In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to | |
768 | individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char- | |
769 | acters, each of which is represented by a two-byte sequence. Similarly, | |
770 | when Unicode property support is available, \X{3} matches three Unicode | |
771 | extended sequences, each of which may be several bytes long (and they | |
772 | may be of different lengths). | |
773 | ||
774 | The quantifier {0} is permitted, causing the expression to behave as if | |
775 | the previous item and the quantifier were not present. | |
776 | ||
777 | For convenience (and historical compatibility) the three most common | |
778 | quantifiers have single-character abbreviations: | |
779 | ||
780 | * is equivalent to {0,} | |
781 | + is equivalent to {1,} | |
782 | ? is equivalent to {0,1} | |
783 | ||
784 | It is possible to construct infinite loops by following a subpattern | |
785 | that can match no characters with a quantifier that has no upper limit, | |
786 | for example: | |
787 | ||
788 | (a?)* | |
789 | ||
790 | Earlier versions of Perl and PCRE used to give an error at compile time | |
791 | for such patterns. However, because there are cases where this can be | |
792 | useful, such patterns are now accepted, but if any repetition of the | |
793 | subpattern does in fact match no characters, the loop is forcibly bro- | |
794 | ken. | |
795 | ||
796 | By default, the quantifiers are "greedy", that is, they match as much | |
797 | as possible (up to the maximum number of permitted times), without | |
798 | causing the rest of the pattern to fail. The classic example of where | |
799 | this gives problems is in trying to match comments in C programs. These | |
800 | appear between /* and */ and within the comment, individual * and / | |
801 | characters may appear. An attempt to match C comments by applying the | |
802 | pattern | |
803 | ||
804 | /\*.*\*/ | |
805 | ||
806 | to the string | |
807 | ||
808 | /* first comment */ not comment /* second comment */ | |
809 | ||
810 | fails, because it matches the entire string owing to the greediness of | |
811 | the .* item. | |
812 | ||
813 | However, if a quantifier is followed by a question mark, it ceases to | |
814 | be greedy, and instead matches the minimum number of times possible, so | |
815 | the pattern | |
816 | ||
817 | /\*.*?\*/ | |
818 | ||
819 | does the right thing with the C comments. The meaning of the various | |
820 | quantifiers is not otherwise changed, just the preferred number of | |
821 | matches. Do not confuse this use of question mark with its use as a | |
822 | quantifier in its own right. Because it has two uses, it can sometimes | |
823 | appear doubled, as in | |
824 | ||
825 | \d??\d | |
826 | ||
827 | which matches one digit by preference, but can match two if that is the | |
828 | only way the rest of the pattern matches. | |
829 | ||
830 | If the PCRE_UNGREEDY option is set (an option which is not available in | |
831 | Perl), the quantifiers are not greedy by default, but individual ones | |
832 | can be made greedy by following them with a question mark. In other | |
833 | words, it inverts the default behaviour. | |
834 | ||
835 | When a parenthesized subpattern is quantified with a minimum repeat | |
836 | count that is greater than 1 or with a limited maximum, more memory is | |
837 | required for the compiled pattern, in proportion to the size of the | |
838 | minimum or maximum. | |
839 | ||
840 | If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv- | |
841 | alent to Perl's /s) is set, thus allowing the . to match newlines, the | |
842 | pattern is implicitly anchored, because whatever follows will be tried | |
843 | against every character position in the subject string, so there is no | |
844 | point in retrying the overall match at any position after the first. | |
845 | PCRE normally treats such a pattern as though it were preceded by \A. | |
846 | ||
847 | In cases where it is known that the subject string contains no new- | |
848 | lines, it is worth setting PCRE_DOTALL in order to obtain this opti- | |
849 | mization, or alternatively using ^ to indicate anchoring explicitly. | |
850 | ||
851 | However, there is one situation where the optimization cannot be used. | |
852 | When .* is inside capturing parentheses that are the subject of a | |
853 | backreference elsewhere in the pattern, a match at the start may fail, | |
854 | and a later one succeed. Consider, for example: | |
855 | ||
856 | (.*)abc\1 | |
857 | ||
858 | If the subject is "xyz123abc123" the match point is the fourth charac- | |
859 | ter. For this reason, such a pattern is not implicitly anchored. | |
860 | ||
861 | When a capturing subpattern is repeated, the value captured is the sub- | |
862 | string that matched the final iteration. For example, after | |
863 | ||
864 | (tweedle[dume]{3}\s*)+ | |
865 | ||
866 | has matched "tweedledum tweedledee" the value of the captured substring | |
867 | is "tweedledee". However, if there are nested capturing subpatterns, | |
868 | the corresponding captured values may have been set in previous itera- | |
869 | tions. For example, after | |
870 | ||
871 | /(a|(b))+/ | |
872 | ||
873 | matches "aba" the value of the second captured substring is "b". | |
874 | ||
875 | ||
876 | ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS | |
877 | ||
878 | With both maximizing and minimizing repetition, failure of what follows | |
879 | normally causes the repeated item to be re-evaluated to see if a dif- | |
880 | ferent number of repeats allows the rest of the pattern to match. Some- | |
881 | times it is useful to prevent this, either to change the nature of the | |
882 | match, or to cause it fail earlier than it otherwise might, when the | |
883 | author of the pattern knows there is no point in carrying on. | |
884 | ||
885 | Consider, for example, the pattern \d+foo when applied to the subject | |
886 | line | |
887 | ||
888 | 123456bar | |
889 | ||
890 | After matching all 6 digits and then failing to match "foo", the normal | |
891 | action of the matcher is to try again with only 5 digits matching the | |
892 | \d+ item, and then with 4, and so on, before ultimately failing. | |
893 | "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides | |
894 | the means for specifying that once a subpattern has matched, it is not | |
895 | to be re-evaluated in this way. | |
896 | ||
897 | If we use atomic grouping for the previous example, the matcher would | |
898 | give up immediately on failing to match "foo" the first time. The nota- | |
899 | tion is a kind of special parenthesis, starting with (?> as in this | |
900 | example: | |
901 | ||
902 | (?>\d+)foo | |
903 | ||
904 | This kind of parenthesis "locks up" the part of the pattern it con- | |
905 | tains once it has matched, and a failure further into the pattern is | |
906 | prevented from backtracking into it. Backtracking past it to previous | |
907 | items, however, works as normal. | |
908 | ||
909 | An alternative description is that a subpattern of this type matches | |
910 | the string of characters that an identical standalone pattern would | |
911 | match, if anchored at the current point in the subject string. | |
912 | ||
913 | Atomic grouping subpatterns are not capturing subpatterns. Simple cases | |
914 | such as the above example can be thought of as a maximizing repeat that | |
915 | must swallow everything it can. So, while both \d+ and \d+? are pre- | |
916 | pared to adjust the number of digits they match in order to make the | |
917 | rest of the pattern match, (?>\d+) can only match an entire sequence of | |
918 | digits. | |
919 | ||
920 | Atomic groups in general can of course contain arbitrarily complicated | |
921 | subpatterns, and can be nested. However, when the subpattern for an | |
922 | atomic group is just a single repeated item, as in the example above, a | |
923 | simpler notation, called a "possessive quantifier" can be used. This | |
924 | consists of an additional + character following a quantifier. Using | |
925 | this notation, the previous example can be rewritten as | |
926 | ||
927 | \d++foo | |
928 | ||
929 | Possessive quantifiers are always greedy; the setting of the | |
930 | PCRE_UNGREEDY option is ignored. They are a convenient notation for the | |
931 | simpler forms of atomic group. However, there is no difference in the | |
932 | meaning or processing of a possessive quantifier and the equivalent | |
933 | atomic group. | |
934 | ||
935 | The possessive quantifier syntax is an extension to the Perl syntax. It | |
936 | originates in Sun's Java package. | |
937 | ||
938 | When a pattern contains an unlimited repeat inside a subpattern that | |
939 | can itself be repeated an unlimited number of times, the use of an | |
940 | atomic group is the only way to avoid some failing matches taking a | |
941 | very long time indeed. The pattern | |
942 | ||
943 | (\D+|<\d+>)*[!?] | |
944 | ||
945 | matches an unlimited number of substrings that either consist of non- | |
946 | digits, or digits enclosed in <>, followed by either ! or ?. When it | |
947 | matches, it runs quickly. However, if it is applied to | |
948 | ||
949 | aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa | |
950 | ||
951 | it takes a long time before reporting failure. This is because the | |
952 | string can be divided between the internal \D+ repeat and the external | |
953 | * repeat in a large number of ways, and all have to be tried. (The | |
954 | example uses [!?] rather than a single character at the end, because | |
955 | both PCRE and Perl have an optimization that allows for fast failure | |
956 | when a single character is used. They remember the last single charac- | |
957 | ter that is required for a match, and fail early if it is not present | |
958 | in the string.) If the pattern is changed so that it uses an atomic | |
959 | group, like this: | |
960 | ||
961 | ((?>\D+)|<\d+>)*[!?] | |
962 | ||
963 | sequences of non-digits cannot be broken, and failure happens quickly. | |
964 | ||
965 | ||
966 | BACK REFERENCES | |
967 | ||
968 | Outside a character class, a backslash followed by a digit greater than | |
969 | 0 (and possibly further digits) is a back reference to a capturing sub- | |
970 | pattern earlier (that is, to its left) in the pattern, provided there | |
971 | have been that many previous capturing left parentheses. | |
972 | ||
973 | However, if the decimal number following the backslash is less than 10, | |
974 | it is always taken as a back reference, and causes an error only if | |
975 | there are not that many capturing left parentheses in the entire pat- | |
976 | tern. In other words, the parentheses that are referenced need not be | |
977 | to the left of the reference for numbers less than 10. See the subsec- | |
978 | tion entitled "Non-printing characters" above for further details of | |
979 | the handling of digits following a backslash. | |
980 | ||
981 | A back reference matches whatever actually matched the capturing sub- | |
982 | pattern in the current subject string, rather than anything matching | |
983 | the subpattern itself (see "Subpatterns as subroutines" below for a way | |
984 | of doing that). So the pattern | |
985 | ||
986 | (sens|respons)e and \1ibility | |
987 | ||
988 | matches "sense and sensibility" and "response and responsibility", but | |
989 | not "sense and responsibility". If caseful matching is in force at the | |
990 | time of the back reference, the case of letters is relevant. For exam- | |
991 | ple, | |
992 | ||
993 | ((?i)rah)\s+\1 | |
994 | ||
995 | matches "rah rah" and "RAH RAH", but not "RAH rah", even though the | |
996 | original capturing subpattern is matched caselessly. | |
997 | ||
998 | Back references to named subpatterns use the Python syntax (?P=name). | |
999 | We could rewrite the above example as follows: | |
1000 | ||
1001 | (?<p1>(?i)rah)\s+(?P=p1) | |
1002 | ||
1003 | There may be more than one back reference to the same subpattern. If a | |
1004 | subpattern has not actually been used in a particular match, any back | |
1005 | references to it always fail. For example, the pattern | |
1006 | ||
1007 | (a|(bc))\2 | |
1008 | ||
1009 | always fails if it starts to match "a" rather than "bc". Because there | |
1010 | may be many capturing parentheses in a pattern, all digits following | |
1011 | the backslash are taken as part of a potential back reference number. | |
1012 | If the pattern continues with a digit character, some delimiter must be | |
1013 | used to terminate the back reference. If the PCRE_EXTENDED option is | |
1014 | set, this can be whitespace. Otherwise an empty comment (see "Com- | |
1015 | ments" below) can be used. | |
1016 | ||
1017 | A back reference that occurs inside the parentheses to which it refers | |
1018 | fails when the subpattern is first used, so, for example, (a\1) never | |
1019 | matches. However, such references can be useful inside repeated sub- | |
1020 | patterns. For example, the pattern | |
1021 | ||
1022 | (a|b\1)+ | |
1023 | ||
1024 | matches any number of "a"s and also "aba", "ababbaa" etc. At each iter- | |
1025 | ation of the subpattern, the back reference matches the character | |
1026 | string corresponding to the previous iteration. In order for this to | |
1027 | work, the pattern must be such that the first iteration does not need | |
1028 | to match the back reference. This can be done using alternation, as in | |
1029 | the example above, or by a quantifier with a minimum of zero. | |
1030 | ||
1031 | ||
1032 | ASSERTIONS | |
1033 | ||
1034 | An assertion is a test on the characters following or preceding the | |
1035 | current matching point that does not actually consume any characters. | |
1036 | The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are | |
1037 | described above. | |
1038 | ||
1039 | More complicated assertions are coded as subpatterns. There are two | |
1040 | kinds: those that look ahead of the current position in the subject | |
1041 | string, and those that look behind it. An assertion subpattern is | |
1042 | matched in the normal way, except that it does not cause the current | |
1043 | matching position to be changed. | |
1044 | ||
1045 | Assertion subpatterns are not capturing subpatterns, and may not be | |
1046 | repeated, because it makes no sense to assert the same thing several | |
1047 | times. If any kind of assertion contains capturing subpatterns within | |
1048 | it, these are counted for the purposes of numbering the capturing sub- | |
1049 | patterns in the whole pattern. However, substring capturing is carried | |
1050 | out only for positive assertions, because it does not make sense for | |
1051 | negative assertions. | |
1052 | ||
1053 | Lookahead assertions | |
1054 | ||
1055 | Lookahead assertions start with (?= for positive assertions and (?! for | |
1056 | negative assertions. For example, | |
1057 | ||
1058 | \w+(?=;) | |
1059 | ||
1060 | matches a word followed by a semicolon, but does not include the semi- | |
1061 | colon in the match, and | |
1062 | ||
1063 | foo(?!bar) | |
1064 | ||
1065 | matches any occurrence of "foo" that is not followed by "bar". Note | |
1066 | that the apparently similar pattern | |
1067 | ||
1068 | (?!foo)bar | |
1069 | ||
1070 | does not find an occurrence of "bar" that is preceded by something | |
1071 | other than "foo"; it finds any occurrence of "bar" whatsoever, because | |
1072 | the assertion (?!foo) is always true when the next three characters are | |
1073 | "bar". A lookbehind assertion is needed to achieve the other effect. | |
1074 | ||
1075 | If you want to force a matching failure at some point in a pattern, the | |
1076 | most convenient way to do it is with (?!) because an empty string | |
1077 | always matches, so an assertion that requires there not to be an empty | |
1078 | string must always fail. | |
1079 | ||
1080 | Lookbehind assertions | |
1081 | ||
1082 | Lookbehind assertions start with (?<= for positive assertions and (?<! | |
1083 | for negative assertions. For example, | |
1084 | ||
1085 | (?<!foo)bar | |
1086 | ||
1087 | does find an occurrence of "bar" that is not preceded by "foo". The | |
1088 | contents of a lookbehind assertion are restricted such that all the | |
1089 | strings it matches must have a fixed length. However, if there are sev- | |
1090 | eral alternatives, they do not all have to have the same fixed length. | |
1091 | Thus | |
1092 | ||
1093 | (?<=bullock|donkey) | |
1094 | ||
1095 | is permitted, but | |
1096 | ||
1097 | (?<!dogs?|cats?) | |
1098 | ||
1099 | causes an error at compile time. Branches that match different length | |
1100 | strings are permitted only at the top level of a lookbehind assertion. | |
1101 | This is an extension compared with Perl (at least for 5.8), which | |
1102 | requires all branches to match the same length of string. An assertion | |
1103 | such as | |
1104 | ||
1105 | (?<=ab(c|de)) | |
1106 | ||
1107 | is not permitted, because its single top-level branch can match two | |
1108 | different lengths, but it is acceptable if rewritten to use two top- | |
1109 | level branches: | |
1110 | ||
1111 | (?<=abc|abde) | |
1112 | ||
1113 | The implementation of lookbehind assertions is, for each alternative, | |
1114 | to temporarily move the current position back by the fixed width and | |
1115 | then try to match. If there are insufficient characters before the cur- | |
1116 | rent position, the match is deemed to fail. | |
1117 | ||
1118 | PCRE does not allow the \C escape (which matches a single byte in UTF-8 | |
1119 | mode) to appear in lookbehind assertions, because it makes it impossi- | |
1120 | ble to calculate the length of the lookbehind. The \X escape, which can | |
1121 | match different numbers of bytes, is also not permitted. | |
1122 | ||
1123 | Atomic groups can be used in conjunction with lookbehind assertions to | |
1124 | specify efficient matching at the end of the subject string. Consider a | |
1125 | simple pattern such as | |
1126 | ||
1127 | abcd$ | |
1128 | ||
1129 | when applied to a long string that does not match. Because matching | |
1130 | proceeds from left to right, PCRE will look for each "a" in the subject | |
1131 | and then see if what follows matches the rest of the pattern. If the | |
1132 | pattern is specified as | |
1133 | ||
1134 | ^.*abcd$ | |
1135 | ||
1136 | the initial .* matches the entire string at first, but when this fails | |
1137 | (because there is no following "a"), it backtracks to match all but the | |
1138 | last character, then all but the last two characters, and so on. Once | |
1139 | again the search for "a" covers the entire string, from right to left, | |
1140 | so we are no better off. However, if the pattern is written as | |
1141 | ||
1142 | ^(?>.*)(?<=abcd) | |
1143 | ||
1144 | or, equivalently, using the possessive quantifier syntax, | |
1145 | ||
1146 | ^.*+(?<=abcd) | |
1147 | ||
1148 | there can be no backtracking for the .* item; it can match only the | |
1149 | entire string. The subsequent lookbehind assertion does a single test | |
1150 | on the last four characters. If it fails, the match fails immediately. | |
1151 | For long strings, this approach makes a significant difference to the | |
1152 | processing time. | |
1153 | ||
1154 | Using multiple assertions | |
1155 | ||
1156 | Several assertions (of any sort) may occur in succession. For example, | |
1157 | ||
1158 | (?<=\d{3})(?<!999)foo | |
1159 | ||
1160 | matches "foo" preceded by three digits that are not "999". Notice that | |
1161 | each of the assertions is applied independently at the same point in | |
1162 | the subject string. First there is a check that the previous three | |
1163 | characters are all digits, and then there is a check that the same | |
1164 | three characters are not "999". This pattern does not match "foo" pre- | |
1165 | ceded by six characters, the first of which are digits and the last | |
1166 | three of which are not "999". For example, it doesn't match "123abc- | |
1167 | foo". A pattern to do that is | |
1168 | ||
1169 | (?<=\d{3}...)(?<!999)foo | |
1170 | ||
1171 | This time the first assertion looks at the preceding six characters, | |
1172 | checking that the first three are digits, and then the second assertion | |
1173 | checks that the preceding three characters are not "999". | |
1174 | ||
1175 | Assertions can be nested in any combination. For example, | |
1176 | ||
1177 | (?<=(?<!foo)bar)baz | |
1178 | ||
1179 | matches an occurrence of "baz" that is preceded by "bar" which in turn | |
1180 | is not preceded by "foo", while | |
1181 | ||
1182 | (?<=\d{3}(?!999)...)foo | |
1183 | ||
1184 | is another pattern that matches "foo" preceded by three digits and any | |
1185 | three characters that are not "999". | |
1186 | ||
1187 | ||
1188 | CONDITIONAL SUBPATTERNS | |
1189 | ||
1190 | It is possible to cause the matching process to obey a subpattern con- | |
1191 | ditionally or to choose between two alternative subpatterns, depending | |
1192 | on the result of an assertion, or whether a previous capturing subpat- | |
1193 | tern matched or not. The two possible forms of conditional subpattern | |
1194 | are | |
1195 | ||
1196 | (?(condition)yes-pattern) | |
1197 | (?(condition)yes-pattern|no-pattern) | |
1198 | ||
1199 | If the condition is satisfied, the yes-pattern is used; otherwise the | |
1200 | no-pattern (if present) is used. If there are more than two alterna- | |
1201 | tives in the subpattern, a compile-time error occurs. | |
1202 | ||
1203 | There are three kinds of condition. If the text between the parentheses | |
1204 | consists of a sequence of digits, the condition is satisfied if the | |
1205 | capturing subpattern of that number has previously matched. The number | |
1206 | must be greater than zero. Consider the following pattern, which con- | |
1207 | tains non-significant white space to make it more readable (assume the | |
1208 | PCRE_EXTENDED option) and to divide it into three parts for ease of | |
1209 | discussion: | |
1210 | ||
1211 | ( \( )? [^()]+ (?(1) \) ) | |
1212 | ||
1213 | The first part matches an optional opening parenthesis, and if that | |
1214 | character is present, sets it as the first captured substring. The sec- | |
1215 | ond part matches one or more characters that are not parentheses. The | |
1216 | third part is a conditional subpattern that tests whether the first set | |
1217 | of parentheses matched or not. If they did, that is, if subject started | |
1218 | with an opening parenthesis, the condition is true, and so the yes-pat- | |
1219 | tern is executed and a closing parenthesis is required. Otherwise, | |
1220 | since no-pattern is not present, the subpattern matches nothing. In | |
1221 | other words, this pattern matches a sequence of non-parentheses, | |
1222 | optionally enclosed in parentheses. | |
1223 | ||
1224 | If the condition is the string (R), it is satisfied if a recursive call | |
1225 | to the pattern or subpattern has been made. At "top level", the condi- | |
1226 | tion is false. This is a PCRE extension. Recursive patterns are | |
1227 | described in the next section. | |
1228 | ||
1229 | If the condition is not a sequence of digits or (R), it must be an | |
1230 | assertion. This may be a positive or negative lookahead or lookbehind | |
1231 | assertion. Consider this pattern, again containing non-significant | |
1232 | white space, and with the two alternatives on the second line: | |
1233 | ||
1234 | (?(?=[^a-z]*[a-z]) | |
1235 | \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) | |
1236 | ||
1237 | The condition is a positive lookahead assertion that matches an | |
1238 | optional sequence of non-letters followed by a letter. In other words, | |
1239 | it tests for the presence of at least one letter in the subject. If a | |
1240 | letter is found, the subject is matched against the first alternative; | |
1241 | otherwise it is matched against the second. This pattern matches | |
1242 | strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are | |
1243 | letters and dd are digits. | |
1244 | ||
1245 | ||
1246 | COMMENTS | |
1247 | ||
1248 | The sequence (?# marks the start of a comment that continues up to the | |
1249 | next closing parenthesis. Nested parentheses are not permitted. The | |
1250 | characters that make up a comment play no part in the pattern matching | |
1251 | at all. | |
1252 | ||
1253 | If the PCRE_EXTENDED option is set, an unescaped # character outside a | |
1254 | character class introduces a comment that continues up to the next new- | |
1255 | line character in the pattern. | |
1256 | ||
1257 | ||
1258 | RECURSIVE PATTERNS | |
1259 | ||
1260 | Consider the problem of matching a string in parentheses, allowing for | |
1261 | unlimited nested parentheses. Without the use of recursion, the best | |
1262 | that can be done is to use a pattern that matches up to some fixed | |
1263 | depth of nesting. It is not possible to handle an arbitrary nesting | |
1264 | depth. Perl provides a facility that allows regular expressions to | |
1265 | recurse (amongst other things). It does this by interpolating Perl code | |
1266 | in the expression at run time, and the code can refer to the expression | |
1267 | itself. A Perl pattern to solve the parentheses problem can be created | |
1268 | like this: | |
1269 | ||
1270 | $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x; | |
1271 | ||
1272 | The (?p{...}) item interpolates Perl code at run time, and in this case | |
1273 | refers recursively to the pattern in which it appears. Obviously, PCRE | |
1274 | cannot support the interpolation of Perl code. Instead, it supports | |
1275 | some special syntax for recursion of the entire pattern, and also for | |
1276 | individual subpattern recursion. | |
1277 | ||
1278 | The special item that consists of (? followed by a number greater than | |
1279 | zero and a closing parenthesis is a recursive call of the subpattern of | |
1280 | the given number, provided that it occurs inside that subpattern. (If | |
1281 | not, it is a "subroutine" call, which is described in the next sec- | |
1282 | tion.) The special item (?R) is a recursive call of the entire regular | |
1283 | expression. | |
1284 | ||
1285 | For example, this PCRE pattern solves the nested parentheses problem | |
1286 | (assume the PCRE_EXTENDED option is set so that white space is | |
1287 | ignored): | |
1288 | ||
1289 | \( ( (?>[^()]+) | (?R) )* \) | |
1290 | ||
1291 | First it matches an opening parenthesis. Then it matches any number of | |
1292 | substrings which can either be a sequence of non-parentheses, or a | |
1293 | recursive match of the pattern itself (that is a correctly parenthe- | |
1294 | sized substring). Finally there is a closing parenthesis. | |
1295 | ||
1296 | If this were part of a larger pattern, you would not want to recurse | |
1297 | the entire pattern, so instead you could use this: | |
1298 | ||
1299 | ( \( ( (?>[^()]+) | (?1) )* \) ) | |
1300 | ||
1301 | We have put the pattern into parentheses, and caused the recursion to | |
1302 | refer to them instead of the whole pattern. In a larger pattern, keep- | |
1303 | ing track of parenthesis numbers can be tricky. It may be more conve- | |
1304 | nient to use named parentheses instead. For this, PCRE uses (?P>name), | |
1305 | which is an extension to the Python syntax that PCRE uses for named | |
1306 | parentheses (Perl does not provide named parentheses). We could rewrite | |
1307 | the above example as follows: | |
1308 | ||
1309 | (?P<pn> \( ( (?>[^()]+) | (?P>pn) )* \) ) | |
1310 | ||
1311 | This particular example pattern contains nested unlimited repeats, and | |
1312 | so the use of atomic grouping for matching strings of non-parentheses | |
1313 | is important when applying the pattern to strings that do not match. | |
1314 | For example, when this pattern is applied to | |
1315 | ||
1316 | (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() | |
1317 | ||
1318 | it yields "no match" quickly. However, if atomic grouping is not used, | |
1319 | the match runs for a very long time indeed because there are so many | |
1320 | different ways the + and * repeats can carve up the subject, and all | |
1321 | have to be tested before failure can be reported. | |
1322 | ||
1323 | At the end of a match, the values set for any capturing subpatterns are | |
1324 | those from the outermost level of the recursion at which the subpattern | |
1325 | value is set. If you want to obtain intermediate values, a callout | |
1326 | function can be used (see the next section and the pcrecallout documen- | |
1327 | tation). If the pattern above is matched against | |
1328 | ||
1329 | (ab(cd)ef) | |
1330 | ||
1331 | the value for the capturing parentheses is "ef", which is the last | |
1332 | value taken on at the top level. If additional parentheses are added, | |
1333 | giving | |
1334 | ||
1335 | \( ( ( (?>[^()]+) | (?R) )* ) \) | |
1336 | ^ ^ | |
1337 | ^ ^ | |
1338 | ||
1339 | the string they capture is "ab(cd)ef", the contents of the top level | |
1340 | parentheses. If there are more than 15 capturing parentheses in a pat- | |
1341 | tern, PCRE has to obtain extra memory to store data during a recursion, | |
1342 | which it does by using pcre_malloc, freeing it via pcre_free after- | |
1343 | wards. If no memory can be obtained, the match fails with the | |
1344 | PCRE_ERROR_NOMEMORY error. | |
1345 | ||
1346 | Do not confuse the (?R) item with the condition (R), which tests for | |
1347 | recursion. Consider this pattern, which matches text in angle brack- | |
1348 | ets, allowing for arbitrary nesting. Only digits are allowed in nested | |
1349 | brackets (that is, when recursing), whereas any characters are permit- | |
1350 | ted at the outer level. | |
1351 | ||
1352 | < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > | |
1353 | ||
1354 | In this pattern, (?(R) is the start of a conditional subpattern, with | |
1355 | two different alternatives for the recursive and non-recursive cases. | |
1356 | The (?R) item is the actual recursive call. | |
1357 | ||
1358 | ||
1359 | SUBPATTERNS AS SUBROUTINES | |
1360 | ||
1361 | If the syntax for a recursive subpattern reference (either by number or | |
1362 | by name) is used outside the parentheses to which it refers, it oper- | |
1363 | ates like a subroutine in a programming language. An earlier example | |
1364 | pointed out that the pattern | |
1365 | ||
1366 | (sens|respons)e and \1ibility | |
1367 | ||
1368 | matches "sense and sensibility" and "response and responsibility", but | |
1369 | not "sense and responsibility". If instead the pattern | |
1370 | ||
1371 | (sens|respons)e and (?1)ibility | |
1372 | ||
1373 | is used, it does match "sense and responsibility" as well as the other | |
1374 | two strings. Such references must, however, follow the subpattern to | |
1375 | which they refer. | |
1376 | ||
1377 | ||
1378 | CALLOUTS | |
1379 | ||
1380 | Perl has a feature whereby using the sequence (?{...}) causes arbitrary | |
1381 | Perl code to be obeyed in the middle of matching a regular expression. | |
1382 | This makes it possible, amongst other things, to extract different sub- | |
1383 | strings that match the same pair of parentheses when there is a repeti- | |
1384 | tion. | |
1385 | ||
1386 | PCRE provides a similar feature, but of course it cannot obey arbitrary | |
1387 | Perl code. The feature is called "callout". The caller of PCRE provides | |
1388 | an external function by putting its entry point in the global variable | |
1389 | pcre_callout. By default, this variable contains NULL, which disables | |
1390 | all calling out. | |
1391 | ||
1392 | Within a regular expression, (?C) indicates the points at which the | |
1393 | external function is to be called. If you want to identify different | |
1394 | callout points, you can put a number less than 256 after the letter C. | |
1395 | The default value is zero. For example, this pattern has two callout | |
1396 | points: | |
1397 | ||
1398 | (?C1)abc(?C2)def | |
1399 | ||
1400 | If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are | |
1401 | automatically installed before each item in the pattern. They are all | |
1402 | numbered 255. | |
1403 | ||
1404 | During matching, when PCRE reaches a callout point (and pcre_callout is | |
1405 | set), the external function is called. It is provided with the number | |
1406 | of the callout, the position in the pattern, and, optionally, one item | |
1407 | of data originally supplied by the caller of pcre_exec(). The callout | |
1408 | function may cause matching to proceed, to backtrack, or to fail alto- | |
1409 | gether. A complete description of the interface to the callout function | |
1410 | is given in the pcrecallout documentation. | |
1411 | ||
1412 | Last updated: 09 September 2004 | |
1413 | Copyright (c) 1997-2004 University of Cambridge. |