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1 nigel 63 .TH PCRE 3
2     .SH NAME
3     PCRE - Perl-compatible regular expressions
5 nigel 63 .rs
6     .sp
7     The syntax and semantics of the regular expressions supported by PCRE are
8     described below. Regular expressions are also described in the Perl
9 nigel 75 documentation and in a number of books, some of which have copious examples.
10     Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers
11     regular expressions in great detail. This description of PCRE's regular
12     expressions is intended as reference material.
13     .P
14     The original operation of PCRE was on strings of one-byte characters. However,
15     there is now also support for UTF-8 character strings. To use this, you must
16     build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with
17     the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
18     places below. There is also a summary of UTF-8 features in the
19 nigel 63 .\" HTML <a href="pcre.html#utf8support">
20     .\" </a>
21     section on UTF-8 support
22     .\"
23     in the main
24     .\" HREF
25 nigel 75 \fBpcre\fP
26 nigel 63 .\"
27     page.
28 nigel 75 .P
29 nigel 77 The remainder of this document discusses the patterns that are supported by
30     PCRE when its main matching function, \fBpcre_exec()\fP, is used.
31     From release 6.0, PCRE offers a second matching function,
32     \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
33     Perl-compatible. The advantages and disadvantages of the alternative function,
34     and how it differs from the normal function, are discussed in the
35     .\" HREF
36     \fBpcrematching\fP
37     .\"
38     page.
39     .P
40 nigel 63 A regular expression is a pattern that is matched against a subject string from
41     left to right. Most characters stand for themselves in a pattern, and match the
42     corresponding characters in the subject. As a trivial example, the pattern
43 nigel 75 .sp
44 nigel 63 The quick brown fox
45 nigel 75 .sp
46 nigel 77 matches a portion of a subject string that is identical to itself. When
47     caseless matching is specified (the PCRE_CASELESS option), letters are matched
48     independently of case. In UTF-8 mode, PCRE always understands the concept of
49     case for characters whose values are less than 128, so caseless matching is
50     always possible. For characters with higher values, the concept of case is
51     supported if PCRE is compiled with Unicode property support, but not otherwise.
52     If you want to use caseless matching for characters 128 and above, you must
53     ensure that PCRE is compiled with Unicode property support as well as with
54     UTF-8 support.
55     .P
56     The power of regular expressions comes from the ability to include alternatives
57     and repetitions in the pattern. These are encoded in the pattern by the use of
58 nigel 75 \fImetacharacters\fP, which do not stand for themselves but instead are
59 nigel 63 interpreted in some special way.
60 nigel 75 .P
61     There are two different sets of metacharacters: those that are recognized
62 nigel 63 anywhere in the pattern except within square brackets, and those that are
63 nigel 75 recognized in square brackets. Outside square brackets, the metacharacters are
64 nigel 63 as follows:
65 nigel 75 .sp
66     \e general escape character with several uses
67 nigel 63 ^ assert start of string (or line, in multiline mode)
68     $ assert end of string (or line, in multiline mode)
69     . match any character except newline (by default)
70     [ start character class definition
71     | start of alternative branch
72     ( start subpattern
73     ) end subpattern
74     ? extends the meaning of (
75     also 0 or 1 quantifier
76     also quantifier minimizer
77     * 0 or more quantifier
78     + 1 or more quantifier
79     also "possessive quantifier"
80     { start min/max quantifier
81 nigel 75 .sp
82 nigel 63 Part of a pattern that is in square brackets is called a "character class". In
83 nigel 75 a character class the only metacharacters are:
84     .sp
85     \e general escape character
86 nigel 63 ^ negate the class, but only if the first character
87     - indicates character range
88 nigel 75 .\" JOIN
89 nigel 63 [ POSIX character class (only if followed by POSIX
90     syntax)
91     ] terminates the character class
92 nigel 75 .sp
93     The following sections describe the use of each of the metacharacters.
94     .
95 nigel 63 .SH BACKSLASH
96     .rs
97     .sp
98     The backslash character has several uses. Firstly, if it is followed by a
99 nigel 75 non-alphanumeric character, it takes away any special meaning that character may
100 nigel 63 have. This use of backslash as an escape character applies both inside and
101     outside character classes.
102 nigel 75 .P
103     For example, if you want to match a * character, you write \e* in the pattern.
104 nigel 63 This escaping action applies whether or not the following character would
105 nigel 75 otherwise be interpreted as a metacharacter, so it is always safe to precede a
106     non-alphanumeric with backslash to specify that it stands for itself. In
107     particular, if you want to match a backslash, you write \e\e.
108     .P
109 nigel 63 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
110     pattern (other than in a character class) and characters between a # outside
111     a character class and the next newline character are ignored. An escaping
112     backslash can be used to include a whitespace or # character as part of the
113     pattern.
114 nigel 75 .P
115 nigel 63 If you want to remove the special meaning from a sequence of characters, you
116 nigel 75 can do so by putting them between \eQ and \eE. This is different from Perl in
117     that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
118 nigel 63 Perl, $ and @ cause variable interpolation. Note the following examples:
119 nigel 75 .sp
120 nigel 63 Pattern PCRE matches Perl matches
121 nigel 75 .sp
122     .\" JOIN
123     \eQabc$xyz\eE abc$xyz abc followed by the
124 nigel 63 contents of $xyz
125 nigel 75 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
126     \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
127     .sp
128     The \eQ...\eE sequence is recognized both inside and outside character classes.
129     .
130     .
131     .\" HTML <a name="digitsafterbackslash"></a>
132     .SS "Non-printing characters"
133     .rs
134     .sp
135 nigel 63 A second use of backslash provides a way of encoding non-printing characters
136     in patterns in a visible manner. There is no restriction on the appearance of
137     non-printing characters, apart from the binary zero that terminates a pattern,
138     but when a pattern is being prepared by text editing, it is usually easier to
139     use one of the following escape sequences than the binary character it
140     represents:
141 nigel 75 .sp
142     \ea alarm, that is, the BEL character (hex 07)
143     \ecx "control-x", where x is any character
144     \ee escape (hex 1B)
145     \ef formfeed (hex 0C)
146     \en newline (hex 0A)
147     \er carriage return (hex 0D)
148     \et tab (hex 09)
149     \eddd character with octal code ddd, or backreference
150     \exhh character with hex code hh
151     \ex{hhh..} character with hex code hhh... (UTF-8 mode only)
152     .sp
153     The precise effect of \ecx is as follows: if x is a lower case letter, it
154 nigel 63 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
155 nigel 75 Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex
156 nigel 63 7B.
157 nigel 75 .P
158     After \ex, from zero to two hexadecimal digits are read (letters can be in
159 nigel 63 upper or lower case). In UTF-8 mode, any number of hexadecimal digits may
160 nigel 75 appear between \ex{ and }, but the value of the character code must be less
161 nigel 63 than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters
162 nigel 75 other than hexadecimal digits appear between \ex{ and }, or if there is no
163 nigel 63 terminating }, this form of escape is not recognized. Instead, the initial
164 nigel 75 \ex will be interpreted as a basic hexadecimal escape, with no following
165     digits, giving a character whose value is zero.
166     .P
167 nigel 63 Characters whose value is less than 256 can be defined by either of the two
168 nigel 75 syntaxes for \ex when PCRE is in UTF-8 mode. There is no difference in the
169     way they are handled. For example, \exdc is exactly the same as \ex{dc}.
170     .P
171     After \e0 up to two further octal digits are read. In both cases, if there
172 nigel 63 are fewer than two digits, just those that are present are used. Thus the
173 nigel 75 sequence \e0\ex\e07 specifies two binary zeros followed by a BEL character
174 nigel 63 (code value 7). Make sure you supply two digits after the initial zero if the
175 nigel 75 pattern character that follows is itself an octal digit.
176     .P
177 nigel 63 The handling of a backslash followed by a digit other than 0 is complicated.
178     Outside a character class, PCRE reads it and any following digits as a decimal
179     number. If the number is less than 10, or if there have been at least that many
180     previous capturing left parentheses in the expression, the entire sequence is
181 nigel 75 taken as a \fIback reference\fP. A description of how this works is given
182     .\" HTML <a href="#backreferences">
183     .\" </a>
184     later,
185     .\"
186     following the discussion of
187     .\" HTML <a href="#subpattern">
188     .\" </a>
189     parenthesized subpatterns.
190     .\"
191     .P
192 nigel 63 Inside a character class, or if the decimal number is greater than 9 and there
193     have not been that many capturing subpatterns, PCRE re-reads up to three octal
194     digits following the backslash, and generates a single byte from the least
195     significant 8 bits of the value. Any subsequent digits stand for themselves.
196     For example:
197 nigel 75 .sp
198     \e040 is another way of writing a space
199     .\" JOIN
200     \e40 is the same, provided there are fewer than 40
201 nigel 63 previous capturing subpatterns
202 nigel 75 \e7 is always a back reference
203     .\" JOIN
204     \e11 might be a back reference, or another way of
205 nigel 63 writing a tab
206 nigel 75 \e011 is always a tab
207     \e0113 is a tab followed by the character "3"
208     .\" JOIN
209     \e113 might be a back reference, otherwise the
210 nigel 63 character with octal code 113
211 nigel 75 .\" JOIN
212     \e377 might be a back reference, otherwise
213 nigel 63 the byte consisting entirely of 1 bits
214 nigel 75 .\" JOIN
215     \e81 is either a back reference, or a binary zero
216 nigel 63 followed by the two characters "8" and "1"
217 nigel 75 .sp
218 nigel 63 Note that octal values of 100 or greater must not be introduced by a leading
219     zero, because no more than three octal digits are ever read.
220 nigel 75 .P
221 nigel 63 All the sequences that define a single byte value or a single UTF-8 character
222     (in UTF-8 mode) can be used both inside and outside character classes. In
223 nigel 75 addition, inside a character class, the sequence \eb is interpreted as the
224     backspace character (hex 08), and the sequence \eX is interpreted as the
225     character "X". Outside a character class, these sequences have different
226     meanings
227     .\" HTML <a href="#uniextseq">
228     .\" </a>
229     (see below).
230     .\"
231     .
232     .
233     .SS "Generic character types"
234     .rs
235     .sp
236     The third use of backslash is for specifying generic character types. The
237     following are always recognized:
238     .sp
239     \ed any decimal digit
240     \eD any character that is not a decimal digit
241     \es any whitespace character
242     \eS any character that is not a whitespace character
243     \ew any "word" character
244     \eW any "non-word" character
245     .sp
246 nigel 63 Each pair of escape sequences partitions the complete set of characters into
247     two disjoint sets. Any given character matches one, and only one, of each pair.
248 nigel 75 .P
249     These character type sequences can appear both inside and outside character
250     classes. They each match one character of the appropriate type. If the current
251     matching point is at the end of the subject string, all of them fail, since
252     there is no character to match.
253     .P
254     For compatibility with Perl, \es does not match the VT character (code 11).
255     This makes it different from the the POSIX "space" class. The \es characters
256 nigel 63 are HT (9), LF (10), FF (12), CR (13), and space (32).
257 nigel 75 .P
258     A "word" character is an underscore or any character less than 256 that is a
259     letter or digit. The definition of letters and digits is controlled by PCRE's
260     low-valued character tables, and may vary if locale-specific matching is taking
261     place (see
262 nigel 63 .\" HTML <a href="pcreapi.html#localesupport">
263     .\" </a>
264     "Locale support"
265     .\"
266     in the
267     .\" HREF
268 nigel 75 \fBpcreapi\fP
269 nigel 63 .\"
270 nigel 75 page). For example, in the "fr_FR" (French) locale, some character codes
271     greater than 128 are used for accented letters, and these are matched by \ew.
272     .P
273     In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
274     \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
275     character property support is available.
276     .
277     .
278     .\" HTML <a name="uniextseq"></a>
279     .SS Unicode character properties
280     .rs
281     .sp
282     When PCRE is built with Unicode character property support, three additional
283     escape sequences to match generic character types are available when UTF-8 mode
284     is selected. They are:
285     .sp
286     \ep{\fIxx\fP} a character with the \fIxx\fP property
287     \eP{\fIxx\fP} a character without the \fIxx\fP property
288     \eX an extended Unicode sequence
289     .sp
290     The property names represented by \fIxx\fP above are limited to the
291     Unicode general category properties. Each character has exactly one such
292     property, specified by a two-letter abbreviation. For compatibility with Perl,
293     negation can be specified by including a circumflex between the opening brace
294     and the property name. For example, \ep{^Lu} is the same as \eP{Lu}.
295     .P
296     If only one letter is specified with \ep or \eP, it includes all the properties
297     that start with that letter. In this case, in the absence of negation, the
298     curly brackets in the escape sequence are optional; these two examples have
299     the same effect:
300     .sp
301     \ep{L}
302     \epL
303     .sp
304     The following property codes are supported:
305     .sp
306     C Other
307     Cc Control
308     Cf Format
309     Cn Unassigned
310     Co Private use
311     Cs Surrogate
312     .sp
313     L Letter
314     Ll Lower case letter
315     Lm Modifier letter
316     Lo Other letter
317     Lt Title case letter
318     Lu Upper case letter
319     .sp
320     M Mark
321     Mc Spacing mark
322     Me Enclosing mark
323     Mn Non-spacing mark
324     .sp
325     N Number
326     Nd Decimal number
327     Nl Letter number
328     No Other number
329     .sp
330     P Punctuation
331     Pc Connector punctuation
332     Pd Dash punctuation
333     Pe Close punctuation
334     Pf Final punctuation
335     Pi Initial punctuation
336     Po Other punctuation
337     Ps Open punctuation
338     .sp
339     S Symbol
340     Sc Currency symbol
341     Sk Modifier symbol
342     Sm Mathematical symbol
343     So Other symbol
344     .sp
345     Z Separator
346     Zl Line separator
347     Zp Paragraph separator
348     Zs Space separator
349     .sp
350     Extended properties such as "Greek" or "InMusicalSymbols" are not supported by
351     PCRE.
352     .P
353     Specifying caseless matching does not affect these escape sequences. For
354     example, \ep{Lu} always matches only upper case letters.
355     .P
356     The \eX escape matches any number of Unicode characters that form an extended
357     Unicode sequence. \eX is equivalent to
358     .sp
359     (?>\ePM\epM*)
360     .sp
361     That is, it matches a character without the "mark" property, followed by zero
362     or more characters with the "mark" property, and treats the sequence as an
363     atomic group
364     .\" HTML <a href="#atomicgroup">
365     .\" </a>
366     (see below).
367     .\"
368     Characters with the "mark" property are typically accents that affect the
369     preceding character.
370     .P
371     Matching characters by Unicode property is not fast, because PCRE has to search
372     a structure that contains data for over fifteen thousand characters. That is
373     why the traditional escape sequences such as \ed and \ew do not use Unicode
374     properties in PCRE.
375     .
376     .
377     .\" HTML <a name="smallassertions"></a>
378     .SS "Simple assertions"
379     .rs
380     .sp
381 nigel 63 The fourth use of backslash is for certain simple assertions. An assertion
382     specifies a condition that has to be met at a particular point in a match,
383     without consuming any characters from the subject string. The use of
384 nigel 75 subpatterns for more complicated assertions is described
385     .\" HTML <a href="#bigassertions">
386     .\" </a>
387     below.
388     .\"
389     The backslashed
390     assertions are:
391     .sp
392     \eb matches at a word boundary
393     \eB matches when not at a word boundary
394     \eA matches at start of subject
395     \eZ matches at end of subject or before newline at end
396     \ez matches at end of subject
397     \eG matches at first matching position in subject
398     .sp
399     These assertions may not appear in character classes (but note that \eb has a
400 nigel 63 different meaning, namely the backspace character, inside a character class).
401 nigel 75 .P
402 nigel 63 A word boundary is a position in the subject string where the current character
403 nigel 75 and the previous character do not both match \ew or \eW (i.e. one matches
404     \ew and the other matches \eW), or the start or end of the string if the
405     first or last character matches \ew, respectively.
406     .P
407     The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
408     dollar (described in the next section) in that they only ever match at the very
409     start and end of the subject string, whatever options are set. Thus, they are
410     independent of multiline mode. These three assertions are not affected by the
411     PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
412     circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
413     argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
414     at a point other than the beginning of the subject, \eA can never match. The
415     difference between \eZ and \ez is that \eZ matches before a newline that is the
416     last character of the string as well as at the end of the string, whereas \ez
417     matches only at the end.
418     .P
419     The \eG assertion is true only when the current matching position is at the
420     start point of the match, as specified by the \fIstartoffset\fP argument of
421     \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
422     non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
423 nigel 63 arguments, you can mimic Perl's /g option, and it is in this kind of
424 nigel 75 implementation where \eG can be useful.
425     .P
426     Note, however, that PCRE's interpretation of \eG, as the start of the current
427 nigel 63 match, is subtly different from Perl's, which defines it as the end of the
428     previous match. In Perl, these can be different when the previously matched
429     string was empty. Because PCRE does just one match at a time, it cannot
430     reproduce this behaviour.
431 nigel 75 .P
432     If all the alternatives of a pattern begin with \eG, the expression is anchored
433 nigel 63 to the starting match position, and the "anchored" flag is set in the compiled
434     regular expression.
435 nigel 75 .
436     .
438 nigel 63 .rs
439     .sp
440     Outside a character class, in the default matching mode, the circumflex
441 nigel 75 character is an assertion that is true only if the current matching point is
442     at the start of the subject string. If the \fIstartoffset\fP argument of
443     \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
444 nigel 63 option is unset. Inside a character class, circumflex has an entirely different
445 nigel 75 meaning
446     .\" HTML <a href="#characterclass">
447     .\" </a>
448     (see below).
449     .\"
450     .P
451 nigel 63 Circumflex need not be the first character of the pattern if a number of
452     alternatives are involved, but it should be the first thing in each alternative
453     in which it appears if the pattern is ever to match that branch. If all
454     possible alternatives start with a circumflex, that is, if the pattern is
455     constrained to match only at the start of the subject, it is said to be an
456     "anchored" pattern. (There are also other constructs that can cause a pattern
457     to be anchored.)
458 nigel 75 .P
459     A dollar character is an assertion that is true only if the current matching
460 nigel 63 point is at the end of the subject string, or immediately before a newline
461     character that is the last character in the string (by default). Dollar need
462     not be the last character of the pattern if a number of alternatives are
463     involved, but it should be the last item in any branch in which it appears.
464     Dollar has no special meaning in a character class.
465 nigel 75 .P
466 nigel 63 The meaning of dollar can be changed so that it matches only at the very end of
467     the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
468 nigel 75 does not affect the \eZ assertion.
469     .P
470 nigel 63 The meanings of the circumflex and dollar characters are changed if the
471     PCRE_MULTILINE option is set. When this is the case, they match immediately
472     after and immediately before an internal newline character, respectively, in
473     addition to matching at the start and end of the subject string. For example,
474 nigel 75 the pattern /^abc$/ matches the subject string "def\enabc" (where \en
475     represents a newline character) in multiline mode, but not otherwise.
476     Consequently, patterns that are anchored in single line mode because all
477     branches start with ^ are not anchored in multiline mode, and a match for
478     circumflex is possible when the \fIstartoffset\fP argument of \fBpcre_exec()\fP
479     is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is
480     set.
481     .P
482     Note that the sequences \eA, \eZ, and \ez can be used to match the start and
483 nigel 63 end of the subject in both modes, and if all branches of a pattern start with
484 nigel 75 \eA it is always anchored, whether PCRE_MULTILINE is set or not.
485     .
486     .
488 nigel 63 .rs
489     .sp
490     Outside a character class, a dot in the pattern matches any one character in
491     the subject, including a non-printing character, but not (by default) newline.
492     In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one
493 nigel 75 byte long, except (by default) newline. If the PCRE_DOTALL option is set,
494 nigel 63 dots match newlines as well. The handling of dot is entirely independent of the
495     handling of circumflex and dollar, the only relationship being that they both
496     involve newline characters. Dot has no special meaning in a character class.
497 nigel 75 .
498     .
500 nigel 63 .rs
501     .sp
502 nigel 75 Outside a character class, the escape sequence \eC matches any one byte, both
503     in and out of UTF-8 mode. Unlike a dot, it can match a newline. The feature is
504     provided in Perl in order to match individual bytes in UTF-8 mode. Because it
505     breaks up UTF-8 characters into individual bytes, what remains in the string
506     may be a malformed UTF-8 string. For this reason, the \eC escape sequence is
507     best avoided.
508     .P
509     PCRE does not allow \eC to appear in lookbehind assertions
510     .\" HTML <a href="#lookbehind">
511     .\" </a>
512     (described below),
513     .\"
514     because in UTF-8 mode this would make it impossible to calculate the length of
515     the lookbehind.
516     .
517     .
518     .\" HTML <a name="characterclass"></a>
520 nigel 63 .rs
521     .sp
522     An opening square bracket introduces a character class, terminated by a closing
523     square bracket. A closing square bracket on its own is not special. If a
524     closing square bracket is required as a member of the class, it should be the
525     first data character in the class (after an initial circumflex, if present) or
526     escaped with a backslash.
527 nigel 75 .P
528 nigel 63 A character class matches a single character in the subject. In UTF-8 mode, the
529     character may occupy more than one byte. A matched character must be in the set
530     of characters defined by the class, unless the first character in the class
531     definition is a circumflex, in which case the subject character must not be in
532     the set defined by the class. If a circumflex is actually required as a member
533     of the class, ensure it is not the first character, or escape it with a
534     backslash.
535 nigel 75 .P
536 nigel 63 For example, the character class [aeiou] matches any lower case vowel, while
537     [^aeiou] matches any character that is not a lower case vowel. Note that a
538 nigel 75 circumflex is just a convenient notation for specifying the characters that
539     are in the class by enumerating those that are not. A class that starts with a
540     circumflex is not an assertion: it still consumes a character from the subject
541     string, and therefore it fails if the current pointer is at the end of the
542     string.
543     .P
544 nigel 63 In UTF-8 mode, characters with values greater than 255 can be included in a
545 nigel 75 class as a literal string of bytes, or by using the \ex{ escaping mechanism.
546     .P
547 nigel 63 When caseless matching is set, any letters in a class represent both their
548     upper case and lower case versions, so for example, a caseless [aeiou] matches
549     "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
550 nigel 77 caseful version would. In UTF-8 mode, PCRE always understands the concept of
551     case for characters whose values are less than 128, so caseless matching is
552     always possible. For characters with higher values, the concept of case is
553     supported if PCRE is compiled with Unicode property support, but not otherwise.
554     If you want to use caseless matching for characters 128 and above, you must
555     ensure that PCRE is compiled with Unicode property support as well as with
556     UTF-8 support.
557 nigel 75 .P
558 nigel 63 The newline character is never treated in any special way in character classes,
559     whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class
560     such as [^a] will always match a newline.
561 nigel 75 .P
562 nigel 63 The minus (hyphen) character can be used to specify a range of characters in a
563     character class. For example, [d-m] matches any letter between d and m,
564     inclusive. If a minus character is required in a class, it must be escaped with
565     a backslash or appear in a position where it cannot be interpreted as
566     indicating a range, typically as the first or last character in the class.
567 nigel 75 .P
568 nigel 63 It is not possible to have the literal character "]" as the end character of a
569     range. A pattern such as [W-]46] is interpreted as a class of two characters
570     ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
571     "-46]". However, if the "]" is escaped with a backslash it is interpreted as
572 nigel 75 the end of range, so [W-\e]46] is interpreted as a class containing a range
573     followed by two other characters. The octal or hexadecimal representation of
574     "]" can also be used to end a range.
575     .P
576 nigel 63 Ranges operate in the collating sequence of character values. They can also be
577 nigel 75 used for characters specified numerically, for example [\e000-\e037]. In UTF-8
578 nigel 63 mode, ranges can include characters whose values are greater than 255, for
579 nigel 75 example [\ex{100}-\ex{2ff}].
580     .P
581 nigel 63 If a range that includes letters is used when caseless matching is set, it
582     matches the letters in either case. For example, [W-c] is equivalent to
583 nigel 75 [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
584     tables for the "fr_FR" locale are in use, [\exc8-\excb] matches accented E
585     characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
586     characters with values greater than 128 only when it is compiled with Unicode
587     property support.
588     .P
589     The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
590     in a character class, and add the characters that they match to the class. For
591     example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
592 nigel 63 conveniently be used with the upper case character types to specify a more
593     restricted set of characters than the matching lower case type. For example,
594 nigel 75 the class [^\eW_] matches any letter or digit, but not underscore.
595     .P
596     The only metacharacters that are recognized in character classes are backslash,
597     hyphen (only where it can be interpreted as specifying a range), circumflex
598     (only at the start), opening square bracket (only when it can be interpreted as
599     introducing a POSIX class name - see the next section), and the terminating
600     closing square bracket. However, escaping other non-alphanumeric characters
601     does no harm.
602     .
603     .
605 nigel 63 .rs
606     .sp
607 nigel 75 Perl supports the POSIX notation for character classes. This uses names
608 nigel 63 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
609     this notation. For example,
610 nigel 75 .sp
611 nigel 63 [01[:alpha:]%]
612 nigel 75 .sp
613 nigel 63 matches "0", "1", any alphabetic character, or "%". The supported class names
614     are
615 nigel 75 .sp
616 nigel 63 alnum letters and digits
617     alpha letters
618     ascii character codes 0 - 127
619     blank space or tab only
620     cntrl control characters
621 nigel 75 digit decimal digits (same as \ed)
622 nigel 63 graph printing characters, excluding space
623     lower lower case letters
624     print printing characters, including space
625     punct printing characters, excluding letters and digits
626 nigel 75 space white space (not quite the same as \es)
627 nigel 63 upper upper case letters
628 nigel 75 word "word" characters (same as \ew)
629 nigel 63 xdigit hexadecimal digits
630 nigel 75 .sp
631 nigel 63 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
632     space (32). Notice that this list includes the VT character (code 11). This
633 nigel 75 makes "space" different to \es, which does not include VT (for Perl
634 nigel 63 compatibility).
635 nigel 75 .P
636 nigel 63 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
637     5.8. Another Perl extension is negation, which is indicated by a ^ character
638     after the colon. For example,
639 nigel 75 .sp
640 nigel 63 [12[:^digit:]]
641 nigel 75 .sp
642 nigel 63 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
643     syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
644     supported, and an error is given if they are encountered.
645 nigel 75 .P
646     In UTF-8 mode, characters with values greater than 128 do not match any of
647 nigel 63 the POSIX character classes.
648 nigel 75 .
649     .
650     .SH "VERTICAL BAR"
651 nigel 63 .rs
652     .sp
653     Vertical bar characters are used to separate alternative patterns. For example,
654     the pattern
655 nigel 75 .sp
656 nigel 63 gilbert|sullivan
657 nigel 75 .sp
658 nigel 63 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
659     and an empty alternative is permitted (matching the empty string).
660     The matching process tries each alternative in turn, from left to right,
661     and the first one that succeeds is used. If the alternatives are within a
662 nigel 75 subpattern
663     .\" HTML <a href="#subpattern">
664     .\" </a>
665     (defined below),
666     .\"
667     "succeeds" means matching the rest of the main pattern as well as the
668     alternative in the subpattern.
669     .
670     .
672 nigel 63 .rs
673     .sp
674     The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
675     PCRE_EXTENDED options can be changed from within the pattern by a sequence of
676     Perl option letters enclosed between "(?" and ")". The option letters are
677 nigel 75 .sp
678 nigel 63 i for PCRE_CASELESS
679     m for PCRE_MULTILINE
680     s for PCRE_DOTALL
681     x for PCRE_EXTENDED
682 nigel 75 .sp
683 nigel 63 For example, (?im) sets caseless, multiline matching. It is also possible to
684     unset these options by preceding the letter with a hyphen, and a combined
685     setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
686     PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
687     permitted. If a letter appears both before and after the hyphen, the option is
688     unset.
689 nigel 75 .P
690 nigel 63 When an option change occurs at top level (that is, not inside subpattern
691     parentheses), the change applies to the remainder of the pattern that follows.
692     If the change is placed right at the start of a pattern, PCRE extracts it into
693     the global options (and it will therefore show up in data extracted by the
694 nigel 75 \fBpcre_fullinfo()\fP function).
695     .P
696 nigel 63 An option change within a subpattern affects only that part of the current
697     pattern that follows it, so
698 nigel 75 .sp
699 nigel 63 (a(?i)b)c
700 nigel 75 .sp
701 nigel 63 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
702     By this means, options can be made to have different settings in different
703     parts of the pattern. Any changes made in one alternative do carry on
704     into subsequent branches within the same subpattern. For example,
705 nigel 75 .sp
706 nigel 63 (a(?i)b|c)
707 nigel 75 .sp
708 nigel 63 matches "ab", "aB", "c", and "C", even though when matching "C" the first
709     branch is abandoned before the option setting. This is because the effects of
710     option settings happen at compile time. There would be some very weird
711     behaviour otherwise.
712 nigel 75 .P
713 nigel 63 The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the
714     same way as the Perl-compatible options by using the characters U and X
715     respectively. The (?X) flag setting is special in that it must always occur
716     earlier in the pattern than any of the additional features it turns on, even
717 nigel 75 when it is at top level. It is best to put it at the start.
718     .
719     .
720     .\" HTML <a name="subpattern"></a>
721 nigel 63 .SH SUBPATTERNS
722     .rs
723     .sp
724     Subpatterns are delimited by parentheses (round brackets), which can be nested.
725 nigel 75 Turning part of a pattern into a subpattern does two things:
726     .sp
727 nigel 63 1. It localizes a set of alternatives. For example, the pattern
728 nigel 75 .sp
729 nigel 63 cat(aract|erpillar|)
730 nigel 75 .sp
731 nigel 63 matches one of the words "cat", "cataract", or "caterpillar". Without the
732     parentheses, it would match "cataract", "erpillar" or the empty string.
733 nigel 75 .sp
734     2. It sets up the subpattern as a capturing subpattern. This means that, when
735     the whole pattern matches, that portion of the subject string that matched the
736     subpattern is passed back to the caller via the \fIovector\fP argument of
737     \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
738     from 1) to obtain numbers for the capturing subpatterns.
739     .P
740 nigel 63 For example, if the string "the red king" is matched against the pattern
741 nigel 75 .sp
742 nigel 63 the ((red|white) (king|queen))
743 nigel 75 .sp
744 nigel 63 the captured substrings are "red king", "red", and "king", and are numbered 1,
745     2, and 3, respectively.
746 nigel 75 .P
747 nigel 63 The fact that plain parentheses fulfil two functions is not always helpful.
748     There are often times when a grouping subpattern is required without a
749     capturing requirement. If an opening parenthesis is followed by a question mark
750     and a colon, the subpattern does not do any capturing, and is not counted when
751     computing the number of any subsequent capturing subpatterns. For example, if
752     the string "the white queen" is matched against the pattern
753 nigel 75 .sp
754 nigel 63 the ((?:red|white) (king|queen))
755 nigel 75 .sp
756 nigel 63 the captured substrings are "white queen" and "queen", and are numbered 1 and
757     2. The maximum number of capturing subpatterns is 65535, and the maximum depth
758     of nesting of all subpatterns, both capturing and non-capturing, is 200.
759 nigel 75 .P
760 nigel 63 As a convenient shorthand, if any option settings are required at the start of
761     a non-capturing subpattern, the option letters may appear between the "?" and
762     the ":". Thus the two patterns
763 nigel 75 .sp
764 nigel 63 (?i:saturday|sunday)
765     (?:(?i)saturday|sunday)
766 nigel 75 .sp
767 nigel 63 match exactly the same set of strings. Because alternative branches are tried
768     from left to right, and options are not reset until the end of the subpattern
769     is reached, an option setting in one branch does affect subsequent branches, so
770     the above patterns match "SUNDAY" as well as "Saturday".
771 nigel 75 .
772     .
774 nigel 63 .rs
775     .sp
776     Identifying capturing parentheses by number is simple, but it can be very hard
777     to keep track of the numbers in complicated regular expressions. Furthermore,
778 nigel 75 if an expression is modified, the numbers may change. To help with this
779 nigel 63 difficulty, PCRE supports the naming of subpatterns, something that Perl does
780     not provide. The Python syntax (?P<name>...) is used. Names consist of
781     alphanumeric characters and underscores, and must be unique within a pattern.
782 nigel 75 .P
783 nigel 63 Named capturing parentheses are still allocated numbers as well as names. The
784     PCRE API provides function calls for extracting the name-to-number translation
785 nigel 75 table from a compiled pattern. There is also a convenience function for
786     extracting a captured substring by name. For further details see the
787 nigel 63 .\" HREF
788 nigel 75 \fBpcreapi\fP
789 nigel 63 .\"
790     documentation.
791 nigel 75 .
792     .
793 nigel 63 .SH REPETITION
794     .rs
795     .sp
796     Repetition is specified by quantifiers, which can follow any of the following
797     items:
798 nigel 75 .sp
799 nigel 63 a literal data character
800     the . metacharacter
801 nigel 75 the \eC escape sequence
802     the \eX escape sequence (in UTF-8 mode with Unicode properties)
803     an escape such as \ed that matches a single character
804 nigel 63 a character class
805     a back reference (see next section)
806     a parenthesized subpattern (unless it is an assertion)
807 nigel 75 .sp
808 nigel 63 The general repetition quantifier specifies a minimum and maximum number of
809     permitted matches, by giving the two numbers in curly brackets (braces),
810     separated by a comma. The numbers must be less than 65536, and the first must
811     be less than or equal to the second. For example:
812 nigel 75 .sp
813 nigel 63 z{2,4}
814 nigel 75 .sp
815 nigel 63 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
816     character. If the second number is omitted, but the comma is present, there is
817     no upper limit; if the second number and the comma are both omitted, the
818     quantifier specifies an exact number of required matches. Thus
819 nigel 75 .sp
820 nigel 63 [aeiou]{3,}
821 nigel 75 .sp
822 nigel 63 matches at least 3 successive vowels, but may match many more, while
823 nigel 75 .sp
824     \ed{8}
825     .sp
826 nigel 63 matches exactly 8 digits. An opening curly bracket that appears in a position
827     where a quantifier is not allowed, or one that does not match the syntax of a
828     quantifier, is taken as a literal character. For example, {,6} is not a
829     quantifier, but a literal string of four characters.
830 nigel 75 .P
831 nigel 63 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
832 nigel 75 bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
833     which is represented by a two-byte sequence. Similarly, when Unicode property
834     support is available, \eX{3} matches three Unicode extended sequences, each of
835     which may be several bytes long (and they may be of different lengths).
836     .P
837 nigel 63 The quantifier {0} is permitted, causing the expression to behave as if the
838     previous item and the quantifier were not present.
839 nigel 75 .P
840 nigel 63 For convenience (and historical compatibility) the three most common
841     quantifiers have single-character abbreviations:
842 nigel 75 .sp
843 nigel 63 * is equivalent to {0,}
844     + is equivalent to {1,}
845     ? is equivalent to {0,1}
846 nigel 75 .sp
847 nigel 63 It is possible to construct infinite loops by following a subpattern that can
848     match no characters with a quantifier that has no upper limit, for example:
849 nigel 75 .sp
850 nigel 63 (a?)*
851 nigel 75 .sp
852 nigel 63 Earlier versions of Perl and PCRE used to give an error at compile time for
853     such patterns. However, because there are cases where this can be useful, such
854     patterns are now accepted, but if any repetition of the subpattern does in fact
855     match no characters, the loop is forcibly broken.
856 nigel 75 .P
857 nigel 63 By default, the quantifiers are "greedy", that is, they match as much as
858     possible (up to the maximum number of permitted times), without causing the
859     rest of the pattern to fail. The classic example of where this gives problems
860 nigel 75 is in trying to match comments in C programs. These appear between /* and */
861     and within the comment, individual * and / characters may appear. An attempt to
862     match C comments by applying the pattern
863     .sp
864     /\e*.*\e*/
865     .sp
866 nigel 63 to the string
867 nigel 75 .sp
868     /* first comment */ not comment /* second comment */
869     .sp
870 nigel 63 fails, because it matches the entire string owing to the greediness of the .*
871     item.
872 nigel 75 .P
873 nigel 63 However, if a quantifier is followed by a question mark, it ceases to be
874     greedy, and instead matches the minimum number of times possible, so the
875     pattern
876 nigel 75 .sp
877     /\e*.*?\e*/
878     .sp
879 nigel 63 does the right thing with the C comments. The meaning of the various
880     quantifiers is not otherwise changed, just the preferred number of matches.
881     Do not confuse this use of question mark with its use as a quantifier in its
882     own right. Because it has two uses, it can sometimes appear doubled, as in
883 nigel 75 .sp
884     \ed??\ed
885     .sp
886 nigel 63 which matches one digit by preference, but can match two if that is the only
887     way the rest of the pattern matches.
888 nigel 75 .P
889 nigel 63 If the PCRE_UNGREEDY option is set (an option which is not available in Perl),
890     the quantifiers are not greedy by default, but individual ones can be made
891     greedy by following them with a question mark. In other words, it inverts the
892     default behaviour.
893 nigel 75 .P
894 nigel 63 When a parenthesized subpattern is quantified with a minimum repeat count that
895 nigel 75 is greater than 1 or with a limited maximum, more memory is required for the
896 nigel 63 compiled pattern, in proportion to the size of the minimum or maximum.
897 nigel 75 .P
898 nigel 63 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
899     to Perl's /s) is set, thus allowing the . to match newlines, the pattern is
900     implicitly anchored, because whatever follows will be tried against every
901     character position in the subject string, so there is no point in retrying the
902     overall match at any position after the first. PCRE normally treats such a
903 nigel 75 pattern as though it were preceded by \eA.
904     .P
905 nigel 63 In cases where it is known that the subject string contains no newlines, it is
906     worth setting PCRE_DOTALL in order to obtain this optimization, or
907     alternatively using ^ to indicate anchoring explicitly.
908 nigel 75 .P
909 nigel 63 However, there is one situation where the optimization cannot be used. When .*
910     is inside capturing parentheses that are the subject of a backreference
911     elsewhere in the pattern, a match at the start may fail, and a later one
912     succeed. Consider, for example:
913 nigel 75 .sp
914     (.*)abc\e1
915     .sp
916 nigel 63 If the subject is "xyz123abc123" the match point is the fourth character. For
917     this reason, such a pattern is not implicitly anchored.
918 nigel 75 .P
919 nigel 63 When a capturing subpattern is repeated, the value captured is the substring
920     that matched the final iteration. For example, after
921 nigel 75 .sp
922     (tweedle[dume]{3}\es*)+
923     .sp
924 nigel 63 has matched "tweedledum tweedledee" the value of the captured substring is
925     "tweedledee". However, if there are nested capturing subpatterns, the
926     corresponding captured values may have been set in previous iterations. For
927     example, after
928 nigel 75 .sp
929 nigel 63 /(a|(b))+/
930 nigel 75 .sp
931 nigel 63 matches "aba" the value of the second captured substring is "b".
932 nigel 75 .
933     .
934     .\" HTML <a name="atomicgroup"></a>
936 nigel 63 .rs
937     .sp
938     With both maximizing and minimizing repetition, failure of what follows
939     normally causes the repeated item to be re-evaluated to see if a different
940     number of repeats allows the rest of the pattern to match. Sometimes it is
941     useful to prevent this, either to change the nature of the match, or to cause
942     it fail earlier than it otherwise might, when the author of the pattern knows
943     there is no point in carrying on.
944 nigel 75 .P
945     Consider, for example, the pattern \ed+foo when applied to the subject line
946     .sp
947 nigel 63 123456bar
948 nigel 75 .sp
949 nigel 63 After matching all 6 digits and then failing to match "foo", the normal
950 nigel 75 action of the matcher is to try again with only 5 digits matching the \ed+
951 nigel 63 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
952     (a term taken from Jeffrey Friedl's book) provides the means for specifying
953     that once a subpattern has matched, it is not to be re-evaluated in this way.
954 nigel 75 .P
955 nigel 63 If we use atomic grouping for the previous example, the matcher would give up
956     immediately on failing to match "foo" the first time. The notation is a kind of
957     special parenthesis, starting with (?> as in this example:
958 nigel 75 .sp
959     (?>\ed+)foo
960     .sp
961 nigel 63 This kind of parenthesis "locks up" the part of the pattern it contains once
962     it has matched, and a failure further into the pattern is prevented from
963     backtracking into it. Backtracking past it to previous items, however, works as
964     normal.
965 nigel 75 .P
966 nigel 63 An alternative description is that a subpattern of this type matches the string
967     of characters that an identical standalone pattern would match, if anchored at
968     the current point in the subject string.
969 nigel 75 .P
970 nigel 63 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
971     the above example can be thought of as a maximizing repeat that must swallow
972 nigel 75 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
973 nigel 63 number of digits they match in order to make the rest of the pattern match,
974 nigel 75 (?>\ed+) can only match an entire sequence of digits.
975     .P
976 nigel 63 Atomic groups in general can of course contain arbitrarily complicated
977     subpatterns, and can be nested. However, when the subpattern for an atomic
978     group is just a single repeated item, as in the example above, a simpler
979     notation, called a "possessive quantifier" can be used. This consists of an
980     additional + character following a quantifier. Using this notation, the
981     previous example can be rewritten as
982 nigel 75 .sp
983     \ed++foo
984     .sp
985 nigel 63 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
986     option is ignored. They are a convenient notation for the simpler forms of
987     atomic group. However, there is no difference in the meaning or processing of a
988     possessive quantifier and the equivalent atomic group.
989 nigel 75 .P
990 nigel 63 The possessive quantifier syntax is an extension to the Perl syntax. It
991     originates in Sun's Java package.
992 nigel 75 .P
993 nigel 63 When a pattern contains an unlimited repeat inside a subpattern that can itself
994     be repeated an unlimited number of times, the use of an atomic group is the
995     only way to avoid some failing matches taking a very long time indeed. The
996     pattern
997 nigel 75 .sp
998     (\eD+|<\ed+>)*[!?]
999     .sp
1000 nigel 63 matches an unlimited number of substrings that either consist of non-digits, or
1001     digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1002     quickly. However, if it is applied to
1003 nigel 75 .sp
1004 nigel 63 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1005 nigel 75 .sp
1006 nigel 63 it takes a long time before reporting failure. This is because the string can
1007 nigel 75 be divided between the internal \eD+ repeat and the external * repeat in a
1008     large number of ways, and all have to be tried. (The example uses [!?] rather
1009     than a single character at the end, because both PCRE and Perl have an
1010     optimization that allows for fast failure when a single character is used. They
1011     remember the last single character that is required for a match, and fail early
1012     if it is not present in the string.) If the pattern is changed so that it uses
1013     an atomic group, like this:
1014     .sp
1015     ((?>\eD+)|<\ed+>)*[!?]
1016     .sp
1017 nigel 63 sequences of non-digits cannot be broken, and failure happens quickly.
1018 nigel 75 .
1019     .
1020     .\" HTML <a name="backreferences"></a>
1022 nigel 63 .rs
1023     .sp
1024     Outside a character class, a backslash followed by a digit greater than 0 (and
1025     possibly further digits) is a back reference to a capturing subpattern earlier
1026     (that is, to its left) in the pattern, provided there have been that many
1027     previous capturing left parentheses.
1028 nigel 75 .P
1029 nigel 63 However, if the decimal number following the backslash is less than 10, it is
1030     always taken as a back reference, and causes an error only if there are not
1031     that many capturing left parentheses in the entire pattern. In other words, the
1032     parentheses that are referenced need not be to the left of the reference for
1033 nigel 75 numbers less than 10. See the subsection entitled "Non-printing characters"
1034     .\" HTML <a href="#digitsafterbackslash">
1035     .\" </a>
1036     above
1037     .\"
1038     for further details of the handling of digits following a backslash.
1039     .P
1040 nigel 63 A back reference matches whatever actually matched the capturing subpattern in
1041     the current subject string, rather than anything matching the subpattern
1042     itself (see
1043     .\" HTML <a href="#subpatternsassubroutines">
1044     .\" </a>
1045     "Subpatterns as subroutines"
1046     .\"
1047     below for a way of doing that). So the pattern
1048 nigel 75 .sp
1049     (sens|respons)e and \e1ibility
1050     .sp
1051 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
1052     "sense and responsibility". If caseful matching is in force at the time of the
1053     back reference, the case of letters is relevant. For example,
1054 nigel 75 .sp
1055     ((?i)rah)\es+\e1
1056     .sp
1057 nigel 63 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1058     capturing subpattern is matched caselessly.
1059 nigel 75 .P
1060 nigel 63 Back references to named subpatterns use the Python syntax (?P=name). We could
1061     rewrite the above example as follows:
1062 nigel 75 .sp
1063     (?<p1>(?i)rah)\es+(?P=p1)
1064     .sp
1065 nigel 63 There may be more than one back reference to the same subpattern. If a
1066     subpattern has not actually been used in a particular match, any back
1067     references to it always fail. For example, the pattern
1068 nigel 75 .sp
1069     (a|(bc))\e2
1070     .sp
1071 nigel 63 always fails if it starts to match "a" rather than "bc". Because there may be
1072     many capturing parentheses in a pattern, all digits following the backslash are
1073     taken as part of a potential back reference number. If the pattern continues
1074     with a digit character, some delimiter must be used to terminate the back
1075     reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1076 nigel 75 Otherwise an empty comment (see
1077     .\" HTML <a href="#comments">
1078     .\" </a>
1079     "Comments"
1080     .\"
1081     below) can be used.
1082     .P
1083 nigel 63 A back reference that occurs inside the parentheses to which it refers fails
1084 nigel 75 when the subpattern is first used, so, for example, (a\e1) never matches.
1085 nigel 63 However, such references can be useful inside repeated subpatterns. For
1086     example, the pattern
1087 nigel 75 .sp
1088     (a|b\e1)+
1089     .sp
1090 nigel 63 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1091     the subpattern, the back reference matches the character string corresponding
1092     to the previous iteration. In order for this to work, the pattern must be such
1093     that the first iteration does not need to match the back reference. This can be
1094     done using alternation, as in the example above, or by a quantifier with a
1095     minimum of zero.
1096 nigel 75 .
1097     .
1098     .\" HTML <a name="bigassertions"></a>
1099 nigel 63 .SH ASSERTIONS
1100     .rs
1101     .sp
1102     An assertion is a test on the characters following or preceding the current
1103     matching point that does not actually consume any characters. The simple
1104 nigel 75 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1105     .\" HTML <a href="#smallassertions">
1106     .\" </a>
1107     above.
1108     .\"
1109     .P
1110 nigel 63 More complicated assertions are coded as subpatterns. There are two kinds:
1111     those that look ahead of the current position in the subject string, and those
1112 nigel 75 that look behind it. An assertion subpattern is matched in the normal way,
1113     except that it does not cause the current matching position to be changed.
1114     .P
1115     Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1116     because it makes no sense to assert the same thing several times. If any kind
1117     of assertion contains capturing subpatterns within it, these are counted for
1118     the purposes of numbering the capturing subpatterns in the whole pattern.
1119     However, substring capturing is carried out only for positive assertions,
1120     because it does not make sense for negative assertions.
1121     .
1122     .
1123     .SS "Lookahead assertions"
1124     .rs
1125     .sp
1126     Lookahead assertions start
1127 nigel 63 with (?= for positive assertions and (?! for negative assertions. For example,
1128 nigel 75 .sp
1129     \ew+(?=;)
1130     .sp
1131 nigel 63 matches a word followed by a semicolon, but does not include the semicolon in
1132     the match, and
1133 nigel 75 .sp
1134 nigel 63 foo(?!bar)
1135 nigel 75 .sp
1136 nigel 63 matches any occurrence of "foo" that is not followed by "bar". Note that the
1137     apparently similar pattern
1138 nigel 75 .sp
1139 nigel 63 (?!foo)bar
1140 nigel 75 .sp
1141 nigel 63 does not find an occurrence of "bar" that is preceded by something other than
1142     "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1143     (?!foo) is always true when the next three characters are "bar". A
1144 nigel 75 lookbehind assertion is needed to achieve the other effect.
1145     .P
1146 nigel 63 If you want to force a matching failure at some point in a pattern, the most
1147     convenient way to do it is with (?!) because an empty string always matches, so
1148     an assertion that requires there not to be an empty string must always fail.
1149 nigel 75 .
1150     .
1151     .\" HTML <a name="lookbehind"></a>
1152     .SS "Lookbehind assertions"
1153     .rs
1154     .sp
1155 nigel 63 Lookbehind assertions start with (?<= for positive assertions and (?<! for
1156     negative assertions. For example,
1157 nigel 75 .sp
1158 nigel 63 (?<!foo)bar
1159 nigel 75 .sp
1160 nigel 63 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1161     a lookbehind assertion are restricted such that all the strings it matches must
1162     have a fixed length. However, if there are several alternatives, they do not
1163     all have to have the same fixed length. Thus
1164 nigel 75 .sp
1165 nigel 63 (?<=bullock|donkey)
1166 nigel 75 .sp
1167 nigel 63 is permitted, but
1168 nigel 75 .sp
1169 nigel 63 (?<!dogs?|cats?)
1170 nigel 75 .sp
1171 nigel 63 causes an error at compile time. Branches that match different length strings
1172     are permitted only at the top level of a lookbehind assertion. This is an
1173     extension compared with Perl (at least for 5.8), which requires all branches to
1174     match the same length of string. An assertion such as
1175 nigel 75 .sp
1176 nigel 63 (?<=ab(c|de))
1177 nigel 75 .sp
1178 nigel 63 is not permitted, because its single top-level branch can match two different
1179     lengths, but it is acceptable if rewritten to use two top-level branches:
1180 nigel 75 .sp
1181 nigel 63 (?<=abc|abde)
1182 nigel 75 .sp
1183 nigel 63 The implementation of lookbehind assertions is, for each alternative, to
1184     temporarily move the current position back by the fixed width and then try to
1185     match. If there are insufficient characters before the current position, the
1186     match is deemed to fail.
1187 nigel 75 .P
1188     PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1189 nigel 63 to appear in lookbehind assertions, because it makes it impossible to calculate
1190 nigel 75 the length of the lookbehind. The \eX escape, which can match different numbers
1191     of bytes, is also not permitted.
1192     .P
1193 nigel 63 Atomic groups can be used in conjunction with lookbehind assertions to specify
1194     efficient matching at the end of the subject string. Consider a simple pattern
1195     such as
1196 nigel 75 .sp
1197 nigel 63 abcd$
1198 nigel 75 .sp
1199 nigel 63 when applied to a long string that does not match. Because matching proceeds
1200     from left to right, PCRE will look for each "a" in the subject and then see if
1201     what follows matches the rest of the pattern. If the pattern is specified as
1202 nigel 75 .sp
1203 nigel 63 ^.*abcd$
1204 nigel 75 .sp
1205 nigel 63 the initial .* matches the entire string at first, but when this fails (because
1206     there is no following "a"), it backtracks to match all but the last character,
1207     then all but the last two characters, and so on. Once again the search for "a"
1208     covers the entire string, from right to left, so we are no better off. However,
1209     if the pattern is written as
1210 nigel 75 .sp
1211 nigel 63 ^(?>.*)(?<=abcd)
1212 nigel 75 .sp
1213     or, equivalently, using the possessive quantifier syntax,
1214     .sp
1215 nigel 63 ^.*+(?<=abcd)
1216 nigel 75 .sp
1217 nigel 63 there can be no backtracking for the .* item; it can match only the entire
1218     string. The subsequent lookbehind assertion does a single test on the last four
1219     characters. If it fails, the match fails immediately. For long strings, this
1220     approach makes a significant difference to the processing time.
1221 nigel 75 .
1222     .
1223     .SS "Using multiple assertions"
1224     .rs
1225     .sp
1226 nigel 63 Several assertions (of any sort) may occur in succession. For example,
1227 nigel 75 .sp
1228     (?<=\ed{3})(?<!999)foo
1229     .sp
1230 nigel 63 matches "foo" preceded by three digits that are not "999". Notice that each of
1231     the assertions is applied independently at the same point in the subject
1232     string. First there is a check that the previous three characters are all
1233     digits, and then there is a check that the same three characters are not "999".
1234 nigel 75 This pattern does \fInot\fP match "foo" preceded by six characters, the first
1235 nigel 63 of which are digits and the last three of which are not "999". For example, it
1236     doesn't match "123abcfoo". A pattern to do that is
1237 nigel 75 .sp
1238     (?<=\ed{3}...)(?<!999)foo
1239     .sp
1240 nigel 63 This time the first assertion looks at the preceding six characters, checking
1241     that the first three are digits, and then the second assertion checks that the
1242     preceding three characters are not "999".
1243 nigel 75 .P
1244 nigel 63 Assertions can be nested in any combination. For example,
1245 nigel 75 .sp
1246 nigel 63 (?<=(?<!foo)bar)baz
1247 nigel 75 .sp
1248 nigel 63 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1249     preceded by "foo", while
1250 nigel 75 .sp
1251     (?<=\ed{3}(?!999)...)foo
1252     .sp
1253     is another pattern that matches "foo" preceded by three digits and any three
1254 nigel 63 characters that are not "999".
1255 nigel 75 .
1256     .
1258 nigel 63 .rs
1259     .sp
1260     It is possible to cause the matching process to obey a subpattern
1261     conditionally or to choose between two alternative subpatterns, depending on
1262     the result of an assertion, or whether a previous capturing subpattern matched
1263     or not. The two possible forms of conditional subpattern are
1264 nigel 75 .sp
1265 nigel 63 (?(condition)yes-pattern)
1266     (?(condition)yes-pattern|no-pattern)
1267 nigel 75 .sp
1268 nigel 63 If the condition is satisfied, the yes-pattern is used; otherwise the
1269     no-pattern (if present) is used. If there are more than two alternatives in the
1270     subpattern, a compile-time error occurs.
1271 nigel 75 .P
1272 nigel 63 There are three kinds of condition. If the text between the parentheses
1273     consists of a sequence of digits, the condition is satisfied if the capturing
1274     subpattern of that number has previously matched. The number must be greater
1275     than zero. Consider the following pattern, which contains non-significant white
1276     space to make it more readable (assume the PCRE_EXTENDED option) and to divide
1277     it into three parts for ease of discussion:
1278 nigel 75 .sp
1279     ( \e( )? [^()]+ (?(1) \e) )
1280     .sp
1281 nigel 63 The first part matches an optional opening parenthesis, and if that
1282     character is present, sets it as the first captured substring. The second part
1283     matches one or more characters that are not parentheses. The third part is a
1284     conditional subpattern that tests whether the first set of parentheses matched
1285     or not. If they did, that is, if subject started with an opening parenthesis,
1286     the condition is true, and so the yes-pattern is executed and a closing
1287     parenthesis is required. Otherwise, since no-pattern is not present, the
1288     subpattern matches nothing. In other words, this pattern matches a sequence of
1289     non-parentheses, optionally enclosed in parentheses.
1290 nigel 75 .P
1291 nigel 63 If the condition is the string (R), it is satisfied if a recursive call to the
1292     pattern or subpattern has been made. At "top level", the condition is false.
1293     This is a PCRE extension. Recursive patterns are described in the next section.
1294 nigel 75 .P
1295 nigel 63 If the condition is not a sequence of digits or (R), it must be an assertion.
1296     This may be a positive or negative lookahead or lookbehind assertion. Consider
1297     this pattern, again containing non-significant white space, and with the two
1298     alternatives on the second line:
1299 nigel 75 .sp
1300 nigel 63 (?(?=[^a-z]*[a-z])
1301 nigel 75 \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
1302     .sp
1303 nigel 63 The condition is a positive lookahead assertion that matches an optional
1304     sequence of non-letters followed by a letter. In other words, it tests for the
1305     presence of at least one letter in the subject. If a letter is found, the
1306     subject is matched against the first alternative; otherwise it is matched
1307     against the second. This pattern matches strings in one of the two forms
1308     dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1309 nigel 75 .
1310     .
1311     .\" HTML <a name="comments"></a>
1312 nigel 63 .SH COMMENTS
1313     .rs
1314     .sp
1315 nigel 75 The sequence (?# marks the start of a comment that continues up to the next
1316 nigel 63 closing parenthesis. Nested parentheses are not permitted. The characters
1317     that make up a comment play no part in the pattern matching at all.
1318 nigel 75 .P
1319 nigel 63 If the PCRE_EXTENDED option is set, an unescaped # character outside a
1320     character class introduces a comment that continues up to the next newline
1321     character in the pattern.
1322 nigel 75 .
1323     .
1325 nigel 63 .rs
1326     .sp
1327     Consider the problem of matching a string in parentheses, allowing for
1328     unlimited nested parentheses. Without the use of recursion, the best that can
1329     be done is to use a pattern that matches up to some fixed depth of nesting. It
1330 nigel 75 is not possible to handle an arbitrary nesting depth. Perl provides a facility
1331     that allows regular expressions to recurse (amongst other things). It does this
1332     by interpolating Perl code in the expression at run time, and the code can
1333     refer to the expression itself. A Perl pattern to solve the parentheses problem
1334     can be created like this:
1335     .sp
1336     $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1337     .sp
1338 nigel 63 The (?p{...}) item interpolates Perl code at run time, and in this case refers
1339     recursively to the pattern in which it appears. Obviously, PCRE cannot support
1340     the interpolation of Perl code. Instead, it supports some special syntax for
1341     recursion of the entire pattern, and also for individual subpattern recursion.
1342 nigel 75 .P
1343 nigel 63 The special item that consists of (? followed by a number greater than zero and
1344     a closing parenthesis is a recursive call of the subpattern of the given
1345     number, provided that it occurs inside that subpattern. (If not, it is a
1346     "subroutine" call, which is described in the next section.) The special item
1347     (?R) is a recursive call of the entire regular expression.
1348 nigel 75 .P
1349 nigel 63 For example, this PCRE pattern solves the nested parentheses problem (assume
1350     the PCRE_EXTENDED option is set so that white space is ignored):
1351 nigel 75 .sp
1352     \e( ( (?>[^()]+) | (?R) )* \e)
1353     .sp
1354 nigel 63 First it matches an opening parenthesis. Then it matches any number of
1355     substrings which can either be a sequence of non-parentheses, or a recursive
1356     match of the pattern itself (that is a correctly parenthesized substring).
1357     Finally there is a closing parenthesis.
1358 nigel 75 .P
1359 nigel 63 If this were part of a larger pattern, you would not want to recurse the entire
1360     pattern, so instead you could use this:
1361 nigel 75 .sp
1362     ( \e( ( (?>[^()]+) | (?1) )* \e) )
1363     .sp
1364 nigel 63 We have put the pattern into parentheses, and caused the recursion to refer to
1365     them instead of the whole pattern. In a larger pattern, keeping track of
1366     parenthesis numbers can be tricky. It may be more convenient to use named
1367     parentheses instead. For this, PCRE uses (?P>name), which is an extension to
1368     the Python syntax that PCRE uses for named parentheses (Perl does not provide
1369     named parentheses). We could rewrite the above example as follows:
1370 nigel 75 .sp
1371     (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )
1372     .sp
1373 nigel 63 This particular example pattern contains nested unlimited repeats, and so the
1374     use of atomic grouping for matching strings of non-parentheses is important
1375     when applying the pattern to strings that do not match. For example, when this
1376     pattern is applied to
1377 nigel 75 .sp
1378 nigel 63 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1379 nigel 75 .sp
1380 nigel 63 it yields "no match" quickly. However, if atomic grouping is not used,
1381     the match runs for a very long time indeed because there are so many different
1382     ways the + and * repeats can carve up the subject, and all have to be tested
1383     before failure can be reported.
1384 nigel 75 .P
1385 nigel 63 At the end of a match, the values set for any capturing subpatterns are those
1386     from the outermost level of the recursion at which the subpattern value is set.
1387     If you want to obtain intermediate values, a callout function can be used (see
1388 nigel 75 the next section and the
1389 nigel 63 .\" HREF
1390 nigel 75 \fBpcrecallout\fP
1391 nigel 63 .\"
1392     documentation). If the pattern above is matched against
1393 nigel 75 .sp
1394 nigel 63 (ab(cd)ef)
1395 nigel 75 .sp
1396 nigel 63 the value for the capturing parentheses is "ef", which is the last value taken
1397     on at the top level. If additional parentheses are added, giving
1398 nigel 75 .sp
1399     \e( ( ( (?>[^()]+) | (?R) )* ) \e)
1400 nigel 63 ^ ^
1401     ^ ^
1402 nigel 75 .sp
1403 nigel 63 the string they capture is "ab(cd)ef", the contents of the top level
1404     parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
1405     has to obtain extra memory to store data during a recursion, which it does by
1406 nigel 75 using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no
1407 nigel 63 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
1408 nigel 75 .P
1409 nigel 63 Do not confuse the (?R) item with the condition (R), which tests for recursion.
1410     Consider this pattern, which matches text in angle brackets, allowing for
1411     arbitrary nesting. Only digits are allowed in nested brackets (that is, when
1412     recursing), whereas any characters are permitted at the outer level.
1413 nigel 75 .sp
1414     < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
1415     .sp
1416 nigel 63 In this pattern, (?(R) is the start of a conditional subpattern, with two
1417     different alternatives for the recursive and non-recursive cases. The (?R) item
1418     is the actual recursive call.
1419 nigel 75 .
1420     .
1421 nigel 63 .\" HTML <a name="subpatternsassubroutines"></a>
1423 nigel 63 .rs
1424     .sp
1425     If the syntax for a recursive subpattern reference (either by number or by
1426     name) is used outside the parentheses to which it refers, it operates like a
1427     subroutine in a programming language. An earlier example pointed out that the
1428     pattern
1429 nigel 75 .sp
1430     (sens|respons)e and \e1ibility
1431     .sp
1432 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
1433     "sense and responsibility". If instead the pattern
1434 nigel 75 .sp
1435 nigel 63 (sens|respons)e and (?1)ibility
1436 nigel 75 .sp
1437 nigel 63 is used, it does match "sense and responsibility" as well as the other two
1438     strings. Such references must, however, follow the subpattern to which they
1439     refer.
1440 nigel 75 .
1441     .
1442 nigel 63 .SH CALLOUTS
1443     .rs
1444     .sp
1445     Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
1446     code to be obeyed in the middle of matching a regular expression. This makes it
1447     possible, amongst other things, to extract different substrings that match the
1448     same pair of parentheses when there is a repetition.
1449 nigel 75 .P
1450 nigel 63 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
1451     code. The feature is called "callout". The caller of PCRE provides an external
1452 nigel 75 function by putting its entry point in the global variable \fIpcre_callout\fP.
1453 nigel 63 By default, this variable contains NULL, which disables all calling out.
1454 nigel 75 .P
1455 nigel 63 Within a regular expression, (?C) indicates the points at which the external
1456     function is to be called. If you want to identify different callout points, you
1457     can put a number less than 256 after the letter C. The default value is zero.
1458     For example, this pattern has two callout points:
1459 nigel 75 .sp
1460 nigel 63 (?C1)\dabc(?C2)def
1461 nigel 75 .sp
1462     If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
1463     automatically installed before each item in the pattern. They are all numbered
1464     255.
1465     .P
1466     During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
1467 nigel 63 set), the external function is called. It is provided with the number of the
1468 nigel 75 callout, the position in the pattern, and, optionally, one item of data
1469     originally supplied by the caller of \fBpcre_exec()\fP. The callout function
1470     may cause matching to proceed, to backtrack, or to fail altogether. A complete
1471     description of the interface to the callout function is given in the
1472 nigel 63 .\" HREF
1473 nigel 75 \fBpcrecallout\fP
1474 nigel 63 .\"
1475     documentation.
1476 nigel 75 .P
1477 nigel 63 .in 0
1478 nigel 77 Last updated: 28 February 2005
1479 nigel 63 .br
1480 nigel 77 Copyright (c) 1997-2005 University of Cambridge.

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