/[pcre]/code/trunk/doc/pcrepattern.3
ViewVC logotype

Contents of /code/trunk/doc/pcrepattern.3

Parent Directory Parent Directory | Revision Log Revision Log


Revision 1459 - (hide annotations) (download)
Tue Mar 4 10:45:15 2014 UTC (7 weeks, 2 days ago) by ph10
File size: 135252 byte(s)
Preparations for next release.

1 ph10 1436 .TH PCREPATTERN 3 "08 January 2014" "PCRE 8.35"
2 nigel 63 .SH NAME
3     PCRE - Perl-compatible regular expressions
4 nigel 75 .SH "PCRE REGULAR EXPRESSION DETAILS"
5 nigel 63 .rs
6     .sp
7 ph10 208 The syntax and semantics of the regular expressions that are supported by PCRE
8     are described in detail below. There is a quick-reference syntax summary in the
9     .\" HREF
10     \fBpcresyntax\fP
11     .\"
12 ph10 333 page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13     also supports some alternative regular expression syntax (which does not
14     conflict with the Perl syntax) in order to provide some compatibility with
15     regular expressions in Python, .NET, and Oniguruma.
16     .P
17     Perl's regular expressions are described in its own documentation, and
18 ph10 208 regular expressions in general are covered in a number of books, some of which
19     have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20     published by O'Reilly, covers regular expressions in great detail. This
21     description of PCRE's regular expressions is intended as reference material.
22 nigel 75 .P
23 ph10 1314 This document discusses the patterns that are supported by PCRE when one its
24     main matching functions, \fBpcre_exec()\fP (8-bit) or \fBpcre[16|32]_exec()\fP
25     (16- or 32-bit), is used. PCRE also has alternative matching functions,
26     \fBpcre_dfa_exec()\fP and \fBpcre[16|32_dfa_exec()\fP, which match using a
27     different algorithm that is not Perl-compatible. Some of the features discussed
28     below are not available when DFA matching is used. The advantages and
29     disadvantages of the alternative functions, and how they differ from the normal
30     functions, are discussed in the
31     .\" HREF
32     \fBpcrematching\fP
33     .\"
34     page.
35     .
36     .
37     .SH "SPECIAL START-OF-PATTERN ITEMS"
38     .rs
39     .sp
40 ph10 1335 A number of options that can be passed to \fBpcre_compile()\fP can also be set
41 ph10 1314 by special items at the start of a pattern. These are not Perl-compatible, but
42     are provided to make these options accessible to pattern writers who are not
43     able to change the program that processes the pattern. Any number of these
44     items may appear, but they must all be together right at the start of the
45     pattern string, and the letters must be in upper case.
46     .
47     .
48     .SS "UTF support"
49     .rs
50     .sp
51 nigel 75 The original operation of PCRE was on strings of one-byte characters. However,
52 chpe 1055 there is now also support for UTF-8 strings in the original library, an
53 ph10 1219 extra library that supports 16-bit and UTF-16 character strings, and a
54     third library that supports 32-bit and UTF-32 character strings. To use these
55 ph10 859 features, PCRE must be built to include appropriate support. When using UTF
56 chpe 1055 strings you must either call the compiling function with the PCRE_UTF8,
57 ph10 1219 PCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of
58 chpe 1055 these special sequences:
59 ph10 412 .sp
60     (*UTF8)
61 ph10 903 (*UTF16)
62 chpe 1055 (*UTF32)
63 ph10 1221 (*UTF)
64 ph10 416 .sp
65 ph10 1219 (*UTF) is a generic sequence that can be used with any of the libraries.
66 ph10 859 Starting a pattern with such a sequence is equivalent to setting the relevant
67 ph10 1314 option. How setting a UTF mode affects pattern matching is mentioned in several
68     places below. There is also a summary of features in the
69 nigel 63 .\" HREF
70 ph10 678 \fBpcreunicode\fP
71 nigel 63 .\"
72     page.
73 nigel 75 .P
74 ph10 1335 Some applications that allow their users to supply patterns may wish to
75 ph10 1314 restrict them to non-UTF data for security reasons. If the PCRE_NEVER_UTF
76     option is set at compile time, (*UTF) etc. are not allowed, and their
77     appearance causes an error.
78     .
79     .
80     .SS "Unicode property support"
81     .rs
82 ph10 518 .sp
83 ph10 1395 Another special sequence that may appear at the start of a pattern is (*UCP).
84 ph10 535 This has the same effect as setting the PCRE_UCP option: it causes sequences
85     such as \ed and \ew to use Unicode properties to determine character types,
86     instead of recognizing only characters with codes less than 128 via a lookup
87 ph10 518 table.
88 nigel 93 .
89     .
90 ph10 1395 .SS "Disabling auto-possessification"
91     .rs
92     .sp
93 ph10 1404 If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as setting
94 ph10 1398 the PCRE_NO_AUTO_POSSESS option at compile time. This stops PCRE from making
95 ph10 1395 quantifiers possessive when what follows cannot match the repeated item. For
96     example, by default a+b is treated as a++b. For more details, see the
97     .\" HREF
98     \fBpcreapi\fP
99     .\"
100     documentation.
101     .
102     .
103 ph10 1314 .SS "Disabling start-up optimizations"
104 ph10 1033 .rs
105     .sp
106 ph10 1314 If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
107 ph10 1395 PCRE_NO_START_OPTIMIZE option either at compile or matching time. This disables
108     several optimizations for quickly reaching "no match" results. For more
109     details, see the
110     .\" HREF
111     \fBpcreapi\fP
112     .\"
113     documentation.
114 ph10 1033 .
115     .
116 ph10 556 .\" HTML <a name="newlines"></a>
117 ph10 1314 .SS "Newline conventions"
118 ph10 227 .rs
119     .sp
120     PCRE supports five different conventions for indicating line breaks in
121     strings: a single CR (carriage return) character, a single LF (linefeed)
122     character, the two-character sequence CRLF, any of the three preceding, or any
123     Unicode newline sequence. The
124     .\" HREF
125     \fBpcreapi\fP
126     .\"
127     page has
128     .\" HTML <a href="pcreapi.html#newlines">
129     .\" </a>
130     further discussion
131     .\"
132     about newlines, and shows how to set the newline convention in the
133     \fIoptions\fP arguments for the compiling and matching functions.
134     .P
135     It is also possible to specify a newline convention by starting a pattern
136     string with one of the following five sequences:
137     .sp
138     (*CR) carriage return
139     (*LF) linefeed
140     (*CRLF) carriage return, followed by linefeed
141     (*ANYCRLF) any of the three above
142     (*ANY) all Unicode newline sequences
143     .sp
144 ph10 859 These override the default and the options given to the compiling function. For
145     example, on a Unix system where LF is the default newline sequence, the pattern
146 ph10 227 .sp
147     (*CR)a.b
148     .sp
149     changes the convention to CR. That pattern matches "a\enb" because LF is no
150 ph10 1314 longer a newline. If more than one of these settings is present, the last one
151 ph10 231 is used.
152     .P
153 ph10 1213 The newline convention affects where the circumflex and dollar assertions are
154     true. It also affects the interpretation of the dot metacharacter when
155     PCRE_DOTALL is not set, and the behaviour of \eN. However, it does not affect
156     what the \eR escape sequence matches. By default, this is any Unicode newline
157     sequence, for Perl compatibility. However, this can be changed; see the
158 ph10 514 description of \eR in the section entitled
159 ph10 231 .\" HTML <a href="#newlineseq">
160     .\" </a>
161     "Newline sequences"
162     .\"
163 ph10 247 below. A change of \eR setting can be combined with a change of newline
164 ph10 246 convention.
165 ph10 227 .
166     .
167 ph10 1314 .SS "Setting match and recursion limits"
168     .rs
169     .sp
170 ph10 1335 The caller of \fBpcre_exec()\fP can set a limit on the number of times the
171     internal \fBmatch()\fP function is called and on the maximum depth of
172 ph10 1314 recursive calls. These facilities are provided to catch runaway matches that
173     are provoked by patterns with huge matching trees (a typical example is a
174     pattern with nested unlimited repeats) and to avoid running out of system stack
175     by too much recursion. When one of these limits is reached, \fBpcre_exec()\fP
176 ph10 1335 gives an error return. The limits can also be set by items at the start of the
177 ph10 1314 pattern of the form
178     .sp
179     (*LIMIT_MATCH=d)
180     (*LIMIT_RECURSION=d)
181     .sp
182 ph10 1335 where d is any number of decimal digits. However, the value of the setting must
183 ph10 1391 be less than the value set (or defaulted) by the caller of \fBpcre_exec()\fP
184     for it to have any effect. In other words, the pattern writer can lower the
185     limits set by the programmer, but not raise them. If there is more than one
186     setting of one of these limits, the lower value is used.
187 ph10 1314 .
188     .
189     .SH "EBCDIC CHARACTER CODES"
190     .rs
191     .sp
192     PCRE can be compiled to run in an environment that uses EBCDIC as its character
193     code rather than ASCII or Unicode (typically a mainframe system). In the
194     sections below, character code values are ASCII or Unicode; in an EBCDIC
195     environment these characters may have different code values, and there are no
196     code points greater than 255.
197     .
198     .
199 nigel 93 .SH "CHARACTERS AND METACHARACTERS"
200     .rs
201     .sp
202 nigel 63 A regular expression is a pattern that is matched against a subject string from
203     left to right. Most characters stand for themselves in a pattern, and match the
204     corresponding characters in the subject. As a trivial example, the pattern
205 nigel 75 .sp
206 nigel 63 The quick brown fox
207 nigel 75 .sp
208 nigel 77 matches a portion of a subject string that is identical to itself. When
209     caseless matching is specified (the PCRE_CASELESS option), letters are matched
210 ph10 859 independently of case. In a UTF mode, PCRE always understands the concept of
211 nigel 77 case for characters whose values are less than 128, so caseless matching is
212     always possible. For characters with higher values, the concept of case is
213     supported if PCRE is compiled with Unicode property support, but not otherwise.
214     If you want to use caseless matching for characters 128 and above, you must
215     ensure that PCRE is compiled with Unicode property support as well as with
216 ph10 859 UTF support.
217 nigel 77 .P
218     The power of regular expressions comes from the ability to include alternatives
219     and repetitions in the pattern. These are encoded in the pattern by the use of
220 nigel 75 \fImetacharacters\fP, which do not stand for themselves but instead are
221 nigel 63 interpreted in some special way.
222 nigel 75 .P
223     There are two different sets of metacharacters: those that are recognized
224 nigel 63 anywhere in the pattern except within square brackets, and those that are
225 nigel 93 recognized within square brackets. Outside square brackets, the metacharacters
226     are as follows:
227 nigel 75 .sp
228     \e general escape character with several uses
229 nigel 63 ^ assert start of string (or line, in multiline mode)
230     $ assert end of string (or line, in multiline mode)
231     . match any character except newline (by default)
232     [ start character class definition
233     | start of alternative branch
234     ( start subpattern
235     ) end subpattern
236     ? extends the meaning of (
237     also 0 or 1 quantifier
238     also quantifier minimizer
239     * 0 or more quantifier
240     + 1 or more quantifier
241     also "possessive quantifier"
242     { start min/max quantifier
243 nigel 75 .sp
244 nigel 63 Part of a pattern that is in square brackets is called a "character class". In
245 nigel 75 a character class the only metacharacters are:
246     .sp
247     \e general escape character
248 nigel 63 ^ negate the class, but only if the first character
249     - indicates character range
250 nigel 75 .\" JOIN
251 nigel 63 [ POSIX character class (only if followed by POSIX
252     syntax)
253     ] terminates the character class
254 nigel 75 .sp
255     The following sections describe the use of each of the metacharacters.
256     .
257 nigel 93 .
258 nigel 63 .SH BACKSLASH
259     .rs
260     .sp
261     The backslash character has several uses. Firstly, if it is followed by a
262 ph10 574 character that is not a number or a letter, it takes away any special meaning
263     that character may have. This use of backslash as an escape character applies
264 ph10 579 both inside and outside character classes.
265 nigel 75 .P
266     For example, if you want to match a * character, you write \e* in the pattern.
267 nigel 63 This escaping action applies whether or not the following character would
268 nigel 75 otherwise be interpreted as a metacharacter, so it is always safe to precede a
269     non-alphanumeric with backslash to specify that it stands for itself. In
270     particular, if you want to match a backslash, you write \e\e.
271     .P
272 ph10 859 In a UTF mode, only ASCII numbers and letters have any special meaning after a
273 ph10 579 backslash. All other characters (in particular, those whose codepoints are
274 ph10 574 greater than 127) are treated as literals.
275     .P
276 ph10 1396 If a pattern is compiled with the PCRE_EXTENDED option, most white space in the
277     pattern (other than in a character class), and characters between a # outside a
278     character class and the next newline, inclusive, are ignored. An escaping
279     backslash can be used to include a white space or # character as part of the
280     pattern.
281 nigel 75 .P
282 nigel 63 If you want to remove the special meaning from a sequence of characters, you
283 nigel 75 can do so by putting them between \eQ and \eE. This is different from Perl in
284     that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
285 nigel 63 Perl, $ and @ cause variable interpolation. Note the following examples:
286 nigel 75 .sp
287 nigel 63 Pattern PCRE matches Perl matches
288 nigel 75 .sp
289     .\" JOIN
290     \eQabc$xyz\eE abc$xyz abc followed by the
291 nigel 63 contents of $xyz
292 nigel 75 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
293     \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
294     .sp
295     The \eQ...\eE sequence is recognized both inside and outside character classes.
296 ph10 654 An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
297     by \eE later in the pattern, the literal interpretation continues to the end of
298 ph10 607 the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
299     a character class, this causes an error, because the character class is not
300     terminated.
301 nigel 75 .
302     .
303     .\" HTML <a name="digitsafterbackslash"></a>
304     .SS "Non-printing characters"
305     .rs
306     .sp
307 nigel 63 A second use of backslash provides a way of encoding non-printing characters
308     in patterns in a visible manner. There is no restriction on the appearance of
309     non-printing characters, apart from the binary zero that terminates a pattern,
310 ph10 456 but when a pattern is being prepared by text editing, it is often easier to use
311     one of the following escape sequences than the binary character it represents:
312 nigel 75 .sp
313     \ea alarm, that is, the BEL character (hex 07)
314 ph10 574 \ecx "control-x", where x is any ASCII character
315 nigel 75 \ee escape (hex 1B)
316 ph10 968 \ef form feed (hex 0C)
317 ph10 227 \en linefeed (hex 0A)
318 nigel 75 \er carriage return (hex 0D)
319     \et tab (hex 09)
320 ph10 1404 \e0dd character with octal code 0dd
321 ph10 488 \eddd character with octal code ddd, or back reference
322 ph10 1404 \eo{ddd..} character with octal code ddd..
323 nigel 75 \exhh character with hex code hh
324 ph10 745 \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
325 ph10 836 \euhhhh character with hex code hhhh (JavaScript mode only)
326 nigel 75 .sp
327 ph10 1001 The precise effect of \ecx on ASCII characters is as follows: if x is a lower
328     case letter, it is converted to upper case. Then bit 6 of the character (hex
329     40) is inverted. Thus \ecA to \ecZ become hex 01 to hex 1A (A is 41, Z is 5A),
330     but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the
331     data item (byte or 16-bit value) following \ec has a value greater than 127, a
332     compile-time error occurs. This locks out non-ASCII characters in all modes.
333 nigel 75 .P
334 ph10 1001 The \ec facility was designed for use with ASCII characters, but with the
335     extension to Unicode it is even less useful than it once was. It is, however,
336     recognized when PCRE is compiled in EBCDIC mode, where data items are always
337     bytes. In this mode, all values are valid after \ec. If the next character is a
338     lower case letter, it is converted to upper case. Then the 0xc0 bits of the
339     byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
340 ph10 1221 the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
341 ph10 1001 characters also generate different values.
342     .P
343 nigel 91 After \e0 up to two further octal digits are read. If there are fewer than two
344     digits, just those that are present are used. Thus the sequence \e0\ex\e07
345     specifies two binary zeros followed by a BEL character (code value 7). Make
346     sure you supply two digits after the initial zero if the pattern character that
347     follows is itself an octal digit.
348 nigel 75 .P
349 ph10 1404 The escape \eo must be followed by a sequence of octal digits, enclosed in
350 ph10 1370 braces. An error occurs if this is not the case. This escape is a recent
351     addition to Perl; it provides way of specifying character code points as octal
352     numbers greater than 0777, and it also allows octal numbers and back references
353     to be unambiguously specified.
354     .P
355     For greater clarity and unambiguity, it is best to avoid following \e by a
356     digit greater than zero. Instead, use \eo{} or \ex{} to specify character
357     numbers, and \eg{} to specify back references. The following paragraphs
358     describe the old, ambiguous syntax.
359     .P
360 ph10 1369 The handling of a backslash followed by a digit other than 0 is complicated,
361     and Perl has changed in recent releases, causing PCRE also to change. Outside a
362     character class, PCRE reads the digit and any following digits as a decimal
363     number. If the number is less than 8, or if there have been at least that many
364 nigel 63 previous capturing left parentheses in the expression, the entire sequence is
365 nigel 75 taken as a \fIback reference\fP. A description of how this works is given
366     .\" HTML <a href="#backreferences">
367     .\" </a>
368     later,
369     .\"
370     following the discussion of
371     .\" HTML <a href="#subpattern">
372     .\" </a>
373     parenthesized subpatterns.
374     .\"
375     .P
376 ph10 1369 Inside a character class, or if the decimal number following \e is greater than
377     7 and there have not been that many capturing subpatterns, PCRE handles \e8 and
378     \e9 as the literal characters "8" and "9", and otherwise re-reads up to three
379     octal digits following the backslash, using them to generate a data character.
380 ph10 1370 Any subsequent digits stand for themselves. For example:
381 nigel 75 .sp
382 ph10 1033 \e040 is another way of writing an ASCII space
383 nigel 75 .\" JOIN
384     \e40 is the same, provided there are fewer than 40
385 nigel 63 previous capturing subpatterns
386 nigel 75 \e7 is always a back reference
387     .\" JOIN
388     \e11 might be a back reference, or another way of
389 nigel 63 writing a tab
390 nigel 75 \e011 is always a tab
391     \e0113 is a tab followed by the character "3"
392     .\" JOIN
393     \e113 might be a back reference, otherwise the
394 nigel 63 character with octal code 113
395 nigel 75 .\" JOIN
396     \e377 might be a back reference, otherwise
397 ph10 859 the value 255 (decimal)
398 nigel 75 .\" JOIN
399 ph10 1369 \e81 is either a back reference, or the two
400     characters "8" and "1"
401 nigel 75 .sp
402 ph10 1370 Note that octal values of 100 or greater that are specified using this syntax
403     must not be introduced by a leading zero, because no more than three octal
404     digits are ever read.
405 nigel 75 .P
406 ph10 1370 By default, after \ex that is not followed by {, from zero to two hexadecimal
407     digits are read (letters can be in upper or lower case). Any number of
408     hexadecimal digits may appear between \ex{ and }. If a character other than
409     a hexadecimal digit appears between \ex{ and }, or if there is no terminating
410     }, an error occurs.
411     .P
412     If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
413     as just described only when it is followed by two hexadecimal digits.
414     Otherwise, it matches a literal "x" character. In JavaScript mode, support for
415     code points greater than 256 is provided by \eu, which must be followed by
416     four hexadecimal digits; otherwise it matches a literal "u" character.
417     .P
418     Characters whose value is less than 256 can be defined by either of the two
419     syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
420     way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
421     \eu00dc in JavaScript mode).
422     .
423     .
424     .SS "Constraints on character values"
425     .rs
426     .sp
427     Characters that are specified using octal or hexadecimal numbers are
428     limited to certain values, as follows:
429     .sp
430     8-bit non-UTF mode less than 0x100
431     8-bit UTF-8 mode less than 0x10ffff and a valid codepoint
432     16-bit non-UTF mode less than 0x10000
433     16-bit UTF-16 mode less than 0x10ffff and a valid codepoint
434 chpe 1373 32-bit non-UTF mode less than 0x100000000
435 ph10 1370 32-bit UTF-32 mode less than 0x10ffff and a valid codepoint
436     .sp
437     Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
438 ph10 1404 "surrogate" codepoints), and 0xffef.
439 ph10 1370 .
440     .
441     .SS "Escape sequences in character classes"
442     .rs
443     .sp
444 nigel 91 All the sequences that define a single character value can be used both inside
445 ph10 836 and outside character classes. In addition, inside a character class, \eb is
446     interpreted as the backspace character (hex 08).
447     .P
448     \eN is not allowed in a character class. \eB, \eR, and \eX are not special
449     inside a character class. Like other unrecognized escape sequences, they are
450     treated as the literal characters "B", "R", and "X" by default, but cause an
451     error if the PCRE_EXTRA option is set. Outside a character class, these
452     sequences have different meanings.
453 nigel 75 .
454     .
455 ph10 745 .SS "Unsupported escape sequences"
456     .rs
457     .sp
458     In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
459     handler and used to modify the case of following characters. By default, PCRE
460     does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
461     option is set, \eU matches a "U" character, and \eu can be used to define a
462     character by code point, as described in the previous section.
463     .
464     .
465 nigel 93 .SS "Absolute and relative back references"
466     .rs
467     .sp
468 ph10 208 The sequence \eg followed by an unsigned or a negative number, optionally
469     enclosed in braces, is an absolute or relative back reference. A named back
470     reference can be coded as \eg{name}. Back references are discussed
471 nigel 93 .\" HTML <a href="#backreferences">
472     .\" </a>
473     later,
474     .\"
475     following the discussion of
476     .\" HTML <a href="#subpattern">
477     .\" </a>
478     parenthesized subpatterns.
479     .\"
480     .
481     .
482 ph10 333 .SS "Absolute and relative subroutine calls"
483     .rs
484     .sp
485 ph10 345 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
486     a number enclosed either in angle brackets or single quotes, is an alternative
487     syntax for referencing a subpattern as a "subroutine". Details are discussed
488 ph10 333 .\" HTML <a href="#onigurumasubroutines">
489     .\" </a>
490     later.
491     .\"
492 ph10 345 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
493 ph10 461 synonymous. The former is a back reference; the latter is a
494 ph10 454 .\" HTML <a href="#subpatternsassubroutines">
495     .\" </a>
496     subroutine
497     .\"
498     call.
499 ph10 333 .
500     .
501 ph10 518 .\" HTML <a name="genericchartypes"></a>
502 nigel 75 .SS "Generic character types"
503     .rs
504     .sp
505 ph10 514 Another use of backslash is for specifying generic character types:
506 nigel 75 .sp
507 ph10 182 \ed any decimal digit
508 nigel 75 \eD any character that is not a decimal digit
509 ph10 968 \eh any horizontal white space character
510     \eH any character that is not a horizontal white space character
511     \es any white space character
512     \eS any character that is not a white space character
513     \ev any vertical white space character
514     \eV any character that is not a vertical white space character
515 nigel 75 \ew any "word" character
516     \eW any "non-word" character
517     .sp
518 ph10 535 There is also the single sequence \eN, which matches a non-newline character.
519     This is the same as
520 ph10 514 .\" HTML <a href="#fullstopdot">
521     .\" </a>
522 ph10 535 the "." metacharacter
523 ph10 514 .\"
524 ph10 836 when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
525 ph10 745 PCRE does not support this.
526 nigel 75 .P
527 ph10 514 Each pair of lower and upper case escape sequences partitions the complete set
528     of characters into two disjoint sets. Any given character matches one, and only
529 ph10 518 one, of each pair. The sequences can appear both inside and outside character
530 nigel 75 classes. They each match one character of the appropriate type. If the current
531 ph10 518 matching point is at the end of the subject string, all of them fail, because
532 nigel 75 there is no character to match.
533     .P
534 ph10 1364 For compatibility with Perl, \es did not used to match the VT character (code
535     11), which made it different from the the POSIX "space" class. However, Perl
536 ph10 1401 added VT at release 5.18, and PCRE followed suit at release 8.34. The default
537     \es characters are now HT (9), LF (10), VT (11), FF (12), CR (13), and space
538 ph10 1404 (32), which are defined as white space in the "C" locale. This list may vary if
539 ph10 1405 locale-specific matching is taking place. For example, in some locales the
540 ph10 1412 "non-breaking space" character (\exA0) is recognized as white space, and in
541 ph10 1405 others the VT character is not.
542 nigel 75 .P
543 ph10 518 A "word" character is an underscore or any character that is a letter or digit.
544     By default, the definition of letters and digits is controlled by PCRE's
545     low-valued character tables, and may vary if locale-specific matching is taking
546     place (see
547     .\" HTML <a href="pcreapi.html#localesupport">
548     .\" </a>
549     "Locale support"
550     .\"
551     in the
552     .\" HREF
553     \fBpcreapi\fP
554     .\"
555     page). For example, in a French locale such as "fr_FR" in Unix-like systems,
556 ph10 1401 or "french" in Windows, some character codes greater than 127 are used for
557 ph10 518 accented letters, and these are then matched by \ew. The use of locales with
558     Unicode is discouraged.
559 ph10 178 .P
560 ph10 1401 By default, characters whose code points are greater than 127 never match \ed,
561     \es, or \ew, and always match \eD, \eS, and \eW, although this may vary for
562     characters in the range 128-255 when locale-specific matching is happening.
563     These escape sequences retain their original meanings from before Unicode
564     support was available, mainly for efficiency reasons. If PCRE is compiled with
565     Unicode property support, and the PCRE_UCP option is set, the behaviour is
566     changed so that Unicode properties are used to determine character types, as
567     follows:
568 ph10 518 .sp
569 ph10 1376 \ed any character that matches \ep{Nd} (decimal digit)
570     \es any character that matches \ep{Z} or \eh or \ev
571     \ew any character that matches \ep{L} or \ep{N}, plus underscore
572 ph10 518 .sp
573     The upper case escapes match the inverse sets of characters. Note that \ed
574 ph10 535 matches only decimal digits, whereas \ew matches any Unicode digit, as well as
575 ph10 518 any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
576     \eB because they are defined in terms of \ew and \eW. Matching these sequences
577     is noticeably slower when PCRE_UCP is set.
578     .P
579 ph10 579 The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
580 ph10 572 release 5.10. In contrast to the other sequences, which match only ASCII
581 ph10 1401 characters by default, these always match certain high-valued code points,
582 ph10 859 whether or not PCRE_UCP is set. The horizontal space characters are:
583 ph10 178 .sp
584 ph10 1033 U+0009 Horizontal tab (HT)
585 ph10 178 U+0020 Space
586     U+00A0 Non-break space
587     U+1680 Ogham space mark
588     U+180E Mongolian vowel separator
589     U+2000 En quad
590     U+2001 Em quad
591     U+2002 En space
592     U+2003 Em space
593     U+2004 Three-per-em space
594     U+2005 Four-per-em space
595     U+2006 Six-per-em space
596     U+2007 Figure space
597     U+2008 Punctuation space
598     U+2009 Thin space
599     U+200A Hair space
600     U+202F Narrow no-break space
601     U+205F Medium mathematical space
602     U+3000 Ideographic space
603     .sp
604     The vertical space characters are:
605     .sp
606 ph10 1033 U+000A Linefeed (LF)
607     U+000B Vertical tab (VT)
608     U+000C Form feed (FF)
609     U+000D Carriage return (CR)
610     U+0085 Next line (NEL)
611 ph10 178 U+2028 Line separator
612     U+2029 Paragraph separator
613 ph10 859 .sp
614 ph10 903 In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
615     relevant.
616 nigel 75 .
617     .
618 ph10 231 .\" HTML <a name="newlineseq"></a>
619 nigel 93 .SS "Newline sequences"
620     .rs
621     .sp
622 ph10 231 Outside a character class, by default, the escape sequence \eR matches any
623 ph10 859 Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
624     following:
625 nigel 93 .sp
626     (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
627     .sp
628     This is an example of an "atomic group", details of which are given
629     .\" HTML <a href="#atomicgroup">
630     .\" </a>
631     below.
632     .\"
633     This particular group matches either the two-character sequence CR followed by
634     LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
635 ph10 968 U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
636 nigel 93 line, U+0085). The two-character sequence is treated as a single unit that
637     cannot be split.
638     .P
639 ph10 859 In other modes, two additional characters whose codepoints are greater than 255
640 nigel 93 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
641     Unicode character property support is not needed for these characters to be
642     recognized.
643     .P
644 ph10 231 It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
645     complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
646 ph10 247 either at compile time or when the pattern is matched. (BSR is an abbrevation
647 ph10 246 for "backslash R".) This can be made the default when PCRE is built; if this is
648     the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
649     It is also possible to specify these settings by starting a pattern string with
650     one of the following sequences:
651 ph10 231 .sp
652     (*BSR_ANYCRLF) CR, LF, or CRLF only
653     (*BSR_UNICODE) any Unicode newline sequence
654     .sp
655 ph10 859 These override the default and the options given to the compiling function, but
656     they can themselves be overridden by options given to a matching function. Note
657     that these special settings, which are not Perl-compatible, are recognized only
658     at the very start of a pattern, and that they must be in upper case. If more
659     than one of them is present, the last one is used. They can be combined with a
660     change of newline convention; for example, a pattern can start with:
661 ph10 246 .sp
662     (*ANY)(*BSR_ANYCRLF)
663     .sp
664 ph10 1219 They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
665     (*UCP) special sequences. Inside a character class, \eR is treated as an
666     unrecognized escape sequence, and so matches the letter "R" by default, but
667     causes an error if PCRE_EXTRA is set.
668 nigel 93 .
669     .
670 nigel 75 .\" HTML <a name="uniextseq"></a>
671     .SS Unicode character properties
672     .rs
673     .sp
674     When PCRE is built with Unicode character property support, three additional
675 ph10 184 escape sequences that match characters with specific properties are available.
676 ph10 859 When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
677 ph10 184 characters whose codepoints are less than 256, but they do work in this mode.
678     The extra escape sequences are:
679 nigel 75 .sp
680 nigel 87 \ep{\fIxx\fP} a character with the \fIxx\fP property
681     \eP{\fIxx\fP} a character without the \fIxx\fP property
682 ph10 1011 \eX a Unicode extended grapheme cluster
683 nigel 75 .sp
684 nigel 87 The property names represented by \fIxx\fP above are limited to the Unicode
685 ph10 517 script names, the general category properties, "Any", which matches any
686     character (including newline), and some special PCRE properties (described
687 ph10 535 in the
688 ph10 517 .\" HTML <a href="#extraprops">
689     .\" </a>
690 ph10 535 next section).
691 ph10 517 .\"
692     Other Perl properties such as "InMusicalSymbols" are not currently supported by
693     PCRE. Note that \eP{Any} does not match any characters, so always causes a
694     match failure.
695 nigel 75 .P
696 nigel 87 Sets of Unicode characters are defined as belonging to certain scripts. A
697     character from one of these sets can be matched using a script name. For
698     example:
699 nigel 75 .sp
700 nigel 87 \ep{Greek}
701     \eP{Han}
702     .sp
703     Those that are not part of an identified script are lumped together as
704     "Common". The current list of scripts is:
705     .P
706     Arabic,
707     Armenian,
708 ph10 491 Avestan,
709 nigel 93 Balinese,
710 ph10 491 Bamum,
711 ph10 942 Batak,
712 nigel 87 Bengali,
713     Bopomofo,
714 ph10 942 Brahmi,
715 nigel 87 Braille,
716     Buginese,
717     Buhid,
718     Canadian_Aboriginal,
719 ph10 491 Carian,
720 ph10 942 Chakma,
721 ph10 491 Cham,
722 nigel 87 Cherokee,
723     Common,
724     Coptic,
725 nigel 93 Cuneiform,
726 nigel 87 Cypriot,
727     Cyrillic,
728     Deseret,
729     Devanagari,
730 ph10 491 Egyptian_Hieroglyphs,
731 nigel 87 Ethiopic,
732     Georgian,
733     Glagolitic,
734     Gothic,
735     Greek,
736     Gujarati,
737     Gurmukhi,
738     Han,
739     Hangul,
740     Hanunoo,
741     Hebrew,
742     Hiragana,
743 ph10 491 Imperial_Aramaic,
744 nigel 87 Inherited,
745 ph10 491 Inscriptional_Pahlavi,
746     Inscriptional_Parthian,
747     Javanese,
748     Kaithi,
749 nigel 87 Kannada,
750     Katakana,
751 ph10 491 Kayah_Li,
752 nigel 87 Kharoshthi,
753     Khmer,
754     Lao,
755     Latin,
756 ph10 491 Lepcha,
757 nigel 87 Limbu,
758     Linear_B,
759 ph10 491 Lisu,
760     Lycian,
761     Lydian,
762 nigel 87 Malayalam,
763 ph10 942 Mandaic,
764 ph10 491 Meetei_Mayek,
765 ph10 942 Meroitic_Cursive,
766     Meroitic_Hieroglyphs,
767     Miao,
768 nigel 87 Mongolian,
769     Myanmar,
770     New_Tai_Lue,
771 nigel 93 Nko,
772 nigel 87 Ogham,
773     Old_Italic,
774     Old_Persian,
775 ph10 491 Old_South_Arabian,
776     Old_Turkic,
777     Ol_Chiki,
778 nigel 87 Oriya,
779     Osmanya,
780 nigel 93 Phags_Pa,
781     Phoenician,
782 ph10 491 Rejang,
783 nigel 87 Runic,
784 ph10 491 Samaritan,
785     Saurashtra,
786 ph10 942 Sharada,
787 nigel 87 Shavian,
788     Sinhala,
789 ph10 942 Sora_Sompeng,
790 ph10 491 Sundanese,
791 nigel 87 Syloti_Nagri,
792     Syriac,
793     Tagalog,
794     Tagbanwa,
795     Tai_Le,
796 ph10 491 Tai_Tham,
797     Tai_Viet,
798 ph10 942 Takri,
799 nigel 87 Tamil,
800     Telugu,
801     Thaana,
802     Thai,
803     Tibetan,
804     Tifinagh,
805     Ugaritic,
806 ph10 491 Vai,
807 nigel 87 Yi.
808     .P
809 ph10 517 Each character has exactly one Unicode general category property, specified by
810     a two-letter abbreviation. For compatibility with Perl, negation can be
811     specified by including a circumflex between the opening brace and the property
812     name. For example, \ep{^Lu} is the same as \eP{Lu}.
813 nigel 87 .P
814     If only one letter is specified with \ep or \eP, it includes all the general
815     category properties that start with that letter. In this case, in the absence
816     of negation, the curly brackets in the escape sequence are optional; these two
817     examples have the same effect:
818     .sp
819 nigel 75 \ep{L}
820     \epL
821     .sp
822 nigel 87 The following general category property codes are supported:
823 nigel 75 .sp
824     C Other
825     Cc Control
826     Cf Format
827     Cn Unassigned
828     Co Private use
829     Cs Surrogate
830     .sp
831     L Letter
832     Ll Lower case letter
833     Lm Modifier letter
834     Lo Other letter
835     Lt Title case letter
836     Lu Upper case letter
837     .sp
838     M Mark
839     Mc Spacing mark
840     Me Enclosing mark
841     Mn Non-spacing mark
842     .sp
843     N Number
844     Nd Decimal number
845     Nl Letter number
846     No Other number
847     .sp
848     P Punctuation
849     Pc Connector punctuation
850     Pd Dash punctuation
851     Pe Close punctuation
852     Pf Final punctuation
853     Pi Initial punctuation
854     Po Other punctuation
855     Ps Open punctuation
856     .sp
857     S Symbol
858     Sc Currency symbol
859     Sk Modifier symbol
860     Sm Mathematical symbol
861     So Other symbol
862     .sp
863     Z Separator
864     Zl Line separator
865     Zp Paragraph separator
866     Zs Space separator
867     .sp
868 nigel 87 The special property L& is also supported: it matches a character that has
869     the Lu, Ll, or Lt property, in other words, a letter that is not classified as
870     a modifier or "other".
871 nigel 75 .P
872 ph10 211 The Cs (Surrogate) property applies only to characters in the range U+D800 to
873 ph10 859 U+DFFF. Such characters are not valid in Unicode strings and so
874     cannot be tested by PCRE, unless UTF validity checking has been turned off
875 chpe 1055 (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
876     PCRE_NO_UTF32_CHECK in the
877 ph10 211 .\" HREF
878     \fBpcreapi\fP
879     .\"
880 ph10 451 page). Perl does not support the Cs property.
881 ph10 211 .P
882 ph10 451 The long synonyms for property names that Perl supports (such as \ep{Letter})
883 nigel 91 are not supported by PCRE, nor is it permitted to prefix any of these
884 nigel 87 properties with "Is".
885     .P
886     No character that is in the Unicode table has the Cn (unassigned) property.
887     Instead, this property is assumed for any code point that is not in the
888     Unicode table.
889     .P
890 nigel 75 Specifying caseless matching does not affect these escape sequences. For
891 ph10 1335 example, \ep{Lu} always matches only upper case letters. This is different from
892 ph10 1258 the behaviour of current versions of Perl.
893 nigel 75 .P
894 ph10 1011 Matching characters by Unicode property is not fast, because PCRE has to do a
895     multistage table lookup in order to find a character's property. That is why
896     the traditional escape sequences such as \ed and \ew do not use Unicode
897     properties in PCRE by default, though you can make them do so by setting the
898     PCRE_UCP option or by starting the pattern with (*UCP).
899     .
900     .
901     .SS Extended grapheme clusters
902     .rs
903 nigel 75 .sp
904 ph10 1011 The \eX escape matches any number of Unicode characters that form an "extended
905     grapheme cluster", and treats the sequence as an atomic group
906 nigel 75 .\" HTML <a href="#atomicgroup">
907     .\" </a>
908     (see below).
909     .\"
910 ph10 1011 Up to and including release 8.31, PCRE matched an earlier, simpler definition
911     that was equivalent to
912     .sp
913     (?>\ePM\epM*)
914     .sp
915     That is, it matched a character without the "mark" property, followed by zero
916     or more characters with the "mark" property. Characters with the "mark"
917     property are typically non-spacing accents that affect the preceding character.
918 nigel 75 .P
919 ph10 1011 This simple definition was extended in Unicode to include more complicated
920     kinds of composite character by giving each character a grapheme breaking
921     property, and creating rules that use these properties to define the boundaries
922     of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
923     one of these clusters.
924 ph10 628 .P
925 ph10 1221 \eX always matches at least one character. Then it decides whether to add
926 ph10 1011 additional characters according to the following rules for ending a cluster:
927     .P
928     1. End at the end of the subject string.
929     .P
930     2. Do not end between CR and LF; otherwise end after any control character.
931     .P
932 ph10 1221 3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
933 ph10 1011 are of five types: L, V, T, LV, and LVT. An L character may be followed by an
934 ph10 1221 L, V, LV, or LVT character; an LV or V character may be followed by a V or T
935 ph10 1011 character; an LVT or T character may be follwed only by a T character.
936     .P
937     4. Do not end before extending characters or spacing marks. Characters with
938     the "mark" property always have the "extend" grapheme breaking property.
939     .P
940     5. Do not end after prepend characters.
941     .P
942     6. Otherwise, end the cluster.
943 nigel 75 .
944     .
945 ph10 517 .\" HTML <a name="extraprops"></a>
946     .SS PCRE's additional properties
947     .rs
948     .sp
949 ph10 1011 As well as the standard Unicode properties described above, PCRE supports four
950     more that make it possible to convert traditional escape sequences such as \ew
951 ph10 1387 and \es to use Unicode properties. PCRE uses these non-standard, non-Perl
952     properties internally when PCRE_UCP is set. However, they may also be used
953     explicitly. These properties are:
954 ph10 517 .sp
955     Xan Any alphanumeric character
956     Xps Any POSIX space character
957     Xsp Any Perl space character
958     Xwd Any Perl "word" character
959     .sp
960 ph10 535 Xan matches characters that have either the L (letter) or the N (number)
961 ph10 968 property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
962 ph10 517 carriage return, and any other character that has the Z (separator) property.
963 ph10 1387 Xsp is the same as Xps; it used to exclude vertical tab, for Perl
964     compatibility, but Perl changed, and so PCRE followed at release 8.34. Xwd
965     matches the same characters as Xan, plus underscore.
966 ph10 1260 .P
967 ph10 1335 There is another non-standard property, Xuc, which matches any character that
968     can be represented by a Universal Character Name in C++ and other programming
969     languages. These are the characters $, @, ` (grave accent), and all characters
970     with Unicode code points greater than or equal to U+00A0, except for the
971     surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
972 ph10 1260 excluded. (Universal Character Names are of the form \euHHHH or \eUHHHHHHHH
973 ph10 1335 where H is a hexadecimal digit. Note that the Xuc property does not match these
974 ph10 1260 sequences but the characters that they represent.)
975 ph10 517 .
976     .
977 ph10 168 .\" HTML <a name="resetmatchstart"></a>
978     .SS "Resetting the match start"
979     .rs
980     .sp
981 ph10 572 The escape sequence \eK causes any previously matched characters not to be
982     included in the final matched sequence. For example, the pattern:
983 ph10 168 .sp
984     foo\eKbar
985     .sp
986 ph10 172 matches "foobar", but reports that it has matched "bar". This feature is
987 ph10 168 similar to a lookbehind assertion
988     .\" HTML <a href="#lookbehind">
989     .\" </a>
990     (described below).
991     .\"
992 ph10 172 However, in this case, the part of the subject before the real match does not
993     have to be of fixed length, as lookbehind assertions do. The use of \eK does
994 ph10 168 not interfere with the setting of
995     .\" HTML <a href="#subpattern">
996     .\" </a>
997     captured substrings.
998 ph10 172 .\"
999 ph10 168 For example, when the pattern
1000     .sp
1001     (foo)\eKbar
1002     .sp
1003 ph10 172 matches "foobar", the first substring is still set to "foo".
1004 ph10 500 .P
1005 ph10 507 Perl documents that the use of \eK within assertions is "not well defined". In
1006     PCRE, \eK is acted upon when it occurs inside positive assertions, but is
1007 ph10 1418 ignored in negative assertions. Note that when a pattern such as (?=ab\eK)
1008 ph10 1459 matches, the reported start of the match can be greater than the end of the
1009 ph10 1418 match.
1010 ph10 168 .
1011     .
1012 nigel 75 .\" HTML <a name="smallassertions"></a>
1013     .SS "Simple assertions"
1014     .rs
1015     .sp
1016 nigel 93 The final use of backslash is for certain simple assertions. An assertion
1017 nigel 63 specifies a condition that has to be met at a particular point in a match,
1018     without consuming any characters from the subject string. The use of
1019 nigel 75 subpatterns for more complicated assertions is described
1020     .\" HTML <a href="#bigassertions">
1021     .\" </a>
1022     below.
1023     .\"
1024 nigel 91 The backslashed assertions are:
1025 nigel 75 .sp
1026     \eb matches at a word boundary
1027     \eB matches when not at a word boundary
1028 nigel 93 \eA matches at the start of the subject
1029     \eZ matches at the end of the subject
1030     also matches before a newline at the end of the subject
1031     \ez matches only at the end of the subject
1032     \eG matches at the first matching position in the subject
1033 nigel 75 .sp
1034 ph10 513 Inside a character class, \eb has a different meaning; it matches the backspace
1035 ph10 535 character. If any other of these assertions appears in a character class, by
1036 ph10 513 default it matches the corresponding literal character (for example, \eB
1037     matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
1038     escape sequence" error is generated instead.
1039 nigel 75 .P
1040 nigel 63 A word boundary is a position in the subject string where the current character
1041 nigel 75 and the previous character do not both match \ew or \eW (i.e. one matches
1042     \ew and the other matches \eW), or the start or end of the string if the
1043 ph10 859 first or last character matches \ew, respectively. In a UTF mode, the meanings
1044 ph10 518 of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
1045     done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
1046     of word" or "end of word" metasequence. However, whatever follows \eb normally
1047     determines which it is. For example, the fragment \eba matches "a" at the start
1048     of a word.
1049 nigel 75 .P
1050     The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
1051     dollar (described in the next section) in that they only ever match at the very
1052     start and end of the subject string, whatever options are set. Thus, they are
1053     independent of multiline mode. These three assertions are not affected by the
1054     PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
1055     circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
1056     argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
1057     at a point other than the beginning of the subject, \eA can never match. The
1058 nigel 91 difference between \eZ and \ez is that \eZ matches before a newline at the end
1059     of the string as well as at the very end, whereas \ez matches only at the end.
1060 nigel 75 .P
1061     The \eG assertion is true only when the current matching position is at the
1062     start point of the match, as specified by the \fIstartoffset\fP argument of
1063     \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
1064     non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
1065 nigel 63 arguments, you can mimic Perl's /g option, and it is in this kind of
1066 nigel 75 implementation where \eG can be useful.
1067     .P
1068     Note, however, that PCRE's interpretation of \eG, as the start of the current
1069 nigel 63 match, is subtly different from Perl's, which defines it as the end of the
1070     previous match. In Perl, these can be different when the previously matched
1071     string was empty. Because PCRE does just one match at a time, it cannot
1072     reproduce this behaviour.
1073 nigel 75 .P
1074     If all the alternatives of a pattern begin with \eG, the expression is anchored
1075 nigel 63 to the starting match position, and the "anchored" flag is set in the compiled
1076     regular expression.
1077 nigel 75 .
1078     .
1079     .SH "CIRCUMFLEX AND DOLLAR"
1080 nigel 63 .rs
1081     .sp
1082 ph10 1221 The circumflex and dollar metacharacters are zero-width assertions. That is,
1083     they test for a particular condition being true without consuming any
1084 ph10 1213 characters from the subject string.
1085     .P
1086 nigel 63 Outside a character class, in the default matching mode, the circumflex
1087 ph10 1213 character is an assertion that is true only if the current matching point is at
1088     the start of the subject string. If the \fIstartoffset\fP argument of
1089 nigel 75 \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
1090 nigel 63 option is unset. Inside a character class, circumflex has an entirely different
1091 nigel 75 meaning
1092     .\" HTML <a href="#characterclass">
1093     .\" </a>
1094     (see below).
1095     .\"
1096     .P
1097 nigel 63 Circumflex need not be the first character of the pattern if a number of
1098     alternatives are involved, but it should be the first thing in each alternative
1099     in which it appears if the pattern is ever to match that branch. If all
1100     possible alternatives start with a circumflex, that is, if the pattern is
1101     constrained to match only at the start of the subject, it is said to be an
1102     "anchored" pattern. (There are also other constructs that can cause a pattern
1103     to be anchored.)
1104 nigel 75 .P
1105 ph10 1213 The dollar character is an assertion that is true only if the current matching
1106     point is at the end of the subject string, or immediately before a newline at
1107     the end of the string (by default). Note, however, that it does not actually
1108     match the newline. Dollar need not be the last character of the pattern if a
1109     number of alternatives are involved, but it should be the last item in any
1110     branch in which it appears. Dollar has no special meaning in a character class.
1111 nigel 75 .P
1112 nigel 63 The meaning of dollar can be changed so that it matches only at the very end of
1113     the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
1114 nigel 75 does not affect the \eZ assertion.
1115     .P
1116 nigel 63 The meanings of the circumflex and dollar characters are changed if the
1117 nigel 91 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
1118     immediately after internal newlines as well as at the start of the subject
1119     string. It does not match after a newline that ends the string. A dollar
1120     matches before any newlines in the string, as well as at the very end, when
1121     PCRE_MULTILINE is set. When newline is specified as the two-character
1122     sequence CRLF, isolated CR and LF characters do not indicate newlines.
1123 nigel 75 .P
1124 nigel 91 For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
1125     \en represents a newline) in multiline mode, but not otherwise. Consequently,
1126     patterns that are anchored in single line mode because all branches start with
1127     ^ are not anchored in multiline mode, and a match for circumflex is possible
1128     when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
1129     PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
1130     .P
1131 nigel 75 Note that the sequences \eA, \eZ, and \ez can be used to match the start and
1132 nigel 63 end of the subject in both modes, and if all branches of a pattern start with
1133 nigel 91 \eA it is always anchored, whether or not PCRE_MULTILINE is set.
1134 nigel 75 .
1135     .
1136 ph10 514 .\" HTML <a name="fullstopdot"></a>
1137     .SH "FULL STOP (PERIOD, DOT) AND \eN"
1138 nigel 63 .rs
1139     .sp
1140     Outside a character class, a dot in the pattern matches any one character in
1141 nigel 91 the subject string except (by default) a character that signifies the end of a
1142 ph10 903 line.
1143 nigel 91 .P
1144 nigel 93 When a line ending is defined as a single character, dot never matches that
1145     character; when the two-character sequence CRLF is used, dot does not match CR
1146     if it is immediately followed by LF, but otherwise it matches all characters
1147     (including isolated CRs and LFs). When any Unicode line endings are being
1148     recognized, dot does not match CR or LF or any of the other line ending
1149     characters.
1150     .P
1151 nigel 91 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
1152 nigel 93 option is set, a dot matches any one character, without exception. If the
1153     two-character sequence CRLF is present in the subject string, it takes two dots
1154     to match it.
1155 nigel 91 .P
1156     The handling of dot is entirely independent of the handling of circumflex and
1157     dollar, the only relationship being that they both involve newlines. Dot has no
1158     special meaning in a character class.
1159 ph10 514 .P
1160 ph10 579 The escape sequence \eN behaves like a dot, except that it is not affected by
1161 ph10 572 the PCRE_DOTALL option. In other words, it matches any character except one
1162 ph10 745 that signifies the end of a line. Perl also uses \eN to match characters by
1163     name; PCRE does not support this.
1164 nigel 75 .
1165     .
1166 ph10 859 .SH "MATCHING A SINGLE DATA UNIT"
1167 nigel 63 .rs
1168     .sp
1169 ph10 859 Outside a character class, the escape sequence \eC matches any one data unit,
1170     whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1171 chpe 1055 byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
1172     a 32-bit unit. Unlike a dot, \eC always
1173 ph10 859 matches line-ending characters. The feature is provided in Perl in order to
1174     match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1175     used. Because \eC breaks up characters into individual data units, matching one
1176     unit with \eC in a UTF mode means that the rest of the string may start with a
1177     malformed UTF character. This has undefined results, because PCRE assumes that
1178     it is dealing with valid UTF strings (and by default it checks this at the
1179 chpe 1055 start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
1180     PCRE_NO_UTF32_CHECK option is used).
1181 nigel 75 .P
1182     PCRE does not allow \eC to appear in lookbehind assertions
1183     .\" HTML <a href="#lookbehind">
1184     .\" </a>
1185 ph10 754 (described below)
1186 nigel 75 .\"
1187 ph10 859 in a UTF mode, because this would make it impossible to calculate the length of
1188 nigel 75 the lookbehind.
1189 ph10 737 .P
1190 ph10 859 In general, the \eC escape sequence is best avoided. However, one
1191     way of using it that avoids the problem of malformed UTF characters is to use a
1192     lookahead to check the length of the next character, as in this pattern, which
1193     could be used with a UTF-8 string (ignore white space and line breaks):
1194 ph10 737 .sp
1195     (?| (?=[\ex00-\ex7f])(\eC) |
1196 ph10 738 (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1197     (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1198 ph10 737 (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1199     .sp
1200 ph10 738 A group that starts with (?| resets the capturing parentheses numbers in each
1201     alternative (see
1202 ph10 737 .\" HTML <a href="#dupsubpatternnumber">
1203     .\" </a>
1204     "Duplicate Subpattern Numbers"
1205     .\"
1206 ph10 738 below). The assertions at the start of each branch check the next UTF-8
1207     character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1208 ph10 737 character's individual bytes are then captured by the appropriate number of
1209     groups.
1210 nigel 75 .
1211     .
1212     .\" HTML <a name="characterclass"></a>
1213     .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
1214 nigel 63 .rs
1215     .sp
1216     An opening square bracket introduces a character class, terminated by a closing
1217 ph10 461 square bracket. A closing square bracket on its own is not special by default.
1218     However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
1219 ph10 456 bracket causes a compile-time error. If a closing square bracket is required as
1220     a member of the class, it should be the first data character in the class
1221     (after an initial circumflex, if present) or escaped with a backslash.
1222 nigel 75 .P
1223 ph10 859 A character class matches a single character in the subject. In a UTF mode, the
1224     character may be more than one data unit long. A matched character must be in
1225     the set of characters defined by the class, unless the first character in the
1226     class definition is a circumflex, in which case the subject character must not
1227     be in the set defined by the class. If a circumflex is actually required as a
1228     member of the class, ensure it is not the first character, or escape it with a
1229 nigel 63 backslash.
1230 nigel 75 .P
1231 nigel 63 For example, the character class [aeiou] matches any lower case vowel, while
1232     [^aeiou] matches any character that is not a lower case vowel. Note that a
1233 nigel 75 circumflex is just a convenient notation for specifying the characters that
1234     are in the class by enumerating those that are not. A class that starts with a
1235 ph10 456 circumflex is not an assertion; it still consumes a character from the subject
1236 nigel 75 string, and therefore it fails if the current pointer is at the end of the
1237     string.
1238     .P
1239 chpe 1055 In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff)
1240     can be included in a class as a literal string of data units, or by using the
1241     \ex{ escaping mechanism.
1242 nigel 75 .P
1243 nigel 63 When caseless matching is set, any letters in a class represent both their
1244     upper case and lower case versions, so for example, a caseless [aeiou] matches
1245     "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1246 ph10 859 caseful version would. In a UTF mode, PCRE always understands the concept of
1247 nigel 77 case for characters whose values are less than 128, so caseless matching is
1248     always possible. For characters with higher values, the concept of case is
1249     supported if PCRE is compiled with Unicode property support, but not otherwise.
1250 ph10 859 If you want to use caseless matching in a UTF mode for characters 128 and
1251     above, you must ensure that PCRE is compiled with Unicode property support as
1252     well as with UTF support.
1253 nigel 75 .P
1254 nigel 93 Characters that might indicate line breaks are never treated in any special way
1255     when matching character classes, whatever line-ending sequence is in use, and
1256     whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1257     such as [^a] always matches one of these characters.
1258 nigel 75 .P
1259 nigel 63 The minus (hyphen) character can be used to specify a range of characters in a
1260     character class. For example, [d-m] matches any letter between d and m,
1261     inclusive. If a minus character is required in a class, it must be escaped with
1262     a backslash or appear in a position where it cannot be interpreted as
1263 ph10 1404 indicating a range, typically as the first or last character in the class, or
1264 ph10 1392 immediately after a range. For example, [b-d-z] matches letters in the range b
1265     to d, a hyphen character, or z.
1266 nigel 75 .P
1267 nigel 63 It is not possible to have the literal character "]" as the end character of a
1268     range. A pattern such as [W-]46] is interpreted as a class of two characters
1269     ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1270     "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1271 nigel 75 the end of range, so [W-\e]46] is interpreted as a class containing a range
1272     followed by two other characters. The octal or hexadecimal representation of
1273     "]" can also be used to end a range.
1274     .P
1275 ph10 1392 An error is generated if a POSIX character class (see below) or an escape
1276     sequence other than one that defines a single character appears at a point
1277     where a range ending character is expected. For example, [z-\exff] is valid,
1278     but [A-\ed] and [A-[:digit:]] are not.
1279     .P
1280 nigel 63 Ranges operate in the collating sequence of character values. They can also be
1281 ph10 903 used for characters specified numerically, for example [\e000-\e037]. Ranges
1282 ph10 859 can include any characters that are valid for the current mode.
1283 nigel 75 .P
1284 nigel 63 If a range that includes letters is used when caseless matching is set, it
1285     matches the letters in either case. For example, [W-c] is equivalent to
1286 ph10 859 [][\e\e^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
1287 ph10 139 tables for a French locale are in use, [\exc8-\excb] matches accented E
1288 ph10 859 characters in both cases. In UTF modes, PCRE supports the concept of case for
1289 nigel 75 characters with values greater than 128 only when it is compiled with Unicode
1290     property support.
1291     .P
1292 ph10 575 The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1293     \eV, \ew, and \eW may appear in a character class, and add the characters that
1294     they match to the class. For example, [\edABCDEF] matches any hexadecimal
1295 ph10 859 digit. In UTF modes, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1296 ph10 575 and their upper case partners, just as it does when they appear outside a
1297     character class, as described in the section entitled
1298     .\" HTML <a href="#genericchartypes">
1299     .\" </a>
1300     "Generic character types"
1301     .\"
1302     above. The escape sequence \eb has a different meaning inside a character
1303     class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1304     are not special inside a character class. Like any other unrecognized escape
1305     sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1306     default, but cause an error if the PCRE_EXTRA option is set.
1307     .P
1308     A circumflex can conveniently be used with the upper case character types to
1309 ph10 518 specify a more restricted set of characters than the matching lower case type.
1310 ph10 575 For example, the class [^\eW_] matches any letter or digit, but not underscore,
1311 ph10 579 whereas [\ew] includes underscore. A positive character class should be read as
1312     "something OR something OR ..." and a negative class as "NOT something AND NOT
1313 ph10 575 something AND NOT ...".
1314 nigel 75 .P
1315     The only metacharacters that are recognized in character classes are backslash,
1316     hyphen (only where it can be interpreted as specifying a range), circumflex
1317     (only at the start), opening square bracket (only when it can be interpreted as
1318 ph10 1408 introducing a POSIX class name, or for a special compatibility feature - see
1319     the next two sections), and the terminating closing square bracket. However,
1320     escaping other non-alphanumeric characters does no harm.
1321 nigel 75 .
1322     .
1323     .SH "POSIX CHARACTER CLASSES"
1324 nigel 63 .rs
1325     .sp
1326 nigel 75 Perl supports the POSIX notation for character classes. This uses names
1327 nigel 63 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1328     this notation. For example,
1329 nigel 75 .sp
1330 nigel 63 [01[:alpha:]%]
1331 nigel 75 .sp
1332 nigel 63 matches "0", "1", any alphabetic character, or "%". The supported class names
1333 ph10 518 are:
1334 nigel 75 .sp
1335 nigel 63 alnum letters and digits
1336     alpha letters
1337     ascii character codes 0 - 127
1338     blank space or tab only
1339     cntrl control characters
1340 nigel 75 digit decimal digits (same as \ed)
1341 nigel 63 graph printing characters, excluding space
1342     lower lower case letters
1343     print printing characters, including space
1344 ph10 518 punct printing characters, excluding letters and digits and space
1345 ph10 1376 space white space (the same as \es from PCRE 8.34)
1346 nigel 63 upper upper case letters
1347 nigel 75 word "word" characters (same as \ew)
1348 nigel 63 xdigit hexadecimal digits
1349 nigel 75 .sp
1350 ph10 1401 The default "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
1351 ph10 1405 and space (32). If locale-specific matching is taking place, the list of space
1352     characters may be different; there may be fewer or more of them. "Space" used
1353     to be different to \es, which did not include VT, for Perl compatibility.
1354     However, Perl changed at release 5.18, and PCRE followed at release 8.34.
1355     "Space" and \es now match the same set of characters.
1356 nigel 75 .P
1357 nigel 63 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1358     5.8. Another Perl extension is negation, which is indicated by a ^ character
1359     after the colon. For example,
1360 nigel 75 .sp
1361 nigel 63 [12[:^digit:]]
1362 nigel 75 .sp
1363 nigel 63 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1364     syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1365     supported, and an error is given if they are encountered.
1366 nigel 75 .P
1367 ph10 1401 By default, characters with values greater than 128 do not match any of the
1368     POSIX character classes. However, if the PCRE_UCP option is passed to
1369     \fBpcre_compile()\fP, some of the classes are changed so that Unicode character
1370     properties are used. This is achieved by replacing certain POSIX classes by
1371     other sequences, as follows:
1372 ph10 518 .sp
1373     [:alnum:] becomes \ep{Xan}
1374     [:alpha:] becomes \ep{L}
1375 ph10 535 [:blank:] becomes \eh
1376 ph10 518 [:digit:] becomes \ep{Nd}
1377     [:lower:] becomes \ep{Ll}
1378 ph10 535 [:space:] becomes \ep{Xps}
1379 ph10 518 [:upper:] becomes \ep{Lu}
1380     [:word:] becomes \ep{Xwd}
1381     .sp
1382 ph10 1404 Negated versions, such as [:^alpha:] use \eP instead of \ep. Three other POSIX
1383 ph10 1387 classes are handled specially in UCP mode:
1384     .TP 10
1385     [:graph:]
1386 ph10 1404 This matches characters that have glyphs that mark the page when printed. In
1387     Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf
1388 ph10 1387 properties, except for:
1389     .sp
1390     U+061C Arabic Letter Mark
1391 ph10 1404 U+180E Mongolian Vowel Separator
1392 ph10 1387 U+2066 - U+2069 Various "isolate"s
1393     .sp
1394     .TP 10
1395     [:print:]
1396 ph10 1404 This matches the same characters as [:graph:] plus space characters that are
1397 ph10 1387 not controls, that is, characters with the Zs property.
1398     .TP 10
1399     [:punct:]
1400     This matches all characters that have the Unicode P (punctuation) property,
1401     plus those characters whose code points are less than 128 that have the S
1402     (Symbol) property.
1403     .P
1404     The other POSIX classes are unchanged, and match only characters with code
1405     points less than 128.
1406 nigel 75 .
1407     .
1408 ph10 1408 .SH "COMPATIBILITY FEATURE FOR WORD BOUNDARIES"
1409     .rs
1410     .sp
1411 ph10 1412 In the POSIX.2 compliant library that was included in 4.4BSD Unix, the ugly
1412     syntax [[:<:]] and [[:>:]] is used for matching "start of word" and "end of
1413 ph10 1408 word". PCRE treats these items as follows:
1414     .sp
1415     [[:<:]] is converted to \eb(?=\ew)
1416     [[:>:]] is converted to \eb(?<=\ew)
1417     .sp
1418     Only these exact character sequences are recognized. A sequence such as
1419 ph10 1412 [a[:<:]b] provokes error for an unrecognized POSIX class name. This support is
1420     not compatible with Perl. It is provided to help migrations from other
1421     environments, and is best not used in any new patterns. Note that \eb matches
1422     at the start and the end of a word (see
1423 ph10 1408 .\" HTML <a href="#smallassertions">
1424     .\" </a>
1425     "Simple assertions"
1426     .\"
1427 ph10 1412 above), and in a Perl-style pattern the preceding or following character
1428     normally shows which is wanted, without the need for the assertions that are
1429 ph10 1408 used above in order to give exactly the POSIX behaviour.
1430     .
1431     .
1432 nigel 75 .SH "VERTICAL BAR"
1433 nigel 63 .rs
1434     .sp
1435     Vertical bar characters are used to separate alternative patterns. For example,
1436     the pattern
1437 nigel 75 .sp
1438 nigel 63 gilbert|sullivan
1439 nigel 75 .sp
1440 nigel 63 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1441 nigel 91 and an empty alternative is permitted (matching the empty string). The matching
1442     process tries each alternative in turn, from left to right, and the first one
1443     that succeeds is used. If the alternatives are within a subpattern
1444 nigel 75 .\" HTML <a href="#subpattern">
1445     .\" </a>
1446     (defined below),
1447     .\"
1448     "succeeds" means matching the rest of the main pattern as well as the
1449     alternative in the subpattern.
1450     .
1451     .
1452     .SH "INTERNAL OPTION SETTING"
1453 nigel 63 .rs
1454     .sp
1455     The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1456 ph10 231 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1457     the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1458     The option letters are
1459 nigel 75 .sp
1460 nigel 63 i for PCRE_CASELESS
1461     m for PCRE_MULTILINE
1462     s for PCRE_DOTALL
1463     x for PCRE_EXTENDED
1464 nigel 75 .sp
1465 nigel 63 For example, (?im) sets caseless, multiline matching. It is also possible to
1466     unset these options by preceding the letter with a hyphen, and a combined
1467     setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1468     PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1469     permitted. If a letter appears both before and after the hyphen, the option is
1470     unset.
1471 nigel 75 .P
1472 ph10 231 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1473     changed in the same way as the Perl-compatible options by using the characters
1474     J, U and X respectively.
1475     .P
1476 ph10 412 When one of these option changes occurs at top level (that is, not inside
1477     subpattern parentheses), the change applies to the remainder of the pattern
1478     that follows. If the change is placed right at the start of a pattern, PCRE
1479     extracts it into the global options (and it will therefore show up in data
1480     extracted by the \fBpcre_fullinfo()\fP function).
1481 nigel 75 .P
1482 nigel 93 An option change within a subpattern (see below for a description of
1483 ph10 572 subpatterns) affects only that part of the subpattern that follows it, so
1484 nigel 75 .sp
1485 nigel 63 (a(?i)b)c
1486 nigel 75 .sp
1487 nigel 63 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1488     By this means, options can be made to have different settings in different
1489     parts of the pattern. Any changes made in one alternative do carry on
1490     into subsequent branches within the same subpattern. For example,
1491 nigel 75 .sp
1492 nigel 63 (a(?i)b|c)
1493 nigel 75 .sp
1494 nigel 63 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1495     branch is abandoned before the option setting. This is because the effects of
1496     option settings happen at compile time. There would be some very weird
1497     behaviour otherwise.
1498 ph10 251 .P
1499     \fBNote:\fP There are other PCRE-specific options that can be set by the
1500 ph10 859 application when the compiling or matching functions are called. In some cases
1501     the pattern can contain special leading sequences such as (*CRLF) to override
1502     what the application has set or what has been defaulted. Details are given in
1503     the section entitled
1504 ph10 251 .\" HTML <a href="#newlineseq">
1505     .\" </a>
1506     "Newline sequences"
1507     .\"
1508 ph10 1219 above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
1509 chpe 1055 sequences that can be used to set UTF and Unicode property modes; they are
1510     equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
1511 ph10 1219 options, respectively. The (*UTF) sequence is a generic version that can be
1512 ph10 1335 used with any of the libraries. However, the application can set the
1513 ph10 1309 PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.
1514 nigel 75 .
1515     .
1516     .\" HTML <a name="subpattern"></a>
1517 nigel 63 .SH SUBPATTERNS
1518     .rs
1519     .sp
1520     Subpatterns are delimited by parentheses (round brackets), which can be nested.
1521 nigel 75 Turning part of a pattern into a subpattern does two things:
1522     .sp
1523 nigel 63 1. It localizes a set of alternatives. For example, the pattern
1524 nigel 75 .sp
1525 nigel 63 cat(aract|erpillar|)
1526 nigel 75 .sp
1527 ph10 572 matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1528     match "cataract", "erpillar" or an empty string.
1529 nigel 75 .sp
1530     2. It sets up the subpattern as a capturing subpattern. This means that, when
1531     the whole pattern matches, that portion of the subject string that matched the
1532 ph10 903 subpattern is passed back to the caller via the \fIovector\fP argument of the
1533     matching function. (This applies only to the traditional matching functions;
1534 ph10 859 the DFA matching functions do not support capturing.)
1535     .P
1536     Opening parentheses are counted from left to right (starting from 1) to obtain
1537     numbers for the capturing subpatterns. For example, if the string "the red
1538     king" is matched against the pattern
1539 nigel 75 .sp
1540 nigel 63 the ((red|white) (king|queen))
1541 nigel 75 .sp
1542 nigel 63 the captured substrings are "red king", "red", and "king", and are numbered 1,
1543     2, and 3, respectively.
1544 nigel 75 .P
1545 nigel 63 The fact that plain parentheses fulfil two functions is not always helpful.
1546     There are often times when a grouping subpattern is required without a
1547     capturing requirement. If an opening parenthesis is followed by a question mark
1548     and a colon, the subpattern does not do any capturing, and is not counted when
1549     computing the number of any subsequent capturing subpatterns. For example, if
1550     the string "the white queen" is matched against the pattern
1551 nigel 75 .sp
1552 nigel 63 the ((?:red|white) (king|queen))
1553 nigel 75 .sp
1554 nigel 63 the captured substrings are "white queen" and "queen", and are numbered 1 and
1555 nigel 93 2. The maximum number of capturing subpatterns is 65535.
1556 nigel 75 .P
1557 nigel 63 As a convenient shorthand, if any option settings are required at the start of
1558     a non-capturing subpattern, the option letters may appear between the "?" and
1559     the ":". Thus the two patterns
1560 nigel 75 .sp
1561 nigel 63 (?i:saturday|sunday)
1562     (?:(?i)saturday|sunday)
1563 nigel 75 .sp
1564 nigel 63 match exactly the same set of strings. Because alternative branches are tried
1565     from left to right, and options are not reset until the end of the subpattern
1566     is reached, an option setting in one branch does affect subsequent branches, so
1567     the above patterns match "SUNDAY" as well as "Saturday".
1568 nigel 75 .
1569     .
1570 ph10 456 .\" HTML <a name="dupsubpatternnumber"></a>
1571 ph10 175 .SH "DUPLICATE SUBPATTERN NUMBERS"
1572     .rs
1573     .sp
1574 ph10 182 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1575     the same numbers for its capturing parentheses. Such a subpattern starts with
1576     (?| and is itself a non-capturing subpattern. For example, consider this
1577 ph10 175 pattern:
1578     .sp
1579     (?|(Sat)ur|(Sun))day
1580 ph10 182 .sp
1581     Because the two alternatives are inside a (?| group, both sets of capturing
1582     parentheses are numbered one. Thus, when the pattern matches, you can look
1583     at captured substring number one, whichever alternative matched. This construct
1584     is useful when you want to capture part, but not all, of one of a number of
1585     alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1586 ph10 175 number is reset at the start of each branch. The numbers of any capturing
1587 ph10 572 parentheses that follow the subpattern start after the highest number used in
1588     any branch. The following example is taken from the Perl documentation. The
1589     numbers underneath show in which buffer the captured content will be stored.
1590 ph10 175 .sp
1591     # before ---------------branch-reset----------- after
1592     / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1593     # 1 2 2 3 2 3 4
1594 ph10 182 .sp
1595 ph10 488 A back reference to a numbered subpattern uses the most recent value that is
1596     set for that number by any subpattern. The following pattern matches "abcabc"
1597     or "defdef":
1598 ph10 456 .sp
1599 ph10 461 /(?|(abc)|(def))\e1/
1600 ph10 456 .sp
1601 ph10 716 In contrast, a subroutine call to a numbered subpattern always refers to the
1602     first one in the pattern with the given number. The following pattern matches
1603     "abcabc" or "defabc":
1604 ph10 456 .sp
1605     /(?|(abc)|(def))(?1)/
1606     .sp
1607 ph10 459 If a
1608     .\" HTML <a href="#conditions">
1609     .\" </a>
1610     condition test
1611     .\"
1612     for a subpattern's having matched refers to a non-unique number, the test is
1613     true if any of the subpatterns of that number have matched.
1614     .P
1615     An alternative approach to using this "branch reset" feature is to use
1616 ph10 175 duplicate named subpatterns, as described in the next section.
1617     .
1618     .
1619 nigel 75 .SH "NAMED SUBPATTERNS"
1620 nigel 63 .rs
1621     .sp
1622     Identifying capturing parentheses by number is simple, but it can be very hard
1623     to keep track of the numbers in complicated regular expressions. Furthermore,
1624 nigel 75 if an expression is modified, the numbers may change. To help with this
1625 nigel 93 difficulty, PCRE supports the naming of subpatterns. This feature was not
1626     added to Perl until release 5.10. Python had the feature earlier, and PCRE
1627     introduced it at release 4.0, using the Python syntax. PCRE now supports both
1628 ph10 459 the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1629     have different names, but PCRE does not.
1630 nigel 93 .P
1631     In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1632     (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1633 nigel 91 parentheses from other parts of the pattern, such as
1634     .\" HTML <a href="#backreferences">
1635     .\" </a>
1636 ph10 488 back references,
1637 nigel 91 .\"
1638     .\" HTML <a href="#recursion">
1639     .\" </a>
1640     recursion,
1641     .\"
1642     and
1643     .\" HTML <a href="#conditions">
1644     .\" </a>
1645     conditions,
1646     .\"
1647     can be made by name as well as by number.
1648 nigel 75 .P
1649 ph10 1404 Names consist of up to 32 alphanumeric characters and underscores, but must
1650 ph10 1394 start with a non-digit. Named capturing parentheses are still allocated numbers
1651     as well as names, exactly as if the names were not present. The PCRE API
1652     provides function calls for extracting the name-to-number translation table
1653     from a compiled pattern. There is also a convenience function for extracting a
1654     captured substring by name.
1655 nigel 91 .P
1656     By default, a name must be unique within a pattern, but it is possible to relax
1657 ph10 457 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1658 ph10 461 names are also always permitted for subpatterns with the same number, set up as
1659 ph10 457 described in the previous section.) Duplicate names can be useful for patterns
1660     where only one instance of the named parentheses can match. Suppose you want to
1661     match the name of a weekday, either as a 3-letter abbreviation or as the full
1662     name, and in both cases you want to extract the abbreviation. This pattern
1663     (ignoring the line breaks) does the job:
1664 nigel 91 .sp
1665 nigel 93 (?<DN>Mon|Fri|Sun)(?:day)?|
1666     (?<DN>Tue)(?:sday)?|
1667     (?<DN>Wed)(?:nesday)?|
1668     (?<DN>Thu)(?:rsday)?|
1669     (?<DN>Sat)(?:urday)?
1670 nigel 91 .sp
1671     There are five capturing substrings, but only one is ever set after a match.
1672 ph10 182 (An alternative way of solving this problem is to use a "branch reset"
1673 ph10 175 subpattern, as described in the previous section.)
1674     .P
1675 nigel 91 The convenience function for extracting the data by name returns the substring
1676 nigel 93 for the first (and in this example, the only) subpattern of that name that
1677 ph10 461 matched. This saves searching to find which numbered subpattern it was.
1678 ph10 459 .P
1679 ph10 488 If you make a back reference to a non-unique named subpattern from elsewhere in
1680 ph10 1404 the pattern, the subpatterns to which the name refers are checked in the order
1681     in which they appear in the overall pattern. The first one that is set is used
1682     for the reference. For example, this pattern matches both "foofoo" and
1683 ph10 1361 "barbar" but not "foobar" or "barfoo":
1684     .sp
1685 ph10 1404 (?:(?<n>foo)|(?<n>bar))\ek<n>
1686 ph10 1361 .sp
1687     .P
1688     If you make a subroutine call to a non-unique named subpattern, the one that
1689     corresponds to the first occurrence of the name is used. In the absence of
1690     duplicate numbers (see the previous section) this is the one with the lowest
1691     number.
1692     .P
1693     If you use a named reference in a condition
1694 ph10 459 test (see the
1695     .\"
1696     .\" HTML <a href="#conditions">
1697     .\" </a>
1698     section about conditions
1699     .\"
1700 ph10 461 below), either to check whether a subpattern has matched, or to check for
1701 ph10 459 recursion, all subpatterns with the same name are tested. If the condition is
1702     true for any one of them, the overall condition is true. This is the same
1703     behaviour as testing by number. For further details of the interfaces for
1704     handling named subpatterns, see the
1705 nigel 63 .\" HREF
1706 nigel 75 \fBpcreapi\fP
1707 nigel 63 .\"
1708     documentation.
1709 ph10 385 .P
1710     \fBWarning:\fP You cannot use different names to distinguish between two
1711 ph10 457 subpatterns with the same number because PCRE uses only the numbers when
1712     matching. For this reason, an error is given at compile time if different names
1713 ph10 1361 are given to subpatterns with the same number. However, you can always give the
1714     same name to subpatterns with the same number, even when PCRE_DUPNAMES is not
1715     set.
1716 nigel 75 .
1717     .
1718 nigel 63 .SH REPETITION
1719     .rs
1720     .sp
1721     Repetition is specified by quantifiers, which can follow any of the following
1722     items:
1723 nigel 75 .sp
1724 nigel 63 a literal data character
1725 nigel 93 the dot metacharacter
1726 nigel 75 the \eC escape sequence
1727 ph10 859 the \eX escape sequence
1728 nigel 93 the \eR escape sequence
1729 ph10 572 an escape such as \ed or \epL that matches a single character
1730 nigel 63 a character class
1731     a back reference (see next section)
1732 ph10 637 a parenthesized subpattern (including assertions)
1733 ph10 716 a subroutine call to a subpattern (recursive or otherwise)
1734 nigel 75 .sp
1735 nigel 63 The general repetition quantifier specifies a minimum and maximum number of
1736     permitted matches, by giving the two numbers in curly brackets (braces),
1737     separated by a comma. The numbers must be less than 65536, and the first must
1738     be less than or equal to the second. For example:
1739 nigel 75 .sp
1740 nigel 63 z{2,4}
1741 nigel 75 .sp
1742 nigel 63 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1743     character. If the second number is omitted, but the comma is present, there is
1744     no upper limit; if the second number and the comma are both omitted, the
1745     quantifier specifies an exact number of required matches. Thus
1746 nigel 75 .sp
1747 nigel 63 [aeiou]{3,}
1748 nigel 75 .sp
1749 nigel 63 matches at least 3 successive vowels, but may match many more, while
1750 nigel 75 .sp
1751     \ed{8}
1752     .sp
1753 nigel 63 matches exactly 8 digits. An opening curly bracket that appears in a position
1754     where a quantifier is not allowed, or one that does not match the syntax of a
1755     quantifier, is taken as a literal character. For example, {,6} is not a
1756     quantifier, but a literal string of four characters.
1757 nigel 75 .P
1758 ph10 859 In UTF modes, quantifiers apply to characters rather than to individual data
1759     units. Thus, for example, \ex{100}{2} matches two characters, each of
1760     which is represented by a two-byte sequence in a UTF-8 string. Similarly,
1761 ph10 1011 \eX{3} matches three Unicode extended grapheme clusters, each of which may be
1762     several data units long (and they may be of different lengths).
1763 nigel 75 .P
1764 nigel 63 The quantifier {0} is permitted, causing the expression to behave as if the
1765 ph10 345 previous item and the quantifier were not present. This may be useful for
1766     subpatterns that are referenced as
1767 ph10 335 .\" HTML <a href="#subpatternsassubroutines">
1768     .\" </a>
1769     subroutines
1770     .\"
1771 ph10 572 from elsewhere in the pattern (but see also the section entitled
1772     .\" HTML <a href="#subdefine">
1773     .\" </a>
1774     "Defining subpatterns for use by reference only"
1775     .\"
1776     below). Items other than subpatterns that have a {0} quantifier are omitted
1777     from the compiled pattern.
1778 nigel 75 .P
1779 nigel 93 For convenience, the three most common quantifiers have single-character
1780     abbreviations:
1781 nigel 75 .sp
1782 nigel 63 * is equivalent to {0,}
1783     + is equivalent to {1,}
1784     ? is equivalent to {0,1}
1785 nigel 75 .sp
1786 nigel 63 It is possible to construct infinite loops by following a subpattern that can
1787     match no characters with a quantifier that has no upper limit, for example:
1788 nigel 75 .sp
1789 nigel 63 (a?)*
1790 nigel 75 .sp
1791 nigel 63 Earlier versions of Perl and PCRE used to give an error at compile time for
1792     such patterns. However, because there are cases where this can be useful, such
1793     patterns are now accepted, but if any repetition of the subpattern does in fact
1794     match no characters, the loop is forcibly broken.
1795 nigel 75 .P
1796 nigel 63 By default, the quantifiers are "greedy", that is, they match as much as
1797     possible (up to the maximum number of permitted times), without causing the
1798     rest of the pattern to fail. The classic example of where this gives problems
1799 nigel 75 is in trying to match comments in C programs. These appear between /* and */
1800     and within the comment, individual * and / characters may appear. An attempt to
1801     match C comments by applying the pattern
1802     .sp
1803     /\e*.*\e*/
1804     .sp
1805 nigel 63 to the string
1806 nigel 75 .sp
1807     /* first comment */ not comment /* second comment */
1808     .sp
1809 nigel 63 fails, because it matches the entire string owing to the greediness of the .*
1810     item.
1811 nigel 75 .P
1812 nigel 63 However, if a quantifier is followed by a question mark, it ceases to be
1813     greedy, and instead matches the minimum number of times possible, so the
1814     pattern
1815 nigel 75 .sp
1816     /\e*.*?\e*/
1817     .sp
1818 nigel 63 does the right thing with the C comments. The meaning of the various
1819     quantifiers is not otherwise changed, just the preferred number of matches.
1820     Do not confuse this use of question mark with its use as a quantifier in its
1821     own right. Because it has two uses, it can sometimes appear doubled, as in
1822 nigel 75 .sp
1823     \ed??\ed
1824     .sp
1825 nigel 63 which matches one digit by preference, but can match two if that is the only
1826     way the rest of the pattern matches.
1827 nigel 75 .P
1828 nigel 93 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1829 nigel 63 the quantifiers are not greedy by default, but individual ones can be made
1830     greedy by following them with a question mark. In other words, it inverts the
1831     default behaviour.
1832 nigel 75 .P
1833 nigel 63 When a parenthesized subpattern is quantified with a minimum repeat count that
1834 nigel 75 is greater than 1 or with a limited maximum, more memory is required for the
1835 nigel 63 compiled pattern, in proportion to the size of the minimum or maximum.
1836 nigel 75 .P
1837 nigel 63 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1838 nigel 93 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1839 nigel 63 implicitly anchored, because whatever follows will be tried against every
1840     character position in the subject string, so there is no point in retrying the
1841     overall match at any position after the first. PCRE normally treats such a
1842 nigel 75 pattern as though it were preceded by \eA.
1843     .P
1844 nigel 63 In cases where it is known that the subject string contains no newlines, it is
1845     worth setting PCRE_DOTALL in order to obtain this optimization, or
1846     alternatively using ^ to indicate anchoring explicitly.
1847 nigel 75 .P
1848 ph10 994 However, there are some cases where the optimization cannot be used. When .*
1849 ph10 488 is inside capturing parentheses that are the subject of a back reference
1850 nigel 93 elsewhere in the pattern, a match at the start may fail where a later one
1851     succeeds. Consider, for example:
1852 nigel 75 .sp
1853     (.*)abc\e1
1854     .sp
1855 nigel 63 If the subject is "xyz123abc123" the match point is the fourth character. For
1856     this reason, such a pattern is not implicitly anchored.
1857 nigel 75 .P
1858 ph10 994 Another case where implicit anchoring is not applied is when the leading .* is
1859     inside an atomic group. Once again, a match at the start may fail where a later
1860     one succeeds. Consider this pattern:
1861     .sp
1862     (?>.*?a)b
1863     .sp
1864 ph10 1221 It matches "ab" in the subject "aab". The use of the backtracking control verbs
1865 ph10 994 (*PRUNE) and (*SKIP) also disable this optimization.
1866     .P
1867 nigel 63 When a capturing subpattern is repeated, the value captured is the substring
1868     that matched the final iteration. For example, after
1869 nigel 75 .sp
1870     (tweedle[dume]{3}\es*)+
1871     .sp
1872 nigel 63 has matched "tweedledum tweedledee" the value of the captured substring is
1873     "tweedledee". However, if there are nested capturing subpatterns, the
1874     corresponding captured values may have been set in previous iterations. For
1875     example, after
1876 nigel 75 .sp
1877 nigel 63 /(a|(b))+/
1878 nigel 75 .sp
1879 nigel 63 matches "aba" the value of the second captured substring is "b".
1880 nigel 75 .
1881     .
1882     .\" HTML <a name="atomicgroup"></a>
1883     .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1884 nigel 63 .rs
1885     .sp
1886 nigel 93 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1887     repetition, failure of what follows normally causes the repeated item to be
1888     re-evaluated to see if a different number of repeats allows the rest of the
1889     pattern to match. Sometimes it is useful to prevent this, either to change the
1890     nature of the match, or to cause it fail earlier than it otherwise might, when
1891     the author of the pattern knows there is no point in carrying on.
1892 nigel 75 .P
1893     Consider, for example, the pattern \ed+foo when applied to the subject line
1894     .sp
1895 nigel 63 123456bar
1896 nigel 75 .sp
1897 nigel 63 After matching all 6 digits and then failing to match "foo", the normal
1898 nigel 75 action of the matcher is to try again with only 5 digits matching the \ed+
1899 nigel 63 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1900     (a term taken from Jeffrey Friedl's book) provides the means for specifying
1901     that once a subpattern has matched, it is not to be re-evaluated in this way.
1902 nigel 75 .P
1903 nigel 93 If we use atomic grouping for the previous example, the matcher gives up
1904 nigel 63 immediately on failing to match "foo" the first time. The notation is a kind of
1905     special parenthesis, starting with (?> as in this example:
1906 nigel 75 .sp
1907     (?>\ed+)foo
1908     .sp
1909 nigel 63 This kind of parenthesis "locks up" the part of the pattern it contains once
1910     it has matched, and a failure further into the pattern is prevented from
1911     backtracking into it. Backtracking past it to previous items, however, works as
1912     normal.
1913 nigel 75 .P
1914 nigel 63 An alternative description is that a subpattern of this type matches the string
1915     of characters that an identical standalone pattern would match, if anchored at
1916     the current point in the subject string.
1917 nigel 75 .P
1918 nigel 63 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1919     the above example can be thought of as a maximizing repeat that must swallow
1920 nigel 75 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1921 nigel 63 number of digits they match in order to make the rest of the pattern match,
1922 nigel 75 (?>\ed+) can only match an entire sequence of digits.
1923     .P
1924 nigel 63 Atomic groups in general can of course contain arbitrarily complicated
1925     subpatterns, and can be nested. However, when the subpattern for an atomic
1926     group is just a single repeated item, as in the example above, a simpler
1927     notation, called a "possessive quantifier" can be used. This consists of an
1928     additional + character following a quantifier. Using this notation, the
1929     previous example can be rewritten as
1930 nigel 75 .sp
1931     \ed++foo
1932     .sp
1933 ph10 208 Note that a possessive quantifier can be used with an entire group, for
1934     example:
1935     .sp
1936     (abc|xyz){2,3}+
1937     .sp
1938 nigel 63 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1939     option is ignored. They are a convenient notation for the simpler forms of
1940 nigel 93 atomic group. However, there is no difference in the meaning of a possessive
1941     quantifier and the equivalent atomic group, though there may be a performance
1942     difference; possessive quantifiers should be slightly faster.
1943 nigel 75 .P
1944 nigel 93 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1945     Jeffrey Friedl originated the idea (and the name) in the first edition of his
1946     book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1947     package, and PCRE copied it from there. It ultimately found its way into Perl
1948     at release 5.10.
1949 nigel 75 .P
1950 nigel 93 PCRE has an optimization that automatically "possessifies" certain simple
1951     pattern constructs. For example, the sequence A+B is treated as A++B because
1952     there is no point in backtracking into a sequence of A's when B must follow.
1953     .P
1954 nigel 63 When a pattern contains an unlimited repeat inside a subpattern that can itself
1955     be repeated an unlimited number of times, the use of an atomic group is the
1956     only way to avoid some failing matches taking a very long time indeed. The
1957     pattern
1958 nigel 75 .sp
1959     (\eD+|<\ed+>)*[!?]
1960     .sp
1961 nigel 63 matches an unlimited number of substrings that either consist of non-digits, or
1962     digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1963     quickly. However, if it is applied to
1964 nigel 75 .sp
1965 nigel 63 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1966 nigel 75 .sp
1967 nigel 63 it takes a long time before reporting failure. This is because the string can
1968 nigel 75 be divided between the internal \eD+ repeat and the external * repeat in a
1969     large number of ways, and all have to be tried. (The example uses [!?] rather
1970     than a single character at the end, because both PCRE and Perl have an
1971     optimization that allows for fast failure when a single character is used. They
1972     remember the last single character that is required for a match, and fail early
1973     if it is not present in the string.) If the pattern is changed so that it uses
1974     an atomic group, like this:
1975     .sp
1976     ((?>\eD+)|<\ed+>)*[!?]
1977     .sp
1978 nigel 63 sequences of non-digits cannot be broken, and failure happens quickly.
1979 nigel 75 .
1980     .
1981     .\" HTML <a name="backreferences"></a>
1982     .SH "BACK REFERENCES"
1983 nigel 63 .rs
1984     .sp
1985     Outside a character class, a backslash followed by a digit greater than 0 (and
1986     possibly further digits) is a back reference to a capturing subpattern earlier
1987     (that is, to its left) in the pattern, provided there have been that many
1988     previous capturing left parentheses.
1989 nigel 75 .P
1990 nigel 63 However, if the decimal number following the backslash is less than 10, it is
1991     always taken as a back reference, and causes an error only if there are not
1992     that many capturing left parentheses in the entire pattern. In other words, the
1993     parentheses that are referenced need not be to the left of the reference for
1994 nigel 91 numbers less than 10. A "forward back reference" of this type can make sense
1995     when a repetition is involved and the subpattern to the right has participated
1996     in an earlier iteration.
1997     .P
1998 nigel 93 It is not possible to have a numerical "forward back reference" to a subpattern
1999     whose number is 10 or more using this syntax because a sequence such as \e50 is
2000     interpreted as a character defined in octal. See the subsection entitled
2001 nigel 91 "Non-printing characters"
2002 nigel 75 .\" HTML <a href="#digitsafterbackslash">
2003     .\" </a>
2004     above
2005     .\"
2006 nigel 93 for further details of the handling of digits following a backslash. There is
2007     no such problem when named parentheses are used. A back reference to any
2008     subpattern is possible using named parentheses (see below).
2009 nigel 75 .P
2010 nigel 93 Another way of avoiding the ambiguity inherent in the use of digits following a
2011 ph10 572 backslash is to use the \eg escape sequence. This escape must be followed by an
2012     unsigned number or a negative number, optionally enclosed in braces. These
2013     examples are all identical:
2014 nigel 93 .sp
2015     (ring), \e1
2016     (ring), \eg1
2017     (ring), \eg{1}
2018     .sp
2019 ph10 208 An unsigned number specifies an absolute reference without the ambiguity that
2020     is present in the older syntax. It is also useful when literal digits follow
2021     the reference. A negative number is a relative reference. Consider this
2022     example:
2023 nigel 93 .sp
2024     (abc(def)ghi)\eg{-1}
2025     .sp
2026     The sequence \eg{-1} is a reference to the most recently started capturing
2027 ph10 572 subpattern before \eg, that is, is it equivalent to \e2 in this example.
2028     Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
2029     can be helpful in long patterns, and also in patterns that are created by
2030     joining together fragments that contain references within themselves.
2031 nigel 93 .P
2032 nigel 63 A back reference matches whatever actually matched the capturing subpattern in
2033     the current subject string, rather than anything matching the subpattern
2034     itself (see
2035     .\" HTML <a href="#subpatternsassubroutines">
2036     .\" </a>
2037     "Subpatterns as subroutines"
2038     .\"
2039     below for a way of doing that). So the pattern
2040 nigel 75 .sp
2041     (sens|respons)e and \e1ibility
2042     .sp
2043 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
2044     "sense and responsibility". If caseful matching is in force at the time of the
2045     back reference, the case of letters is relevant. For example,
2046 nigel 75 .sp
2047     ((?i)rah)\es+\e1
2048     .sp
2049 nigel 63 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
2050     capturing subpattern is matched caselessly.
2051 nigel 75 .P
2052 ph10 171 There are several different ways of writing back references to named
2053     subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
2054     \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
2055     back reference syntax, in which \eg can be used for both numeric and named
2056     references, is also supported. We could rewrite the above example in any of
2057 nigel 93 the following ways:
2058 nigel 75 .sp
2059 nigel 93 (?<p1>(?i)rah)\es+\ek<p1>
2060 ph10 171 (?'p1'(?i)rah)\es+\ek{p1}
2061 nigel 91 (?P<p1>(?i)rah)\es+(?P=p1)
2062 ph10 171 (?<p1>(?i)rah)\es+\eg{p1}
2063 nigel 75 .sp
2064 nigel 91 A subpattern that is referenced by name may appear in the pattern before or
2065     after the reference.
2066     .P
2067 nigel 63 There may be more than one back reference to the same subpattern. If a
2068     subpattern has not actually been used in a particular match, any back
2069 ph10 456 references to it always fail by default. For example, the pattern
2070 nigel 75 .sp
2071     (a|(bc))\e2
2072     .sp
2073 ph10 461 always fails if it starts to match "a" rather than "bc". However, if the
2074     PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
2075 ph10 456 unset value matches an empty string.
2076     .P
2077     Because there may be many capturing parentheses in a pattern, all digits
2078     following a backslash are taken as part of a potential back reference number.
2079     If the pattern continues with a digit character, some delimiter must be used to
2080     terminate the back reference. If the PCRE_EXTENDED option is set, this can be
2081 ph10 968 white space. Otherwise, the \eg{ syntax or an empty comment (see
2082 nigel 75 .\" HTML <a href="#comments">
2083     .\" </a>
2084     "Comments"
2085     .\"
2086     below) can be used.
2087 ph10 488 .
2088     .SS "Recursive back references"
2089     .rs
2090     .sp
2091 nigel 63 A back reference that occurs inside the parentheses to which it refers fails
2092 nigel 75 when the subpattern is first used, so, for example, (a\e1) never matches.
2093 nigel 63 However, such references can be useful inside repeated subpatterns. For
2094     example, the pattern
2095 nigel 75 .sp
2096     (a|b\e1)+
2097     .sp
2098 nigel 63 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
2099     the subpattern, the back reference matches the character string corresponding
2100     to the previous iteration. In order for this to work, the pattern must be such
2101     that the first iteration does not need to match the back reference. This can be
2102     done using alternation, as in the example above, or by a quantifier with a
2103     minimum of zero.
2104 ph10 488 .P
2105     Back references of this type cause the group that they reference to be treated
2106     as an
2107     .\" HTML <a href="#atomicgroup">
2108     .\" </a>
2109     atomic group.
2110     .\"
2111     Once the whole group has been matched, a subsequent matching failure cannot
2112     cause backtracking into the middle of the group.
2113 nigel 75 .
2114     .
2115     .\" HTML <a name="bigassertions"></a>
2116 nigel 63 .SH ASSERTIONS
2117     .rs
2118     .sp
2119     An assertion is a test on the characters following or preceding the current
2120     matching point that does not actually consume any characters. The simple
2121 nigel 75 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
2122     .\" HTML <a href="#smallassertions">
2123     .\" </a>
2124     above.
2125     .\"
2126     .P
2127 nigel 63 More complicated assertions are coded as subpatterns. There are two kinds:
2128     those that look ahead of the current position in the subject string, and those
2129 nigel 75 that look behind it. An assertion subpattern is matched in the normal way,
2130     except that it does not cause the current matching position to be changed.
2131     .P
2132 ph10 637 Assertion subpatterns are not capturing subpatterns. If such an assertion
2133     contains capturing subpatterns within it, these are counted for the purposes of
2134     numbering the capturing subpatterns in the whole pattern. However, substring
2135 ph10 1335 capturing is carried out only for positive assertions. (Perl sometimes, but not
2136 ph10 1292 always, does do capturing in negative assertions.)
2137 ph10 637 .P
2138 ph10 643 For compatibility with Perl, assertion subpatterns may be repeated; though
2139 ph10 654 it makes no sense to assert the same thing several times, the side effect of
2140 ph10 643 capturing parentheses may occasionally be useful. In practice, there only three
2141     cases:
2142 ph10 637 .sp
2143 ph10 654 (1) If the quantifier is {0}, the assertion is never obeyed during matching.
2144     However, it may contain internal capturing parenthesized groups that are called
2145 ph10 637 from elsewhere via the
2146     .\" HTML <a href="#subpatternsassubroutines">
2147     .\" </a>
2148     subroutine mechanism.
2149     .\"
2150     .sp
2151 ph10 654 (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
2152     were {0,1}. At run time, the rest of the pattern match is tried with and
2153 ph10 637 without the assertion, the order depending on the greediness of the quantifier.
2154     .sp
2155 ph10 654 (3) If the minimum repetition is greater than zero, the quantifier is ignored.
2156 ph10 637 The assertion is obeyed just once when encountered during matching.
2157 nigel 75 .
2158     .
2159     .SS "Lookahead assertions"
2160     .rs
2161     .sp
2162 nigel 91 Lookahead assertions start with (?= for positive assertions and (?! for
2163     negative assertions. For example,
2164 nigel 75 .sp
2165     \ew+(?=;)
2166     .sp
2167 nigel 63 matches a word followed by a semicolon, but does not include the semicolon in
2168     the match, and
2169 nigel 75 .sp
2170 nigel 63 foo(?!bar)
2171 nigel 75 .sp
2172 nigel 63 matches any occurrence of "foo" that is not followed by "bar". Note that the
2173     apparently similar pattern
2174 nigel 75 .sp
2175 nigel 63 (?!foo)bar
2176 nigel 75 .sp
2177 nigel 63 does not find an occurrence of "bar" that is preceded by something other than
2178     "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
2179     (?!foo) is always true when the next three characters are "bar". A
2180 nigel 75 lookbehind assertion is needed to achieve the other effect.
2181     .P
2182 nigel 63 If you want to force a matching failure at some point in a pattern, the most
2183     convenient way to do it is with (?!) because an empty string always matches, so
2184     an assertion that requires there not to be an empty string must always fail.
2185 ph10 572 The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
2186 nigel 75 .
2187     .
2188     .\" HTML <a name="lookbehind"></a>
2189     .SS "Lookbehind assertions"
2190     .rs
2191     .sp
2192 nigel 63 Lookbehind assertions start with (?<= for positive assertions and (?<! for
2193     negative assertions. For example,
2194 nigel 75 .sp
2195 nigel 63 (?<!foo)bar
2196 nigel 75 .sp
2197 nigel 63 does find an occurrence of "bar" that is not preceded by "foo". The contents of
2198     a lookbehind assertion are restricted such that all the strings it matches must
2199 nigel 91 have a fixed length. However, if there are several top-level alternatives, they
2200     do not all have to have the same fixed length. Thus
2201 nigel 75 .sp
2202 nigel 63 (?<=bullock|donkey)
2203 nigel 75 .sp
2204 nigel 63 is permitted, but
2205 nigel 75 .sp
2206 nigel 63 (?<!dogs?|cats?)
2207 nigel 75 .sp
2208 nigel 63 causes an error at compile time. Branches that match different length strings
2209     are permitted only at the top level of a lookbehind assertion. This is an
2210 ph10 572 extension compared with Perl, which requires all branches to match the same
2211     length of string. An assertion such as
2212 nigel 75 .sp
2213 nigel 63 (?<=ab(c|de))
2214 nigel 75 .sp
2215 nigel 63 is not permitted, because its single top-level branch can match two different
2216 ph10 454 lengths, but it is acceptable to PCRE if rewritten to use two top-level
2217     branches:
2218 nigel 75 .sp
2219 nigel 63 (?<=abc|abde)
2220 nigel 75 .sp
2221 ph10 572 In some cases, the escape sequence \eK
2222 ph10 168 .\" HTML <a href="#resetmatchstart">
2223     .\" </a>
2224     (see above)
2225     .\"
2226 ph10 461 can be used instead of a lookbehind assertion to get round the fixed-length
2227 ph10 454 restriction.
2228 ph10 168 .P
2229 nigel 63 The implementation of lookbehind assertions is, for each alternative, to
2230 nigel 93 temporarily move the current position back by the fixed length and then try to
2231 nigel 63 match. If there are insufficient characters before the current position, the
2232 nigel 93 assertion fails.
2233 nigel 75 .P
2234 ph10 859 In a UTF mode, PCRE does not allow the \eC escape (which matches a single data
2235     unit even in a UTF mode) to appear in lookbehind assertions, because it makes
2236     it impossible to calculate the length of the lookbehind. The \eX and \eR
2237     escapes, which can match different numbers of data units, are also not
2238     permitted.
2239 nigel 75 .P
2240 ph10 454 .\" HTML <a href="#subpatternsassubroutines">
2241     .\" </a>
2242     "Subroutine"
2243     .\"
2244     calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2245 ph10 461 as the subpattern matches a fixed-length string.
2246 ph10 454 .\" HTML <a href="#recursion">
2247     .\" </a>
2248     Recursion,
2249     .\"
2250     however, is not supported.
2251     .P
2252 nigel 93 Possessive quantifiers can be used in conjunction with lookbehind assertions to
2253 ph10 456 specify efficient matching of fixed-length strings at the end of subject
2254     strings. Consider a simple pattern such as
2255 nigel 75 .sp
2256 nigel 63 abcd$
2257 nigel 75 .sp
2258 nigel 63 when applied to a long string that does not match. Because matching proceeds
2259     from left to right, PCRE will look for each "a" in the subject and then see if
2260     what follows matches the rest of the pattern. If the pattern is specified as
2261 nigel 75 .sp
2262 nigel 63 ^.*abcd$
2263 nigel 75 .sp
2264 nigel 63 the initial .* matches the entire string at first, but when this fails (because
2265     there is no following "a"), it backtracks to match all but the last character,
2266     then all but the last two characters, and so on. Once again the search for "a"
2267     covers the entire string, from right to left, so we are no better off. However,
2268     if the pattern is written as
2269 nigel 75 .sp
2270 nigel 63 ^.*+(?<=abcd)
2271 nigel 75 .sp
2272 nigel 93 there can be no backtracking for the .*+ item; it can match only the entire
2273 nigel 63 string. The subsequent lookbehind assertion does a single test on the last four
2274     characters. If it fails, the match fails immediately. For long strings, this
2275     approach makes a significant difference to the processing time.
2276 nigel 75 .
2277     .
2278     .SS "Using multiple assertions"
2279     .rs
2280     .sp
2281 nigel 63 Several assertions (of any sort) may occur in succession. For example,
2282 nigel 75 .sp
2283     (?<=\ed{3})(?<!999)foo
2284     .sp
2285 nigel 63 matches "foo" preceded by three digits that are not "999". Notice that each of
2286     the assertions is applied independently at the same point in the subject
2287     string. First there is a check that the previous three characters are all
2288     digits, and then there is a check that the same three characters are not "999".
2289 nigel 75 This pattern does \fInot\fP match "foo" preceded by six characters, the first
2290 nigel 63 of which are digits and the last three of which are not "999". For example, it
2291     doesn't match "123abcfoo". A pattern to do that is
2292 nigel 75 .sp
2293     (?<=\ed{3}...)(?<!999)foo
2294     .sp
2295 nigel 63 This time the first assertion looks at the preceding six characters, checking
2296     that the first three are digits, and then the second assertion checks that the
2297     preceding three characters are not "999".
2298 nigel 75 .P
2299 nigel 63 Assertions can be nested in any combination. For example,
2300 nigel 75 .sp
2301 nigel 63 (?<=(?<!foo)bar)baz
2302 nigel 75 .sp
2303 nigel 63 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
2304     preceded by "foo", while
2305 nigel 75 .sp
2306     (?<=\ed{3}(?!999)...)foo
2307     .sp
2308     is another pattern that matches "foo" preceded by three digits and any three
2309 nigel 63 characters that are not "999".
2310 nigel 75 .
2311     .
2312 nigel 91 .\" HTML <a name="conditions"></a>
2313 nigel 75 .SH "CONDITIONAL SUBPATTERNS"
2314 nigel 63 .rs
2315     .sp
2316     It is possible to cause the matching process to obey a subpattern
2317     conditionally or to choose between two alternative subpatterns, depending on
2318 ph10 461 the result of an assertion, or whether a specific capturing subpattern has
2319 ph10 456 already been matched. The two possible forms of conditional subpattern are:
2320 nigel 75 .sp
2321 nigel 63 (?(condition)yes-pattern)
2322     (?(condition)yes-pattern|no-pattern)
2323 nigel 75 .sp
2324 nigel 63 If the condition is satisfied, the yes-pattern is used; otherwise the
2325     no-pattern (if present) is used. If there are more than two alternatives in the
2326 ph10 557 subpattern, a compile-time error occurs. Each of the two alternatives may
2327 ph10 579 itself contain nested subpatterns of any form, including conditional
2328 ph10 557 subpatterns; the restriction to two alternatives applies only at the level of
2329 ph10 579 the condition. This pattern fragment is an example where the alternatives are
2330 ph10 557 complex:
2331     .sp
2332     (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2333     .sp
2334 nigel 75 .P
2335 nigel 93 There are four kinds of condition: references to subpatterns, references to
2336     recursion, a pseudo-condition called DEFINE, and assertions.
2337     .
2338     .SS "Checking for a used subpattern by number"
2339     .rs
2340     .sp
2341     If the text between the parentheses consists of a sequence of digits, the
2342 ph10 456 condition is true if a capturing subpattern of that number has previously
2343 ph10 461 matched. If there is more than one capturing subpattern with the same number
2344     (see the earlier
2345 ph10 456 .\"
2346     .\" HTML <a href="#recursion">
2347     .\" </a>
2348     section about duplicate subpattern numbers),
2349     .\"
2350 ph10 572 the condition is true if any of them have matched. An alternative notation is
2351 ph10 456 to precede the digits with a plus or minus sign. In this case, the subpattern
2352     number is relative rather than absolute. The most recently opened parentheses
2353 ph10 572 can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2354 ph10 579 loops it can also make sense to refer to subsequent groups. The next
2355 ph10 572 parentheses to be opened can be referenced as (?(+1), and so on. (The value
2356     zero in any of these forms is not used; it provokes a compile-time error.)
2357 nigel 91 .P
2358     Consider the following pattern, which contains non-significant white space to
2359     make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2360     three parts for ease of discussion:
2361 nigel 75 .sp
2362     ( \e( )? [^()]+ (?(1) \e) )
2363     .sp
2364 nigel 63 The first part matches an optional opening parenthesis, and if that
2365     character is present, sets it as the first captured substring. The second part
2366     matches one or more characters that are not parentheses. The third part is a
2367 ph10 572 conditional subpattern that tests whether or not the first set of parentheses
2368     matched. If they did, that is, if subject started with an opening parenthesis,
2369 nigel 63 the condition is true, and so the yes-pattern is executed and a closing
2370     parenthesis is required. Otherwise, since no-pattern is not present, the
2371     subpattern matches nothing. In other words, this pattern matches a sequence of
2372 nigel 93 non-parentheses, optionally enclosed in parentheses.
2373 ph10 167 .P
2374 ph10 172 If you were embedding this pattern in a larger one, you could use a relative
2375 ph10 167 reference:
2376     .sp
2377     ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ...
2378     .sp
2379     This makes the fragment independent of the parentheses in the larger pattern.
2380 nigel 93 .
2381     .SS "Checking for a used subpattern by name"
2382     .rs
2383 nigel 91 .sp
2384 nigel 93 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2385     subpattern by name. For compatibility with earlier versions of PCRE, which had
2386 ph10 1404 this facility before Perl, the syntax (?(name)...) is also recognized.
2387 nigel 93 .P
2388     Rewriting the above example to use a named subpattern gives this:
2389 nigel 91 .sp
2390 nigel 93 (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) )
2391     .sp
2392 ph10 461 If the name used in a condition of this kind is a duplicate, the test is
2393     applied to all subpatterns of the same name, and is true if any one of them has
2394 ph10 459 matched.
2395 nigel 93 .
2396     .SS "Checking for pattern recursion"
2397     .rs
2398     .sp
2399 nigel 91 If the condition is the string (R), and there is no subpattern with the name R,
2400 nigel 93 the condition is true if a recursive call to the whole pattern or any
2401     subpattern has been made. If digits or a name preceded by ampersand follow the
2402     letter R, for example:
2403     .sp
2404     (?(R3)...) or (?(R&name)...)
2405     .sp
2406 ph10 456 the condition is true if the most recent recursion is into a subpattern whose
2407 nigel 93 number or name is given. This condition does not check the entire recursion
2408 ph10 461 stack. If the name used in a condition of this kind is a duplicate, the test is
2409     applied to all subpatterns of the same name, and is true if any one of them is
2410     the most recent recursion.
2411 nigel 75 .P
2412 ph10 461 At "top level", all these recursion test conditions are false.
2413 ph10 454 .\" HTML <a href="#recursion">
2414     .\" </a>
2415 ph10 459 The syntax for recursive patterns
2416 ph10 454 .\"
2417 ph10 459 is described below.
2418 nigel 93 .
2419 ph10 572 .\" HTML <a name="subdefine"></a>
2420 nigel 93 .SS "Defining subpatterns for use by reference only"
2421     .rs
2422     .sp
2423     If the condition is the string (DEFINE), and there is no subpattern with the
2424     name DEFINE, the condition is always false. In this case, there may be only one
2425     alternative in the subpattern. It is always skipped if control reaches this
2426     point in the pattern; the idea of DEFINE is that it can be used to define
2427 ph10 716 subroutines that can be referenced from elsewhere. (The use of
2428 ph10 454 .\" HTML <a href="#subpatternsassubroutines">
2429     .\" </a>
2430 ph10 716 subroutines
2431 ph10 454 .\"
2432 ph10 572 is described below.) For example, a pattern to match an IPv4 address such as
2433 ph10 968 "192.168.23.245" could be written like this (ignore white space and line
2434 ph10 572 breaks):
2435 nigel 93 .sp
2436     (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2437     \eb (?&byte) (\e.(?&byte)){3} \eb
2438     .sp
2439     The first part of the pattern is a DEFINE group inside which a another group
2440     named "byte" is defined. This matches an individual component of an IPv4
2441     address (a number less than 256). When matching takes place, this part of the
2442 ph10 456 pattern is skipped because DEFINE acts like a false condition. The rest of the
2443     pattern uses references to the named group to match the four dot-separated
2444     components of an IPv4 address, insisting on a word boundary at each end.
2445 nigel 93 .
2446     .SS "Assertion conditions"
2447     .rs
2448     .sp
2449     If the condition is not in any of the above formats, it must be an assertion.
2450 nigel 63 This may be a positive or negative lookahead or lookbehind assertion. Consider
2451     this pattern, again containing non-significant white space, and with the two
2452     alternatives on the second line:
2453 nigel 75 .sp
2454 nigel 63 (?(?=[^a-z]*[a-z])
2455 nigel 75 \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
2456     .sp
2457 nigel 63 The condition is a positive lookahead assertion that matches an optional
2458     sequence of non-letters followed by a letter. In other words, it tests for the
2459     presence of at least one letter in the subject. If a letter is found, the
2460     subject is matched against the first alternative; otherwise it is matched
2461     against the second. This pattern matches strings in one of the two forms
2462     dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2463 nigel 75 .
2464     .
2465     .\" HTML <a name="comments"></a>
2466 nigel 63 .SH COMMENTS
2467     .rs
2468     .sp
2469 ph10 579 There are two ways of including comments in patterns that are processed by
2470 ph10 562 PCRE. In both cases, the start of the comment must not be in a character class,
2471     nor in the middle of any other sequence of related characters such as (?: or a
2472     subpattern name or number. The characters that make up a comment play no part
2473     in the pattern matching.
2474     .P
2475 nigel 75 The sequence (?# marks the start of a comment that continues up to the next
2476 ph10 562 closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2477     option is set, an unescaped # character also introduces a comment, which in
2478     this case continues to immediately after the next newline character or
2479     character sequence in the pattern. Which characters are interpreted as newlines
2480 ph10 859 is controlled by the options passed to a compiling function or by a special
2481 ph10 562 sequence at the start of the pattern, as described in the section entitled
2482 ph10 572 .\" HTML <a href="#newlines">
2483 ph10 556 .\" </a>
2484     "Newline conventions"
2485     .\"
2486 ph10 572 above. Note that the end of this type of comment is a literal newline sequence
2487     in the pattern; escape sequences that happen to represent a newline do not
2488     count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2489     default newline convention is in force:
2490 ph10 556 .sp
2491     abc #comment \en still comment
2492     .sp
2493 ph10 579 On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2494 ph10 556 a newline in the pattern. The sequence \en is still literal at this stage, so
2495     it does not terminate the comment. Only an actual character with the code value
2496 ph10 562 0x0a (the default newline) does so.
2497 nigel 75 .
2498     .
2499 nigel 91 .\" HTML <a name="recursion"></a>
2500 nigel 75 .SH "RECURSIVE PATTERNS"
2501 nigel 63 .rs
2502     .sp
2503     Consider the problem of matching a string in parentheses, allowing for
2504     unlimited nested parentheses. Without the use of recursion, the best that can
2505     be done is to use a pattern that matches up to some fixed depth of nesting. It
2506 nigel 93 is not possible to handle an arbitrary nesting depth.
2507     .P
2508     For some time, Perl has provided a facility that allows regular expressions to
2509     recurse (amongst other things). It does this by interpolating Perl code in the
2510     expression at run time, and the code can refer to the expression itself. A Perl
2511     pattern using code interpolation to solve the parentheses problem can be
2512     created like this:
2513 nigel 75 .sp
2514     $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2515     .sp
2516 nigel 63 The (?p{...}) item interpolates Perl code at run time, and in this case refers
2517 nigel 93 recursively to the pattern in which it appears.
2518 nigel 75 .P
2519 nigel 93 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2520     supports special syntax for recursion of the entire pattern, and also for
2521     individual subpattern recursion. After its introduction in PCRE and Python,
2522 ph10 453 this kind of recursion was subsequently introduced into Perl at release 5.10.
2523 nigel 75 .P
2524 nigel 93 A special item that consists of (? followed by a number greater than zero and a
2525 ph10 716 closing parenthesis is a recursive subroutine call of the subpattern of the
2526     given number, provided that it occurs inside that subpattern. (If not, it is a
2527 ph10 454 .\" HTML <a href="#subpatternsassubroutines">
2528     .\" </a>
2529 ph10 716 non-recursive subroutine
2530 ph10 454 .\"
2531 nigel 93 call, which is described in the next section.) The special item (?R) or (?0) is
2532     a recursive call of the entire regular expression.
2533 nigel 87 .P
2534     This PCRE pattern solves the nested parentheses problem (assume the
2535     PCRE_EXTENDED option is set so that white space is ignored):
2536 nigel 75 .sp
2537 ph10 456 \e( ( [^()]++ | (?R) )* \e)
2538 nigel 75 .sp
2539 nigel 63 First it matches an opening parenthesis. Then it matches any number of
2540     substrings which can either be a sequence of non-parentheses, or a recursive
2541 nigel 87 match of the pattern itself (that is, a correctly parenthesized substring).
2542 ph10 461 Finally there is a closing parenthesis. Note the use of a possessive quantifier
2543 ph10 456 to avoid backtracking into sequences of non-parentheses.
2544 nigel 75 .P
2545 nigel 63 If this were part of a larger pattern, you would not want to recurse the entire
2546     pattern, so instead you could use this:
2547 nigel 75 .sp
2548 ph10 456 ( \e( ( [^()]++ | (?1) )* \e) )
2549 nigel 75 .sp
2550 nigel 63 We have put the pattern into parentheses, and caused the recursion to refer to
2551 ph10 172 them instead of the whole pattern.
2552 ph10 166 .P
2553     In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2554 ph10 572 is made easier by the use of relative references. Instead of (?1) in the
2555     pattern above you can write (?-2) to refer to the second most recently opened
2556     parentheses preceding the recursion. In other words, a negative number counts
2557     capturing parentheses leftwards from the point at which it is encountered.
2558 ph10 166 .P
2559     It is also possible to refer to subsequently opened parentheses, by writing
2560     references such as (?+2). However, these cannot be recursive because the
2561     reference is not inside the parentheses that are referenced. They are always
2562 ph10 454 .\" HTML <a href="#subpatternsassubroutines">
2563     .\" </a>
2564 ph10 716 non-recursive subroutine
2565 ph10 454 .\"
2566     calls, as described in the next section.
2567 ph10 166 .P
2568     An alternative approach is to use named parentheses instead. The Perl syntax
2569     for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2570     could rewrite the above example as follows:
2571 nigel 75 .sp
2572 ph10 456 (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2573 nigel 75 .sp
2574 nigel 93 If there is more than one subpattern with the same name, the earliest one is
2575 ph10 172 used.
2576 ph10 166 .P
2577     This particular example pattern that we have been looking at contains nested
2578 ph10 456 unlimited repeats, and so the use of a possessive quantifier for matching
2579     strings of non-parentheses is important when applying the pattern to strings
2580     that do not match. For example, when this pattern is applied to
2581 nigel 75 .sp
2582 nigel 63 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2583 nigel 75 .sp
2584 ph10 456 it yields "no match" quickly. However, if a possessive quantifier is not used,
2585 nigel 63 the match runs for a very long time indeed because there are so many different
2586     ways the + and * repeats can carve up the subject, and all have to be tested
2587     before failure can be reported.
2588 nigel 75 .P
2589 ph10 464 At the end of a match, the values of capturing parentheses are those from
2590     the outermost level. If you want to obtain intermediate values, a callout
2591     function can be used (see below and the
2592 nigel 63 .\" HREF
2593 nigel 75 \fBpcrecallout\fP
2594 nigel 63 .\"
2595     documentation). If the pattern above is matched against
2596 nigel 75 .sp
2597 nigel 63 (ab(cd)ef)
2598 nigel 75 .sp
2599 ph10 464 the value for the inner capturing parentheses (numbered 2) is "ef", which is
2600     the last value taken on at the top level. If a capturing subpattern is not
2601 ph10 724 matched at the top level, its final captured value is unset, even if it was
2602     (temporarily) set at a deeper level during the matching process.
2603 nigel 75 .P
2604 ph10 464 If there are more than 15 capturing parentheses in a pattern, PCRE has to
2605     obtain extra memory to store data during a recursion, which it does by using
2606     \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2607     be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2608     .P
2609 nigel 63 Do not confuse the (?R) item with the condition (R), which tests for recursion.
2610     Consider this pattern, which matches text in angle brackets, allowing for
2611     arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2612     recursing), whereas any characters are permitted at the outer level.
2613 nigel 75 .sp
2614     < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
2615     .sp
2616 nigel 63 In this pattern, (?(R) is the start of a conditional subpattern, with two
2617     different alternatives for the recursive and non-recursive cases. The (?R) item
2618     is the actual recursive call.
2619 nigel 75 .
2620     .
2621 ph10 453 .\" HTML <a name="recursiondifference"></a>
2622 ph10 724 .SS "Differences in recursion processing between PCRE and Perl"
2623 ph10 453 .rs
2624     .sp
2625 ph10 724 Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2626     (like Python, but unlike Perl), a recursive subpattern call is always treated
2627     as an atomic group. That is, once it has matched some of the subject string, it
2628     is never re-entered, even if it contains untried alternatives and there is a
2629     subsequent matching failure. This can be illustrated by the following pattern,
2630     which purports to match a palindromic string that contains an odd number of
2631     characters (for example, "a", "aba", "abcba", "abcdcba"):
2632 ph10 453 .sp
2633     ^(.|(.)(?1)\e2)$
2634     .sp
2635 ph10 461 The idea is that it either matches a single character, or two identical
2636     characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2637 ph10 453 it does not if the pattern is longer than three characters. Consider the
2638     subject string "abcba":
2639     .P
2640 ph10 461 At the top level, the first character is matched, but as it is not at the end
2641 ph10 453 of the string, the first alternative fails; the second alternative is taken
2642     and the recursion kicks in. The recursive call to subpattern 1 successfully
2643     matches the next character ("b"). (Note that the beginning and end of line
2644     tests are not part of the recursion).
2645     .P
2646     Back at the top level, the next character ("c") is compared with what
2647 ph10 461 subpattern 2 matched, which was "a". This fails. Because the recursion is
2648 ph10 453 treated as an atomic group, there are now no backtracking points, and so the
2649     entire match fails. (Perl is able, at this point, to re-enter the recursion and
2650     try the second alternative.) However, if the pattern is written with the
2651     alternatives in the other order, things are different:
2652     .sp
2653     ^((.)(?1)\e2|.)$
2654     .sp
2655 ph10 461 This time, the recursing alternative is tried first, and continues to recurse
2656     until it runs out of characters, at which point the recursion fails. But this
2657     time we do have another alternative to try at the higher level. That is the big
2658 ph10 453 difference: in the previous case the remaining alternative is at a deeper
2659     recursion level, which PCRE cannot use.
2660     .P
2661 ph10 572 To change the pattern so that it matches all palindromic strings, not just
2662     those with an odd number of characters, it is tempting to change the pattern to
2663     this:
2664 ph10 453 .sp
2665     ^((.)(?1)\e2|.?)$
2666     .sp
2667 ph10 461 Again, this works in Perl, but not in PCRE, and for the same reason. When a
2668     deeper recursion has matched a single character, it cannot be entered again in
2669     order to match an empty string. The solution is to separate the two cases, and
2670 ph10 453 write out the odd and even cases as alternatives at the higher level:
2671     .sp
2672     ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2673 ph10 461 .sp
2674     If you want to match typical palindromic phrases, the pattern has to ignore all
2675 ph10 453 non-word characters, which can be done like this:
2676     .sp
2677 ph10 461 ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2678 ph10 453 .sp
2679 ph10 461 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2680     man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2681     the use of the possessive quantifier *+ to avoid backtracking into sequences of
2682 ph10 453 non-word characters. Without this, PCRE takes a great deal longer (ten times or
2683     more) to match typical phrases, and Perl takes so long that you think it has
2684     gone into a loop.
2685 ph10 456 .P
2686     \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2687     string does not start with a palindrome that is shorter than the entire string.
2688     For example, although "abcba" is correctly matched, if the subject is "ababa",
2689     PCRE finds the palindrome "aba" at the start, then fails at top level because
2690     the end of the string does not follow. Once again, it cannot jump back into the
2691     recursion to try other alternatives, so the entire match fails.
2692 ph10 724 .P
2693 ph10 733 The second way in which PCRE and Perl differ in their recursion processing is
2694     in the handling of captured values. In Perl, when a subpattern is called
2695     recursively or as a subpattern (see the next section), it has no access to any
2696     values that were captured outside the recursion, whereas in PCRE these values
2697 ph10 724 can be referenced. Consider this pattern:
2698     .sp
2699     ^(.)(\e1|a(?2))
2700     .sp
2701 ph10 733 In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2702 ph10 724 then in the second group, when the back reference \e1 fails to match "b", the
2703     second alternative matches "a" and then recurses. In the recursion, \e1 does
2704     now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2705     match because inside the recursive call \e1 cannot access the externally set
2706     value.
2707 ph10 453 .
2708     .
2709 nigel 63 .\" HTML <a name="subpatternsassubroutines"></a>
2710 nigel 75 .SH "SUBPATTERNS AS SUBROUTINES"
2711 nigel 63 .rs
2712     .sp
2713 ph10 716 If the syntax for a recursive subpattern call (either by number or by
2714 nigel 63 name) is used outside the parentheses to which it refers, it operates like a
2715 ph10 716 subroutine in a programming language. The called subpattern may be defined
2716 ph10 166 before or after the reference. A numbered reference can be absolute or
2717     relative, as in these examples:
2718 nigel 75 .sp
2719 ph10 166 (...(absolute)...)...(?2)...
2720     (...(relative)...)...(?-1)...
2721 ph10 172 (...(?+1)...(relative)...
2722 ph10 166 .sp
2723     An earlier example pointed out that the pattern
2724     .sp
2725 nigel 75 (sens|respons)e and \e1ibility
2726     .sp
2727 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
2728     "sense and responsibility". If instead the pattern
2729 nigel 75 .sp
2730 nigel 63 (sens|respons)e and (?1)ibility
2731 nigel 75 .sp
2732 nigel 63 is used, it does match "sense and responsibility" as well as the other two
2733 nigel 93 strings. Another example is given in the discussion of DEFINE above.
2734 nigel 87 .P
2735 ph10 716 All subroutine calls, whether recursive or not, are always treated as atomic
2736     groups. That is, once a subroutine has matched some of the subject string, it
2737     is never re-entered, even if it contains untried alternatives and there is a
2738     subsequent matching failure. Any capturing parentheses that are set during the
2739     subroutine call revert to their previous values afterwards.
2740 nigel 93 .P
2741 ph10 716 Processing options such as case-independence are fixed when a subpattern is
2742     defined, so if it is used as a subroutine, such options cannot be changed for
2743     different calls. For example, consider this pattern:
2744 nigel 93 .sp
2745 ph10 166 (abc)(?i:(?-1))
2746 nigel 93 .sp
2747     It matches "abcabc". It does not match "abcABC" because the change of
2748     processing option does not affect the called subpattern.
2749 nigel 75 .
2750     .
2751 ph10 333 .\" HTML <a name="onigurumasubroutines"></a>
2752     .SH "ONIGURUMA SUBROUTINE SYNTAX"
2753     .rs
2754     .sp
2755 ph10 345 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2756     a number enclosed either in angle brackets or single quotes, is an alternative
2757     syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2758 ph10 333 are two of the examples used above, rewritten using this syntax:
2759     .sp
2760     (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2761     (sens|respons)e and \eg'1'ibility
2762     .sp
2763 ph10 345 PCRE supports an extension to Oniguruma: if a number is preceded by a
2764 ph10 333 plus or a minus sign it is taken as a relative reference. For example:
2765     .sp
2766     (abc)(?i:\eg<-1>)
2767     .sp
2768 ph10 345 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2769 ph10 333 synonymous. The former is a back reference; the latter is a subroutine call.
2770     .
2771     .
2772 nigel 63 .SH CALLOUTS
2773     .rs
2774     .sp
2775     Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2776     code to be obeyed in the middle of matching a regular expression. This makes it
2777     possible, amongst other things, to extract different substrings that match the
2778     same pair of parentheses when there is a repetition.
2779 nigel 75 .P
2780 nigel 63 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2781     code. The feature is called "callout". The caller of PCRE provides an external
2782 ph10 903 function by putting its entry point in the global variable \fIpcre_callout\fP
2783 chpe 1055 (8-bit library) or \fIpcre[16|32]_callout\fP (16-bit or 32-bit library).
2784     By default, this variable contains NULL, which disables all calling out.
2785 nigel 75 .P
2786 nigel 63 Within a regular expression, (?C) indicates the points at which the external
2787     function is to be called. If you want to identify different callout points, you
2788     can put a number less than 256 after the letter C. The default value is zero.
2789     For example, this pattern has two callout points:
2790 nigel 75 .sp
2791 ph10 155 (?C1)abc(?C2)def
2792 nigel 75 .sp
2793 ph10 859 If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
2794 nigel 75 automatically installed before each item in the pattern. They are all numbered
2795 ph10 1266 255. If there is a conditional group in the pattern whose condition is an
2796     assertion, an additional callout is inserted just before the condition. An
2797     explicit callout may also be set at this position, as in this example:
2798     .sp
2799     (?(?C9)(?=a)abc|def)
2800     .sp
2801 ph10 1335 Note that this applies only to assertion conditions, not to other types of
2802     condition.
2803 nigel 75 .P
2804 ph10 859 During matching, when PCRE reaches a callout point, the external function is
2805     called. It is provided with the number of the callout, the position in the
2806     pattern, and, optionally, one item of data originally supplied by the caller of
2807     the matching function. The callout function may cause matching to proceed, to
2808 ph10 1395 backtrack, or to fail altogether.
2809     .P
2810     By default, PCRE implements a number of optimizations at compile time and
2811     matching time, and one side-effect is that sometimes callouts are skipped. If
2812     you need all possible callouts to happen, you need to set options that disable
2813     the relevant optimizations. More details, and a complete description of the
2814     interface to the callout function, are given in the
2815 nigel 63 .\" HREF
2816 nigel 75 \fBpcrecallout\fP
2817 nigel 63 .\"
2818     documentation.
2819 nigel 93 .
2820     .
2821 ph10 510 .\" HTML <a name="backtrackcontrol"></a>
2822 ph10 235 .SH "BACKTRACKING CONTROL"
2823 ph10 210 .rs
2824     .sp
2825 ph10 211 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2826 ph10 1297 are still described in the Perl documentation as "experimental and subject to
2827     change or removal in a future version of Perl". It goes on to say: "Their usage
2828     in production code should be noted to avoid problems during upgrades." The same
2829 ph10 210 remarks apply to the PCRE features described in this section.
2830     .P
2831 ph10 211 The new verbs make use of what was previously invalid syntax: an opening
2832 ph10 510 parenthesis followed by an asterisk. They are generally of the form
2833 ph10 1335 (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
2834 ph10 1302 differently depending on whether or not a name is present. A name is any
2835     sequence of characters that does not include a closing parenthesis. The maximum
2836     length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
2837     libraries. If the name is empty, that is, if the closing parenthesis
2838     immediately follows the colon, the effect is as if the colon were not there.
2839     Any number of these verbs may occur in a pattern.
2840 ph10 1297 .P
2841     Since these verbs are specifically related to backtracking, most of them can be
2842     used only when the pattern is to be matched using one of the traditional
2843     matching functions, because these use a backtracking algorithm. With the
2844 ph10 1335 exception of (*FAIL), which behaves like a failing negative assertion, the
2845 ph10 1297 backtracking control verbs cause an error if encountered by a DFA matching
2846     function.
2847     .P
2848 ph10 1335 The behaviour of these verbs in
2849 ph10 1298 .\" HTML <a href="#btrepeat">
2850     .\" </a>
2851 ph10 1335 repeated groups,
2852 ph10 1298 .\"
2853     .\" HTML <a href="#btassert">
2854     .\" </a>
2855 ph10 1335 assertions,
2856 ph10 1298 .\"
2857 ph10 1335 and in
2858 ph10 1298 .\" HTML <a href="#btsub">
2859     .\" </a>
2860     subpatterns called as subroutines
2861     .\"
2862     (whether or not recursively) is documented below.
2863 ph10 930 .
2864     .
2865     .\" HTML <a name="nooptimize"></a>
2866     .SS "Optimizations that affect backtracking verbs"
2867     .rs
2868     .sp
2869 ph10 512 PCRE contains some optimizations that are used to speed up matching by running
2870     some checks at the start of each match attempt. For example, it may know the
2871     minimum length of matching subject, or that a particular character must be
2872 ph10 1297 present. When one of these optimizations bypasses the running of a match, any
2873 ph10 512 included backtracking verbs will not, of course, be processed. You can suppress
2874     the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2875 ph10 577 when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2876 ph10 930 pattern with (*NO_START_OPT). There is more discussion of this option in the
2877     section entitled
2878     .\" HTML <a href="pcreapi.html#execoptions">
2879     .\" </a>
2880     "Option bits for \fBpcre_exec()\fP"
2881     .\"
2882     in the
2883     .\" HREF
2884     \fBpcreapi\fP
2885     .\"
2886 ph10 975 documentation.
2887 ph10 836 .P
2888     Experiments with Perl suggest that it too has similar optimizations, sometimes
2889     leading to anomalous results.
2890 ph10 210 .
2891 ph10 510 .
2892 ph10 210 .SS "Verbs that act immediately"
2893     .rs
2894     .sp
2895 ph10 512 The following verbs act as soon as they are encountered. They may not be
2896 ph10 510 followed by a name.
2897 ph10 210 .sp
2898     (*ACCEPT)
2899     .sp
2900     This verb causes the match to end successfully, skipping the remainder of the
2901 ph10 716 pattern. However, when it is inside a subpattern that is called as a
2902     subroutine, only that subpattern is ended successfully. Matching then continues
2903 ph10 1335 at the outer level. If (*ACCEPT) in triggered in a positive assertion, the
2904 ph10 1297 assertion succeeds; in a negative assertion, the assertion fails.
2905     .P
2906     If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
2907     example:
2908 ph10 210 .sp
2909 ph10 447 A((?:A|B(*ACCEPT)|C)D)
2910 ph10 210 .sp
2911 ph10 461 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2912 ph10 447 the outer parentheses.
2913 ph10 210 .sp
2914     (*FAIL) or (*F)
2915     .sp
2916 ph10 716 This verb causes a matching failure, forcing backtracking to occur. It is
2917 ph10 210 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2918     probably useful only when combined with (?{}) or (??{}). Those are, of course,
2919     Perl features that are not present in PCRE. The nearest equivalent is the
2920     callout feature, as for example in this pattern:
2921     .sp
2922     a+(?C)(*FAIL)
2923     .sp
2924 ph10 211 A match with the string "aaaa" always fails, but the callout is taken before
2925     each backtrack happens (in this example, 10 times).
2926 ph10 210 .
2927 ph10 510 .
2928     .SS "Recording which path was taken"
2929     .rs
2930     .sp
2931 ph10 512 There is one verb whose main purpose is to track how a match was arrived at,
2932     though it also has a secondary use in conjunction with advancing the match
2933 ph10 510 starting point (see (*SKIP) below).
2934     .sp
2935     (*MARK:NAME) or (*:NAME)
2936     .sp
2937     A name is always required with this verb. There may be as many instances of
2938     (*MARK) as you like in a pattern, and their names do not have to be unique.
2939     .P
2940 ph10 1287 When a match succeeds, the name of the last-encountered (*MARK:NAME),
2941     (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
2942     caller as described in the section entitled
2943 ph10 510 .\" HTML <a href="pcreapi.html#extradata">
2944     .\" </a>
2945 ph10 859 "Extra data for \fBpcre_exec()\fP"
2946 ph10 510 .\"
2947 ph10 512 in the
2948 ph10 510 .\" HREF
2949     \fBpcreapi\fP
2950     .\"
2951 ph10 836 documentation. Here is an example of \fBpcretest\fP output, where the /K
2952     modifier requests the retrieval and outputting of (*MARK) data:
2953 ph10 510 .sp
2954 ph10 836 re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2955     data> XY
2956 ph10 510 0: XY
2957     MK: A
2958     XZ
2959     0: XZ
2960     MK: B
2961     .sp
2962 ph10 512 The (*MARK) name is tagged with "MK:" in this output, and in this example it
2963     indicates which of the two alternatives matched. This is a more efficient way
2964 ph10 510 of obtaining this information than putting each alternative in its own
2965     capturing parentheses.
2966     .P
2967 ph10 1302 If a verb with a name is encountered in a positive assertion that is true, the
2968     name is recorded and passed back if it is the last-encountered. This does not
2969     happen for negative assertions or failing positive assertions.
2970 ph10 630 .P
2971 ph10 1287 After a partial match or a failed match, the last encountered name in the
2972     entire match process is returned. For example:
2973 ph10 510 .sp
2974 ph10 836 re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2975     data> XP
2976 ph10 510 No match, mark = B
2977     .sp
2978 ph10 836 Note that in this unanchored example the mark is retained from the match
2979 ph10 930 attempt that started at the letter "X" in the subject. Subsequent match
2980     attempts starting at "P" and then with an empty string do not get as far as the
2981     (*MARK) item, but nevertheless do not reset it.
2982     .P
2983     If you are interested in (*MARK) values after failed matches, you should
2984 ph10 975 probably set the PCRE_NO_START_OPTIMIZE option
2985 ph10 930 .\" HTML <a href="#nooptimize">
2986     .\" </a>
2987 ph10 975 (see above)
2988 ph10 930 .\"
2989     to ensure that the match is always attempted.
2990 ph10 510 .
2991     .
2992 ph10 210 .SS "Verbs that act after backtracking"
2993     .rs
2994     .sp
2995 ph10 211 The following verbs do nothing when they are encountered. Matching continues
2996 ph10 510 with what follows, but if there is no subsequent match, causing a backtrack to
2997     the verb, a failure is forced. That is, backtracking cannot pass to the left of
2998 ph10 1335 the verb. However, when one of these verbs appears inside an atomic group or an
2999 ph10 1302 assertion that is true, its effect is confined to that group, because once the
3000     group has been matched, there is never any backtracking into it. In this
3001     situation, backtracking can "jump back" to the left of the entire atomic group
3002     or assertion. (Remember also, as stated above, that this localization also
3003     applies in subroutine calls.)
3004 ph10 510 .P
3005     These verbs differ in exactly what kind of failure occurs when backtracking
3006 ph10 1302 reaches them. The behaviour described below is what happens when the verb is
3007 ph10 1335 not in a subroutine or an assertion. Subsequent sections cover these special
3008 ph10 1302 cases.
3009 ph10 210 .sp
3010     (*COMMIT)
3011     .sp
3012 ph10 510 This verb, which may not be followed by a name, causes the whole match to fail
3013 ph10 1335 outright if there is a later matching failure that causes backtracking to reach
3014 ph10 1297 it. Even if the pattern is unanchored, no further attempts to find a match by
3015     advancing the starting point take place. If (*COMMIT) is the only backtracking
3016     verb that is encountered, once it has been passed \fBpcre_exec()\fP is
3017     committed to finding a match at the current starting point, or not at all. For
3018     example:
3019 ph10 210 .sp
3020     a+(*COMMIT)b
3021     .sp
3022 ph10 211 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
3023 ph10 512 dynamic anchor, or "I've started, so I must finish." The name of the most
3024     recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
3025 ph10 510 match failure.
3026     .P
3027 ph10 1335 If there is more than one backtracking verb in a pattern, a different one that
3028     follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a
3029 ph10 1297 match does not always guarantee that a match must be at this starting point.
3030     .P
3031 ph10 512 Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
3032     unless PCRE's start-of-match optimizations are turned off, as shown in this
3033 ph10 1436 output from \fBpcretest\fP:
3034 ph10 210 .sp
3035 ph10 836 re> /(*COMMIT)abc/
3036     data> xyzabc
3037 ph10 510 0: abc
3038 ph10 1436 data> xyzabc\eY
3039 ph10 510 No match
3040 ph10 210 .sp
3041 ph10 1436 For this pattern, PCRE knows that any match must start with "a", so the
3042     optimization skips along the subject to "a" before applying the pattern to the
3043     first set of data. The match attempt then succeeds. In the second set of data,
3044     the escape sequence \eY is interpreted by the \fBpcretest\fP program. It causes
3045     the PCRE_NO_START_OPTIMIZE option to be set when \fBpcre_exec()\fP is called.
3046     This disables the optimization that skips along to the first character. The
3047     pattern is now applied starting at "x", and so the (*COMMIT) causes the match
3048     to fail without trying any other starting points.
3049 ph10 210 .sp
3050 ph10 510 (*PRUNE) or (*PRUNE:NAME)
3051     .sp
3052 ph10 512 This verb causes the match to fail at the current starting position in the
3053 ph10 1297 subject if there is a later matching failure that causes backtracking to reach
3054     it. If the pattern is unanchored, the normal "bumpalong" advance to the next
3055     starting character then happens. Backtracking can occur as usual to the left of
3056     (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but
3057     if there is no match to the right, backtracking cannot cross (*PRUNE). In
3058     simple cases, the use of (*PRUNE) is just an alternative to an atomic group or
3059     possessive quantifier, but there are some uses of (*PRUNE) that cannot be
3060     expressed in any other way. In an anchored pattern (*PRUNE) has the same effect
3061     as (*COMMIT).
3062 ph10 1287 .P
3063 ph10 1335 The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).
3064 ph10 1287 It is like (*MARK:NAME) in that the name is remembered for passing back to the
3065     caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3066 ph10 510 .sp
3067 ph10 210 (*SKIP)
3068     .sp
3069 ph10 510 This verb, when given without a name, is like (*PRUNE), except that if the
3070     pattern is unanchored, the "bumpalong" advance is not to the next character,
3071     but to the position in the subject where (*SKIP) was encountered. (*SKIP)
3072     signifies that whatever text was matched leading up to it cannot be part of a
3073     successful match. Consider:
3074 ph10 210 .sp
3075     a+(*SKIP)b
3076     .sp
3077 ph10 211 If the subject is "aaaac...", after the first match attempt fails (starting at
3078 ph10 210 the first character in the string), the starting point skips on to start the
3079 ph10 211 next attempt at "c". Note that a possessive quantifer does not have the same
3080 ph10 456 effect as this example; although it would suppress backtracking during the
3081 ph10 210 first match attempt, the second attempt would start at the second character
3082     instead of skipping on to "c".
3083     .sp
3084 ph10 510 (*SKIP:NAME)
3085 ph10 211 .sp
3086 ph10 1335 When (*SKIP) has an associated name, its behaviour is modified. When it is
3087 ph10 1297 triggered, the previous path through the pattern is searched for the most
3088     recent (*MARK) that has the same name. If one is found, the "bumpalong" advance
3089     is to the subject position that corresponds to that (*MARK) instead of to where
3090     (*SKIP) was encountered. If no (*MARK) with a matching name is found, the
3091     (*SKIP) is ignored.
3092 ph10 1287 .P
3093 ph10 1335 Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
3094 ph10 1287 names that are set by (*PRUNE:NAME) or (*THEN:NAME).
3095 ph10 510 .sp
3096     (*THEN) or (*THEN:NAME)
3097     .sp
3098 ph10 1335 This verb causes a skip to the next innermost alternative when backtracking
3099 ph10 1297 reaches it. That is, it cancels any further backtracking within the current
3100     alternative. Its name comes from the observation that it can be used for a
3101     pattern-based if-then-else block:
3102 ph10 210 .sp
3103     ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
3104     .sp
3105 ph10 211 If the COND1 pattern matches, FOO is tried (and possibly further items after
3106 ph10 716 the end of the group if FOO succeeds); on failure, the matcher skips to the
3107 ph10 1302 second alternative and tries COND2, without backtracking into COND1. If that
3108     succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no
3109     more alternatives, so there is a backtrack to whatever came before the entire
3110     group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
3111 ph10 551 .P
3112 ph10 1335 The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).
3113 ph10 1287 It is like (*MARK:NAME) in that the name is remembered for passing back to the
3114     caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3115     .P
3116 ph10 1297 A subpattern that does not contain a | character is just a part of the
3117     enclosing alternative; it is not a nested alternation with only one
3118 ph10 716 alternative. The effect of (*THEN) extends beyond such a subpattern to the
3119     enclosing alternative. Consider this pattern, where A, B, etc. are complex
3120     pattern fragments that do not contain any | characters at this level:
3121     .sp
3122     A (B(*THEN)C) | D
3123     .sp
3124 ph10 733 If A and B are matched, but there is a failure in C, matching does not
3125 ph10 716 backtrack into A; instead it moves to the next alternative, that is, D.
3126     However, if the subpattern containing (*THEN) is given an alternative, it
3127     behaves differently:
3128     .sp
3129     A (B(*THEN)C | (*FAIL)) | D
3130     .sp
3131     The effect of (*THEN) is now confined to the inner subpattern. After a failure
3132 ph10 733 in C, matching moves to (*FAIL), which causes the whole subpattern to fail
3133     because there are no more alternatives to try. In this case, matching does now
3134 ph10 716 backtrack into A.
3135 ph10 551 .P
3136 ph10 1297 Note that a conditional subpattern is not considered as having two
3137 ph10 733 alternatives, because only one is ever used. In other words, the | character in
3138 ph10 716 a conditional subpattern has a different meaning. Ignoring white space,
3139     consider:
3140 ph10 551 .sp
3141 ph10 716 ^.*? (?(?=a) a | b(*THEN)c )
3142     .sp
3143 ph10 733 If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
3144     it initially matches zero characters. The condition (?=a) then fails, the
3145 ph10 716 character "b" is matched, but "c" is not. At this point, matching does not
3146     backtrack to .*? as might perhaps be expected from the presence of the |
3147     character. The conditional subpattern is part of the single alternative that
3148 ph10 733 comprises the whole pattern, and so the match fails. (If there was a backtrack
3149 ph10 716 into .*?, allowing it to match "b", the match would succeed.)
3150     .P
3151     The verbs just described provide four different "strengths" of control when
3152     subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
3153     next alternative. (*PRUNE) comes next, failing the match at the current
3154     starting position, but allowing an advance to the next character (for an
3155     unanchored pattern). (*SKIP) is similar, except that the advance may be more
3156     than one character. (*COMMIT) is the strongest, causing the entire match to
3157     fail.
3158 ph10 1297 .
3159     .
3160     .SS "More than one backtracking verb"
3161     .rs
3162 ph10 716 .sp
3163 ph10 1297 If more than one backtracking verb is present in a pattern, the one that is
3164     backtracked onto first acts. For example, consider this pattern, where A, B,
3165     etc. are complex pattern fragments:
3166     .sp
3167 ph10 1293 (A(*COMMIT)B(*THEN)C|ABD)
3168 ph10 551 .sp
3169 ph10 1335 If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
3170     fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
3171 ph10 1293 the next alternative (ABD) to be tried. This behaviour is consistent, but is
3172 ph10 1297 not always the same as Perl's. It means that if two or more backtracking verbs
3173     appear in succession, all the the last of them has no effect. Consider this
3174     example:
3175     .sp
3176     ...(*COMMIT)(*PRUNE)...
3177     .sp
3178 ph10 1341 If there is a matching failure to the right, backtracking onto (*PRUNE) causes
3179 ph10 1335 it to be triggered, and its action is taken. There can never be a backtrack
3180     onto (*COMMIT).
3181 ph10 210 .
3182 ph10 551 .
3183 ph10 1298 .\" HTML <a name="btrepeat"></a>
3184 ph10 1297 .SS "Backtracking verbs in repeated groups"
3185     .rs
3186     .sp
3187 ph10 1335 PCRE differs from Perl in its handling of backtracking verbs in repeated
3188 ph10 1297 groups. For example, consider:
3189     .sp
3190     /(a(*COMMIT)b)+ac/
3191     .sp
3192 ph10 1335 If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in
3193     the second repeat of the group acts.
3194 ph10 1297 .
3195     .
3196 ph10 1298 .\" HTML <a name="btassert"></a>
3197     .SS "Backtracking verbs in assertions"
3198     .rs
3199     .sp
3200     (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
3201     .P
3202 ph10 1335 (*ACCEPT) in a positive assertion causes the assertion to succeed without any
3203     further processing. In a negative assertion, (*ACCEPT) causes the assertion to
3204 ph10 1298 fail without any further processing.
3205     .P
3206 ph10 1302 The other backtracking verbs are not treated specially if they appear in a
3207     positive assertion. In particular, (*THEN) skips to the next alternative in the
3208 ph10 1298 innermost enclosing group that has alternations, whether or not this is within
3209     the assertion.
3210 ph10 1302 .P
3211     Negative assertions are, however, different, in order to ensure that changing a
3212     positive assertion into a negative assertion changes its result. Backtracking
3213 ph10 1335 into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
3214     without considering any further alternative branches in the assertion.
3215 ph10 1302 Backtracking into (*THEN) causes it to skip to the next enclosing alternative
3216 ph10 1335 within the assertion (the normal behaviour), but if the assertion does not have
3217 ph10 1302 such an alternative, (*THEN) behaves like (*PRUNE).
3218 ph10 1298 .
3219     .
3220     .\" HTML <a name="btsub"></a>
3221     .SS "Backtracking verbs in subroutines"
3222     .rs
3223     .sp
3224 ph10 1335 These behaviours occur whether or not the subpattern is called recursively.
3225 ph10 1298 Perl's treatment of subroutines is different in some cases.
3226     .P
3227     (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
3228     an immediate backtrack.
3229     .P
3230 ph10 1335 (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
3231     succeed without any further processing. Matching then continues after the
3232 ph10 1298 subroutine call.
3233     .P
3234     (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
3235     the subroutine match to fail.
3236     .P
3237     (*THEN) skips to the next alternative in the innermost enclosing group within
3238 ph10 1335 the subpattern that has alternatives. If there is no such group within the
3239 ph10 1298 subpattern, (*THEN) causes the subroutine match to fail.
3240     .
3241     .
3242 nigel 93 .SH "SEE ALSO"
3243     .rs
3244     .sp
3245 ph10 461 \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
3246 chpe 1055 \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP.
3247 ph10 99 .
3248     .
3249     .SH AUTHOR
3250     .rs
3251     .sp
3252     .nf
3253     Philip Hazel
3254     University Computing Service
3255     Cambridge CB2 3QH, England.
3256     .fi
3257     .
3258     .
3259     .SH REVISION
3260     .rs
3261     .sp
3262     .nf
3263 ph10 1436 Last updated: 08 January 2014
3264     Copyright (c) 1997-2014 University of Cambridge.
3265 ph10 99 .fi

Properties

Name Value
svn:eol-style native
svn:keywords "Author Date Id Revision Url"

webmaster@exim.org
ViewVC Help
Powered by ViewVC 1.1.12