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1 nigel 3 .TH PCRE 3
2     .SH NAME
3     pcre - Perl-compatible regular expressions.
5     .B #include <pcre.h>
6     .PP
7     .SM
8     .br
9     .B pcre *pcre_compile(const char *\fIpattern\fR, int \fIoptions\fR,
10     .ti +5n
11 nigel 25 .B const char **\fIerrptr\fR, int *\fIerroffset\fR,
12     .ti +5n
13     .B const unsigned char *\fItableptr\fR);
14 nigel 3 .PP
15     .br
16     .B pcre_extra *pcre_study(const pcre *\fIcode\fR, int \fIoptions\fR,
17     .ti +5n
18 nigel 11 .B const char **\fIerrptr\fR);
19 nigel 3 .PP
20     .br
21     .B int pcre_exec(const pcre *\fIcode\fR, "const pcre_extra *\fIextra\fR,"
22     .ti +5n
23     .B "const char *\fIsubject\fR," int \fIlength\fR, int \fIoptions\fR,
24     .ti +5n
25     .B int *\fIovector\fR, int \fIovecsize\fR);
26     .PP
27     .br
28 nigel 29 .B int pcre_copy_substring(const char *\fIsubject\fR, int *\fIovector\fR,
29     .ti +5n
30     .B int \fIstringcount\fR, int \fIstringnumber\fR, char *\fIbuffer\fR,
31     .ti +5n
32     .B int \fIbuffersize\fR);
33     .PP
34     .br
35     .B int pcre_get_substring(const char *\fIsubject\fR, int *\fIovector\fR,
36     .ti +5n
37     .B int \fIstringcount\fR, int \fIstringnumber\fR,
38     .ti +5n
39     .B const char **\fIstringptr\fR);
40     .PP
41     .br
42     .B int pcre_get_substring_list(const char *\fIsubject\fR,
43     .ti +5n
44     .B int *\fIovector\fR, int \fIstringcount\fR, "const char ***\fIlistptr\fR);"
45     .PP
46     .br
47     .B const unsigned char *pcre_maketables(void);
48     .PP
49     .br
50 nigel 3 .B int pcre_info(const pcre *\fIcode\fR, int *\fIoptptr\fR, int
51     .B *\fIfirstcharptr\fR);
52     .PP
53     .br
54     .B char *pcre_version(void);
55     .PP
56     .br
57     .B void *(*pcre_malloc)(size_t);
58     .PP
59     .br
60     .B void (*pcre_free)(void *);
65     The PCRE library is a set of functions that implement regular expression
66     pattern matching using the same syntax and semantics as Perl 5, with just a few
67 nigel 23 differences (see below). The current implementation corresponds to Perl 5.005.
68 nigel 3
69     PCRE has its own native API, which is described in this man page. There is also
70     a set of wrapper functions that correspond to the POSIX API. See
71     \fBpcreposix (3)\fR.
73 nigel 29 The functions \fBpcre_compile()\fR, \fBpcre_study()\fR, and \fBpcre_exec()\fR
74     are used for compiling and matching regular expressions, while
75     \fBpcre_copy_substring()\fR, \fBpcre_get_substring()\fR, and
76     \fBpcre_get_substring_list()\fR are convenience functions for extracting
77     captured substrings from a matched subject string. The function
78     \fBpcre_maketables()\fR is used (optionally) to build a set of character tables
79     in the current locale for passing to \fBpcre_compile()\fR.
80 nigel 25
81     The function \fBpcre_info()\fR is used to find out information about a compiled
82 nigel 3 pattern, while the function \fBpcre_version()\fR returns a pointer to a string
83     containing the version of PCRE and its date of release.
85     The global variables \fBpcre_malloc\fR and \fBpcre_free\fR initially contain
86     the entry points of the standard \fBmalloc()\fR and \fBfree()\fR functions
87     respectively. PCRE calls the memory management functions via these variables,
88     so a calling program can replace them if it wishes to intercept the calls. This
89     should be done before calling any PCRE functions.
93     The PCRE functions can be used in multi-threading applications, with the
94 nigel 25 proviso that the memory management functions pointed to by \fBpcre_malloc\fR
95     and \fBpcre_free\fR are shared by all threads.
96 nigel 3
97     The compiled form of a regular expression is not altered during matching, so
98     the same compiled pattern can safely be used by several threads at once.
102     The function \fBpcre_compile()\fR is called to compile a pattern into an
103     internal form. The pattern is a C string terminated by a binary zero, and
104 nigel 25 is passed in the argument \fIpattern\fR. A pointer to a single block of memory
105     that is obtained via \fBpcre_malloc\fR is returned. This contains the
106     compiled code and related data. The \fBpcre\fR type is defined for this for
107     convenience, but in fact \fBpcre\fR is just a typedef for \fBvoid\fR, since the
108     contents of the block are not externally defined. It is up to the caller to
109     free the memory when it is no longer required.
110 nigel 3 .PP
111     The size of a compiled pattern is roughly proportional to the length of the
112     pattern string, except that each character class (other than those containing
113     just a single character, negated or not) requires 33 bytes, and repeat
114     quantifiers with a minimum greater than one or a bounded maximum cause the
115     relevant portions of the compiled pattern to be replicated.
116     .PP
117     The \fIoptions\fR argument contains independent bits that affect the
118 nigel 23 compilation. It should be zero if no options are required. Some of the options,
119     in particular, those that are compatible with Perl, can also be set and unset
120     from within the pattern (see the detailed description of regular expressions
121     below). For these options, the contents of the \fIoptions\fR argument specifies
122     their initial settings at the start of compilation and execution. The
123     PCRE_ANCHORED option can be set at the time of matching as well as at compile
124     time.
125 nigel 3 .PP
126     If \fIerrptr\fR is NULL, \fBpcre_compile()\fR returns NULL immediately.
127     Otherwise, if compilation of a pattern fails, \fBpcre_compile()\fR returns
128     NULL, and sets the variable pointed to by \fIerrptr\fR to point to a textual
129 nigel 25 error message. The offset from the start of the pattern to the character where
130     the error was discovered is placed in the variable pointed to by
131     \fIerroffset\fR, which must not be NULL. If it is, an immediate error is given.
132 nigel 3 .PP
133 nigel 25 If the final argument, \fItableptr\fR, is NULL, PCRE uses a default set of
134     character tables which are built when it is compiled, using the default C
135     locale. Otherwise, \fItableptr\fR must be the result of a call to
136     \fBpcre_maketables()\fR. See the section on locale support below.
137     .PP
138 nigel 3 The following option bits are defined in the header file:
142     If this bit is set, the pattern is forced to be "anchored", that is, it is
143     constrained to match only at the start of the string which is being searched
144     (the "subject string"). This effect can also be achieved by appropriate
145     constructs in the pattern itself, which is the only way to do it in Perl.
149     If this bit is set, letters in the pattern match both upper and lower case
150 nigel 23 letters. It is equivalent to Perl's /i option.
151 nigel 3
154     If this bit is set, a dollar metacharacter in the pattern matches only at the
155 nigel 23 end of the subject string. Without this option, a dollar also matches
156     immediately before the final character if it is a newline (but not before any
157     other newlines). The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is
158     set. There is no equivalent to this option in Perl.
159 nigel 3
162     If this bit is set, a dot metacharater in the pattern matches all characters,
163 nigel 23 including newlines. Without it, newlines are excluded. This option is
164 nigel 3 equivalent to Perl's /s option. A negative class such as [^a] always matches a
165     newline character, independent of the setting of this option.
169 nigel 23 If this bit is set, whitespace data characters in the pattern are totally
170     ignored except when escaped or inside a character class, and characters between
171     an unescaped # outside a character class and the next newline character,
172 nigel 3 inclusive, are also ignored. This is equivalent to Perl's /x option, and makes
173 nigel 23 it possible to include comments inside complicated patterns. Note, however,
174     that this applies only to data characters. Whitespace characters may never
175     appear within special character sequences in a pattern, for example within the
176     sequence (?( which introduces a conditional subpattern.
177 nigel 3
178 nigel 23 PCRE_EXTRA
180     This option turns on additional functionality of PCRE that is incompatible with
181     Perl. Any backslash in a pattern that is followed by a letter that has no
182     special meaning causes an error, thus reserving these combinations for future
183     expansion. By default, as in Perl, a backslash followed by a letter with no
184     special meaning is treated as a literal. There are at present no other features
185     controlled by this option.
187 nigel 3 PCRE_MULTILINE
189     By default, PCRE treats the subject string as consisting of a single "line" of
190     characters (even if it actually contains several newlines). The "start of line"
191     metacharacter (^) matches only at the start of the string, while the "end of
192     line" metacharacter ($) matches only at the end of the string, or before a
193 nigel 23 terminating newline (unless PCRE_DOLLAR_ENDONLY is set). This is the same as
194     Perl.
195 nigel 3
196     When PCRE_MULTILINE it is set, the "start of line" and "end of line" constructs
197     match immediately following or immediately before any newline in the subject
198     string, respectively, as well as at the very start and end. This is equivalent
199     to Perl's /m option. If there are no "\\n" characters in a subject string, or
200     no occurrences of ^ or $ in a pattern, setting PCRE_MULTILINE has no
201     effect.
203 nigel 19 PCRE_UNGREEDY
204 nigel 3
205 nigel 19 This option inverts the "greediness" of the quantifiers so that they are not
206     greedy by default, but become greedy if followed by "?". It is not compatible
207     with Perl. It can also be set by a (?U) option setting within the pattern.
208 nigel 3
209 nigel 19
211     When a pattern is going to be used several times, it is worth spending more
212     time analyzing it in order to speed up the time taken for matching. The
213     function \fBpcre_study()\fR takes a pointer to a compiled pattern as its first
214     argument, and returns a pointer to a \fBpcre_extra\fR block (another \fBvoid\fR
215     typedef) containing additional information about the pattern; this can be
216     passed to \fBpcre_exec()\fR. If no additional information is available, NULL
217     is returned.
219 nigel 23 The second argument contains option bits. At present, no options are defined
220     for \fBpcre_study()\fR, and this argument should always be zero.
221 nigel 3
222     The third argument for \fBpcre_study()\fR is a pointer to an error message. If
223     studying succeeds (even if no data is returned), the variable it points to is
224     set to NULL. Otherwise it points to a textual error message.
226     At present, studying a pattern is useful only for non-anchored patterns that do
227     not have a single fixed starting character. A bitmap of possible starting
228     characters is created.
231 nigel 25 .SH LOCALE SUPPORT
232     PCRE handles caseless matching, and determines whether characters are letters,
233     digits, or whatever, by reference to a set of tables. The library contains a
234     default set of tables which is created in the default C locale when PCRE is
235     compiled. This is used when the final argument of \fBpcre_compile()\fR is NULL,
236     and is sufficient for many applications.
238     An alternative set of tables can, however, be supplied. Such tables are built
239     by calling the \fBpcre_maketables()\fR function, which has no arguments, in the
240     relevant locale. The result can then be passed to \fBpcre_compile()\ as often
241     as necessary. For example, to build and use tables that are appropriate for the
242     French locale (where accented characters with codes greater than 128 are
243     treated as letters), the following code could be used:
245     setlocale(LC_CTYPE, "fr");
246     tables = pcre_maketables();
247     re = pcre_compile(..., tables);
249     The tables are built in memory that is obtained via \fBpcre_malloc\fR. The
250     pointer that is passed to \fBpcre_compile\fR is saved with the compiled
251     pattern, and the same tables are used via this pointer by \fBpcre_study()\fR
252     and \fBpcre_match()\fR. Thus for any single pattern, compilation, studying and
253     matching all happen in the same locale, but different patterns can be compiled
254     in different locales. It is the caller's responsibility to ensure that the
255     memory containing the tables remains available for as long as it is needed.
259     The \fBpcre_info()\fR function returns information about a compiled pattern.
260     Its yield is the number of capturing subpatterns, or one of the following
261     negative numbers:
263     PCRE_ERROR_NULL the argument \fIcode\fR was NULL
264     PCRE_ERROR_BADMAGIC the "magic number" was not found
266     If the \fIoptptr\fR argument is not NULL, a copy of the options with which the
267 nigel 33 pattern was compiled is placed in the integer it points to. These option bits
268     are those specified in the call to \fBpcre_compile()\fR, modified by any
269     top-level option settings within the pattern itself, and with the PCRE_ANCHORED
270     bit set if the form of the pattern implies that it can match only at the start
271     of a subject string.
272 nigel 29
273 nigel 33 If the pattern is not anchored and the \fIfirstcharptr\fR argument is not NULL,
274     it is used to pass back information about the first character of any matched
275     string. If there is a fixed first character, e.g. from a pattern such as
276     (cat|cow|coyote), then it is returned in the integer pointed to by
277     \fIfirstcharptr\fR. Otherwise, if either
279     (a) the pattern was compiled with the PCRE_MULTILINE option, and every branch
280     starts with "^", or
282     (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not set
283     (if it were set, the pattern would be anchored),
285     then -1 is returned, indicating that the pattern matches only at the
286 nigel 29 start of a subject string or after any "\\n" within the string. Otherwise -2 is
287     returned.
291     The function \fBpcre_exec()\fR is called to match a subject string against a
292     pre-compiled pattern, which is passed in the \fIcode\fR argument. If the
293     pattern has been studied, the result of the study should be passed in the
294     \fIextra\fR argument. Otherwise this must be NULL.
296     The subject string is passed as a pointer in \fIsubject\fR and a length in
297     \fIlength\fR. Unlike the pattern string, it may contain binary zero characters.
299 nigel 23 The PCRE_ANCHORED option can be passed in the \fIoptions\fR argument, whose
300     unused bits must be zero. However, if a pattern was compiled with
301     PCRE_ANCHORED, or turned out to be anchored by virtue of its contents, it
302     cannot be made unachored at matching time.
303 nigel 3
304     There are also two further options that can be set only at matching time:
308     The first character of the string is not the beginning of a line, so the
309     circumflex metacharacter should not match before it. Setting this without
310 nigel 23 PCRE_MULTILINE (at compile time) causes circumflex never to match.
311 nigel 3
314     The end of the string is not the end of a line, so the dollar metacharacter
315 nigel 23 should not match it nor (except in multiline mode) a newline immediately before
316     it. Setting this without PCRE_MULTILINE (at compile time) causes dollar never
317     to match.
318 nigel 3
319     In general, a pattern matches a certain portion of the subject, and in
320     addition, further substrings from the subject may be picked out by parts of the
321     pattern. Following the usage in Jeffrey Friedl's book, this is called
322     "capturing" in what follows, and the phrase "capturing subpattern" is used for
323     a fragment of a pattern that picks out a substring. PCRE supports several other
324     kinds of parenthesized subpattern that do not cause substrings to be captured.
326     Captured substrings are returned to the caller via a vector of integer offsets
327     whose address is passed in \fIovector\fR. The number of elements in the vector
328 nigel 23 is passed in \fIovecsize\fR. The first two-thirds of the vector is used to pass
329     back captured substrings, each substring using a pair of integers. The
330     remaining third of the vector is used as workspace by \fBpcre_exec()\fR while
331     matching capturing subpatterns, and is not available for passing back
332     information. The length passed in \fIovecsize\fR should always be a multiple of
333     three. If it is not, it is rounded down.
334 nigel 3
335 nigel 23 When a match has been successful, information about captured substrings is
336     returned in pairs of integers, starting at the beginning of \fIovector\fR, and
337     continuing up to two-thirds of its length at the most. The first element of a
338     pair is set to the offset of the first character in a substring, and the second
339     is set to the offset of the first character after the end of a substring. The
340     first pair, \fIovector[0]\fR and \fIovector[1]\fR, identify the portion of the
341     subject string matched by the entire pattern. The next pair is used for the
342     first capturing subpattern, and so on. The value returned by \fBpcre_exec()\fR
343     is the number of pairs that have been set. If there are no capturing
344     subpatterns, the return value from a successful match is 1, indicating that
345     just the first pair of offsets has been set.
346 nigel 3
347 nigel 29 Some convenience functions are provided for extracting the captured substrings
348     as separate strings. These are described in the following section.
350 nigel 3 It is possible for an capturing subpattern number \fIn+1\fR to match some
351     part of the subject when subpattern \fIn\fR has not been used at all. For
352 nigel 23 example, if the string "abc" is matched against the pattern (a|(z))(bc)
353 nigel 3 subpatterns 1 and 3 are matched, but 2 is not. When this happens, both offset
354     values corresponding to the unused subpattern are set to -1.
356     If a capturing subpattern is matched repeatedly, it is the last portion of the
357     string that it matched that gets returned.
359     If the vector is too small to hold all the captured substrings, it is used as
360 nigel 23 far as possible (up to two-thirds of its length), and the function returns a
361     value of zero. In particular, if the substring offsets are not of interest,
362     \fBpcre_exec()\fR may be called with \fIovector\fR passed as NULL and
363     \fIovecsize\fR as zero. However, if the pattern contains back references and
364     the \fIovector\fR isn't big enough to remember the related substrings, PCRE has
365     to get additional memory for use during matching. Thus it is usually advisable
366     to supply an \fIovector\fR.
367 nigel 3
368     Note that \fBpcre_info()\fR can be used to find out how many capturing
369 nigel 23 subpatterns there are in a compiled pattern. The smallest size for
370     \fIovector\fR that will allow for \fIn\fR captured substrings in addition to
371     the offsets of the substring matched by the whole pattern is (\fIn\fR+1)*3.
372 nigel 3
373     If \fBpcre_exec()\fR fails, it returns a negative number. The following are
374     defined in the header file:
378     The subject string did not match the pattern.
380 nigel 23 PCRE_ERROR_NULL (-2)
381 nigel 3
382     Either \fIcode\fR or \fIsubject\fR was passed as NULL, or \fIovector\fR was
383     NULL and \fIovecsize\fR was not zero.
385 nigel 23 PCRE_ERROR_BADOPTION (-3)
386 nigel 3
387     An unrecognized bit was set in the \fIoptions\fR argument.
389 nigel 23 PCRE_ERROR_BADMAGIC (-4)
390 nigel 3
391     PCRE stores a 4-byte "magic number" at the start of the compiled code, to catch
392     the case when it is passed a junk pointer. This is the error it gives when the
393     magic number isn't present.
395 nigel 23 PCRE_ERROR_UNKNOWN_NODE (-5)
396 nigel 3
397     While running the pattern match, an unknown item was encountered in the
398     compiled pattern. This error could be caused by a bug in PCRE or by overwriting
399     of the compiled pattern.
401 nigel 23 PCRE_ERROR_NOMEMORY (-6)
402 nigel 3
403     If a pattern contains back references, but the \fIovector\fR that is passed to
404     \fBpcre_exec()\fR is not big enough to remember the referenced substrings, PCRE
405     gets a block of memory at the start of matching to use for this purpose. If the
406     call via \fBpcre_malloc()\fR fails, this error is given. The memory is freed at
407     the end of matching.
411     Captured substrings can be accessed directly by using the offsets returned by
412     \fBpcre_exec()\fR in \fIovector\fR. For convenience, the functions
413     \fBpcre_copy_substring()\fR, \fBpcre_get_substring()\fR, and
414     \fBpcre_get_substring_list()\fR are provided for extracting captured substrings
415     as new, separate, zero-terminated strings. A substring that contains a binary
416     zero is correctly extracted and has a further zero added on the end, but the
417     result does not, of course, function as a C string.
418 nigel 3
419 nigel 29 The first three arguments are the same for all three functions: \fIsubject\fR
420     is the subject string which has just been successfully matched, \fIovector\fR
421     is a pointer to the vector of integer offsets that was passed to
422     \fBpcre_exec()\fR, and \fIstringcount\fR is the number of substrings that
423     were captured by the match, including the substring that matched the entire
424     regular expression. This is the value returned by \fBpcre_exec\fR if it
425     is greater than zero. If \fBpcre_exec()\fR returned zero, indicating that it
426     ran out of space in \fIovector\fR, then the value passed as
427     \fIstringcount\fR should be the size of the vector divided by three.
428 nigel 3
429 nigel 29 The functions \fBpcre_copy_substring()\fR and \fBpcre_get_substring()\fR
430     extract a single substring, whose number is given as \fIstringnumber\fR. A
431     value of zero extracts the substring that matched the entire pattern, while
432     higher values extract the captured substrings. For \fBpcre_copy_substring()\fR,
433     the string is placed in \fIbuffer\fR, whose length is given by
434     \fIbuffersize\fR, while for \fBpcre_get_substring()\fR a new block of store is
435     obtained via \fBpcre_malloc\fR, and its address is returned via
436     \fIstringptr\fR. The yield of the function is the length of the string, not
437     including the terminating zero, or one of
438 nigel 3
439 nigel 29 PCRE_ERROR_NOMEMORY (-6)
440 nigel 3
441 nigel 29 The buffer was too small for \fBpcre_copy_substring()\fR, or the attempt to get
442     memory failed for \fBpcre_get_substring()\fR.
443 nigel 3
444 nigel 29 PCRE_ERROR_NOSUBSTRING (-7)
446     There is no substring whose number is \fIstringnumber\fR.
448     The \fBpcre_get_substring_list()\fR function extracts all available substrings
449     and builds a list of pointers to them. All this is done in a single block of
450     memory which is obtained via \fBpcre_malloc\fR. The address of the memory block
451     is returned via \fIlistptr\fR, which is also the start of the list of string
452     pointers. The end of the list is marked by a NULL pointer. The yield of the
453     function is zero if all went well, or
457     if the attempt to get the memory block failed.
459     When any of these functions encounter a substring that is unset, which can
460     happen when capturing subpattern number \fIn+1\fR matches some part of the
461     subject, but subpattern \fIn\fR has not been used at all, they return an empty
462     string. This can be distinguished from a genuine zero-length substring by
463     inspecting the appropriate offset in \fIovector\fR, which is negative for unset
464     substrings.
468 nigel 3 .SH LIMITATIONS
469     There are some size limitations in PCRE but it is hoped that they will never in
470     practice be relevant.
471     The maximum length of a compiled pattern is 65539 (sic) bytes.
472     All values in repeating quantifiers must be less than 65536.
473     The maximum number of capturing subpatterns is 99.
474     The maximum number of all parenthesized subpatterns, including capturing
475 nigel 23 subpatterns, assertions, and other types of subpattern, is 200.
476 nigel 3
477     The maximum length of a subject string is the largest positive number that an
478     integer variable can hold. However, PCRE uses recursion to handle subpatterns
479     and indefinite repetition. This means that the available stack space may limit
480     the size of a subject string that can be processed by certain patterns.
484 nigel 23 The differences described here are with respect to Perl 5.005.
485 nigel 3
486     1. By default, a whitespace character is any character that the C library
487     function \fBisspace()\fR recognizes, though it is possible to compile PCRE with
488     alternative character type tables. Normally \fBisspace()\fR matches space,
489     formfeed, newline, carriage return, horizontal tab, and vertical tab. Perl 5
490     no longer includes vertical tab in its set of whitespace characters. The \\v
491     escape that was in the Perl documentation for a long time was never in fact
492     recognized. However, the character itself was treated as whitespace at least
493 nigel 23 up to 5.002. In 5.004 and 5.005 it does not match \\s.
494 nigel 3
495     2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
496 nigel 23 them, but they do not mean what you might think. For example, (?!a){3} does
497 nigel 3 not assert that the next three characters are not "a". It just asserts that the
498     next character is not "a" three times.
500     3. Capturing subpatterns that occur inside negative lookahead assertions are
501     counted, but their entries in the offsets vector are never set. Perl sets its
502     numerical variables from any such patterns that are matched before the
503     assertion fails to match something (thereby succeeding), but only if the
504     negative lookahead assertion contains just one branch.
506     4. Though binary zero characters are supported in the subject string, they are
507     not allowed in a pattern string because it is passed as a normal C string,
508     terminated by zero. The escape sequence "\\0" can be used in the pattern to
509     represent a binary zero.
511     5. The following Perl escape sequences are not supported: \\l, \\u, \\L, \\U,
512     \\E, \\Q. In fact these are implemented by Perl's general string-handling and
513     are not part of its pattern matching engine.
515     6. The Perl \\G assertion is not supported as it is not relevant to single
516     pattern matches.
518 nigel 23 7. Fairly obviously, PCRE does not support the (?{code}) construction.
519 nigel 3
520 nigel 23 8. There are at the time of writing some oddities in Perl 5.005_02 concerned
521     with the settings of captured strings when part of a pattern is repeated. For
522     example, matching "aba" against the pattern /^(a(b)?)+$/ sets $2 to the value
523     "b", but matching "aabbaa" against /^(aa(bb)?)+$/ leaves $2 unset. However, if
524     the pattern is changed to /^(aa(b(b))?)+$/ then $2 (and $3) get set.
525 nigel 3
526 nigel 23 In Perl 5.004 $2 is set in both cases, and that is also true of PCRE. If in the
527     future Perl changes to a consistent state that is different, PCRE may change to
528     follow.
529 nigel 3
530 nigel 23 9. Another as yet unresolved discrepancy is that in Perl 5.005_02 the pattern
531     /^(a)?(?(1)a|b)+$/ matches the string "a", whereas in PCRE it does not.
532     However, in both Perl and PCRE /^(a)?a/ matched against "a" leaves $1 unset.
533 nigel 3
534 nigel 23 10. PCRE provides some extensions to the Perl regular expression facilities:
535 nigel 3
536 nigel 23 (a) Although lookbehind assertions must match fixed length strings, each
537     alternative branch of a lookbehind assertion can match a different length of
538     string. Perl 5.005 requires them all to have the same length.
539 nigel 3
540 nigel 23 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ meta-
541 nigel 3 character matches only at the very end of the string.
543 nigel 23 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
544     meaning is faulted.
545 nigel 3
546 nigel 23 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
547 nigel 19 inverted, that is, by default they are not greedy, but if followed by a
548     question mark they are.
549 nigel 3
550 nigel 19
552     The syntax and semantics of the regular expressions supported by PCRE are
553     described below. Regular expressions are also described in the Perl
554     documentation and in a number of other books, some of which have copious
555     examples. Jeffrey Friedl's "Mastering Regular Expressions", published by
556     O'Reilly (ISBN 1-56592-257-3), covers them in great detail. The description
557     here is intended as reference documentation.
559     A regular expression is a pattern that is matched against a subject string from
560     left to right. Most characters stand for themselves in a pattern, and match the
561     corresponding characters in the subject. As a trivial example, the pattern
563     The quick brown fox
565     matches a portion of a subject string that is identical to itself. The power of
566     regular expressions comes from the ability to include alternatives and
567     repetitions in the pattern. These are encoded in the pattern by the use of
568     \fImeta-characters\fR, which do not stand for themselves but instead are
569     interpreted in some special way.
571     There are two different sets of meta-characters: those that are recognized
572     anywhere in the pattern except within square brackets, and those that are
573     recognized in square brackets. Outside square brackets, the meta-characters are
574     as follows:
576     \\ general escape character with several uses
577     ^ assert start of subject (or line, in multiline mode)
578     $ assert end of subject (or line, in multiline mode)
579     . match any character except newline (by default)
580     [ start character class definition
581     | start of alternative branch
582     ( start subpattern
583     ) end subpattern
584     ? extends the meaning of (
585     also 0 or 1 quantifier
586     also quantifier minimizer
587     * 0 or more quantifier
588     + 1 or more quantifier
589     { start min/max quantifier
591     Part of a pattern that is in square brackets is called a "character class". In
592     a character class the only meta-characters are:
594     \\ general escape character
595     ^ negate the class, but only if the first character
596     - indicates character range
597     ] terminates the character class
599     The following sections describe the use of each of the meta-characters.
603     The backslash character has several uses. Firstly, if it is followed by a
604     non-alphameric character, it takes away any special meaning that character may
605     have. This use of backslash as an escape character applies both inside and
606     outside character classes.
608     For example, if you want to match a "*" character, you write "\\*" in the
609     pattern. This applies whether or not the following character would otherwise be
610     interpreted as a meta-character, so it is always safe to precede a
611     non-alphameric with "\\" to specify that it stands for itself. In particular,
612     if you want to match a backslash, you write "\\\\".
614     If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
615 nigel 23 pattern (other than in a character class) and characters between a "#" outside
616     a character class and the next newline character are ignored. An escaping
617     backslash can be used to include a whitespace or "#" character as part of the
618     pattern.
619 nigel 3
620     A second use of backslash provides a way of encoding non-printing characters
621     in patterns in a visible manner. There is no restriction on the appearance of
622     non-printing characters, apart from the binary zero that terminates a pattern,
623     but when a pattern is being prepared by text editing, it is usually easier to
624     use one of the following escape sequences than the binary character it
625     represents:
627     \\a alarm, that is, the BEL character (hex 07)
628     \\cx "control-x", where x is any character
629     \\e escape (hex 1B)
630     \\f formfeed (hex 0C)
631     \\n newline (hex 0A)
632     \\r carriage return (hex 0D)
633     \\t tab (hex 09)
634     \\xhh character with hex code hh
635 nigel 23 \\ddd character with octal code ddd, or backreference
636 nigel 3
637     The precise effect of "\\cx" is as follows: if "x" is a lower case letter, it
638     is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
639     Thus "\\cz" becomes hex 1A, but "\\c{" becomes hex 3B, while "\\c;" becomes hex
640     7B.
642     After "\\x", up to two hexadecimal digits are read (letters can be in upper or
643     lower case).
645     After "\\0" up to two further octal digits are read. In both cases, if there
646     are fewer than two digits, just those that are present are used. Thus the
647     sequence "\\0\\x\\07" specifies two binary zeros followed by a BEL character.
648 nigel 23 Make sure you supply two digits after the initial zero if the character that
649     follows is itself an octal digit.
650 nigel 3
651     The handling of a backslash followed by a digit other than 0 is complicated.
652     Outside a character class, PCRE reads it and any following digits as a decimal
653     number. If the number is less than 10, or if there have been at least that many
654     previous capturing left parentheses in the expression, the entire sequence is
655     taken as a \fIback reference\fR. A description of how this works is given
656     later, following the discussion of parenthesized subpatterns.
658     Inside a character class, or if the decimal number is greater than 9 and there
659     have not been that many capturing subpatterns, PCRE re-reads up to three octal
660     digits following the backslash, and generates a single byte from the least
661     significant 8 bits of the value. Any subsequent digits stand for themselves.
662     For example:
664     \\040 is another way of writing a space
665     \\40 is the same, provided there are fewer than 40
666     previous capturing subpatterns
667     \\7 is always a back reference
668     \\11 might be a back reference, or another way of
669     writing a tab
670     \\011 is always a tab
671     \\0113 is a tab followed by the character "3"
672     \\113 is the character with octal code 113 (since there
673     can be no more than 99 back references)
674     \\377 is a byte consisting entirely of 1 bits
675     \\81 is either a back reference, or a binary zero
676     followed by the two characters "8" and "1"
678     Note that octal values of 100 or greater must not be introduced by a leading
679     zero, because no more than three octal digits are ever read.
681     All the sequences that define a single byte value can be used both inside and
682     outside character classes. In addition, inside a character class, the sequence
683     "\\b" is interpreted as the backspace character (hex 08). Outside a character
684     class it has a different meaning (see below).
686     The third use of backslash is for specifying generic character types:
688     \\d any decimal digit
689     \\D any character that is not a decimal digit
690     \\s any whitespace character
691     \\S any character that is not a whitespace character
692     \\w any "word" character
693     \\W any "non-word" character
695     Each pair of escape sequences partitions the complete set of characters into
696     two disjoint sets. Any given character matches one, and only one, of each pair.
698     A "word" character is any letter or digit or the underscore character, that is,
699 nigel 25 any character which can be part of a Perl "word". The definition of letters and
700     digits is controlled by PCRE's character tables, and may vary if locale-
701     specific matching is taking place (see "Locale support" above). For example, in
702     the "fr" (French) locale, some character codes greater than 128 are used for
703     accented letters, and these are matched by \\w.
704 nigel 3
705 nigel 25 These character type sequences can appear both inside and outside character
706     classes. They each match one character of the appropriate type. If the current
707     matching point is at the end of the subject string, all of them fail, since
708     there is no character to match.
710 nigel 23 The fourth use of backslash is for certain simple assertions. An assertion
711     specifies a condition that has to be met at a particular point in a match,
712     without consuming any characters from the subject string. The use of
713     subpatterns for more complicated assertions is described below. The backslashed
714     assertions are
715 nigel 3
716     \\b word boundary
717     \\B not a word boundary
718     \\A start of subject (independent of multiline mode)
719 nigel 23 \\Z end of subject or newline at end (independent of multiline mode)
720     \\z end of subject (independent of multiline mode)
721 nigel 3
722 nigel 23 These assertions may not appear in character classes (but note that "\\b" has a
723 nigel 3 different meaning, namely the backspace character, inside a character class).
725     A word boundary is a position in the subject string where the current character
726 nigel 23 and the previous character do not both match \\w or \\W (i.e. one matches
727     \\w and the other matches \\W), or the start or end of the string if the
728     first or last character matches \\w, respectively.
729 nigel 3
730 nigel 23 The \\A, \\Z, and \\z assertions differ from the traditional circumflex and
731     dollar (described below) in that they only ever match at the very start and end
732     of the subject string, whatever options are set. They are not affected by the
733     PCRE_NOTBOL or PCRE_NOTEOL options. The difference between \\Z and \\z is that
734     \\Z matches before a newline that is the last character of the string as well
735     as at the end of the string, whereas \\z matches only at the end.
736 nigel 3
739 nigel 23 Outside a character class, in the default matching mode, the circumflex
740     character is an assertion which is true only if the current matching point is
741     at the start of the subject string. Inside a character class, circumflex has an
742 nigel 3 entirely different meaning (see below).
744     Circumflex need not be the first character of the pattern if a number of
745     alternatives are involved, but it should be the first thing in each alternative
746     in which it appears if the pattern is ever to match that branch. If all
747     possible alternatives start with a circumflex, that is, if the pattern is
748     constrained to match only at the start of the subject, it is said to be an
749     "anchored" pattern. (There are also other constructs that can cause a pattern
750     to be anchored.)
752     A dollar character is an assertion which is true only if the current matching
753     point is at the end of the subject string, or immediately before a newline
754     character that is the last character in the string (by default). Dollar need
755     not be the last character of the pattern if a number of alternatives are
756     involved, but it should be the last item in any branch in which it appears.
757     Dollar has no special meaning in a character class.
759     The meaning of dollar can be changed so that it matches only at the very end of
760     the string, by setting the PCRE_DOLLAR_ENDONLY option at compile or matching
761 nigel 23 time. This does not affect the \\Z assertion.
762 nigel 3
763     The meanings of the circumflex and dollar characters are changed if the
764 nigel 23 PCRE_MULTILINE option is set. When this is the case, they match immediately
765     after and immediately before an internal "\\n" character, respectively, in
766     addition to matching at the start and end of the subject string. For example,
767     the pattern /^abc$/ matches the subject string "def\\nabc" in multiline mode,
768     but not otherwise. Consequently, patterns that are anchored in single line mode
769     because all branches start with "^" are not anchored in multiline mode. The
770     PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
771 nigel 3
772 nigel 23 Note that the sequences \\A, \\Z, and \\z can be used to match the start and
773     end of the subject in both modes, and if all branches of a pattern start with
774     \\A is it always anchored, whether PCRE_MULTILINE is set or not.
775 nigel 3
778     Outside a character class, a dot in the pattern matches any one character in
779     the subject, including a non-printing character, but not (by default) newline.
780     If the PCRE_DOTALL option is set, then dots match newlines as well. The
781     handling of dot is entirely independent of the handling of circumflex and
782     dollar, the only relationship being that they both involve newline characters.
783     Dot has no special meaning in a character class.
787     An opening square bracket introduces a character class, terminated by a closing
788     square bracket. A closing square bracket on its own is not special. If a
789     closing square bracket is required as a member of the class, it should be the
790     first data character in the class (after an initial circumflex, if present) or
791 nigel 23 escaped with a backslash.
792 nigel 3
793     A character class matches a single character in the subject; the character must
794     be in the set of characters defined by the class, unless the first character in
795     the class is a circumflex, in which case the subject character must not be in
796     the set defined by the class. If a circumflex is actually required as a member
797 nigel 23 of the class, ensure it is not the first character, or escape it with a
798     backslash.
799 nigel 3
800     For example, the character class [aeiou] matches any lower case vowel, while
801     [^aeiou] matches any character that is not a lower case vowel. Note that a
802     circumflex is just a convenient notation for specifying the characters which
803     are in the class by enumerating those that are not. It is not an assertion: it
804     still consumes a character from the subject string, and fails if the current
805     pointer is at the end of the string.
807 nigel 25 When caseless matching is set, any letters in a class represent both their
808     upper case and lower case versions, so for example, a caseless [aeiou] matches
809     "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
810     caseful version would.
811 nigel 23
812 nigel 3 The newline character is never treated in any special way in character classes,
813     whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class
814     such as [^a] will always match a newline.
816     The minus (hyphen) character can be used to specify a range of characters in a
817     character class. For example, [d-m] matches any letter between d and m,
818     inclusive. If a minus character is required in a class, it must be escaped with
819 nigel 23 a backslash or appear in a position where it cannot be interpreted as
820 nigel 29 indicating a range, typically as the first or last character in the class.
821 nigel 3
822 nigel 29 It is not possible to have the literal character "]" as the end character of a
823     range. A pattern such as [W-]46] is interpreted as a class of two characters
824     ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
825     "-46]". However, if the "]" is escaped with a backslash it is interpreted as
826     the end of range, so [W-\\]46] is interpreted as a single class containing a
827     range followed by two separate characters. The octal or hexadecimal
828     representation of "]" can also be used to end a range.
830 nigel 3 Ranges operate in ASCII collating sequence. They can also be used for
831 nigel 25 characters specified numerically, for example [\\000-\\037]. If a range that
832     includes letters is used when caseless matching is set, it matches the letters
833     in either case. For example, [W-c] is equivalent to [][\\^_`wxyzabc], matched
834     caselessly, and if character tables for the "fr" locale are in use,
835     [\\xc8-\\xcb] matches accented E characters in both cases.
836 nigel 3
837     The character types \\d, \\D, \\s, \\S, \\w, and \\W may also appear in a
838     character class, and add the characters that they match to the class. For
839 nigel 23 example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can
840     conveniently be used with the upper case character types to specify a more
841     restricted set of characters than the matching lower case type. For example,
842     the class [^\\W_] matches any letter or digit, but not underscore.
843 nigel 3
844     All non-alphameric characters other than \\, -, ^ (at the start) and the
845     terminating ] are non-special in character classes, but it does no harm if they
846     are escaped.
850 nigel 23 Vertical bar characters are used to separate alternative patterns. For example,
851     the pattern
852 nigel 3
853     gilbert|sullivan
855 nigel 23 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
856 nigel 3 and an empty alternative is permitted (matching the empty string).
857 nigel 23 The matching process tries each alternative in turn, from left to right,
858     and the first one that succeeds is used. If the alternatives are within a
859     subpattern (defined below), "succeeds" means matching the rest of the main
860     pattern as well as the alternative in the subpattern.
861 nigel 3
865     can be changed from within the pattern by a sequence of Perl option letters
866     enclosed between "(?" and ")". The option letters are
868     i for PCRE_CASELESS
869     m for PCRE_MULTILINE
870     s for PCRE_DOTALL
871     x for PCRE_EXTENDED
873     For example, (?im) sets caseless, multiline matching. It is also possible to
874     unset these options by preceding the letter with a hyphen, and a combined
875     setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
876     PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
877     permitted. If a letter appears both before and after the hyphen, the option is
878     unset.
880     The scope of these option changes depends on where in the pattern the setting
881     occurs. For settings that are outside any subpattern (defined below), the
882     effect is the same as if the options were set or unset at the start of
883     matching. The following patterns all behave in exactly the same way:
885     (?i)abc
886     a(?i)bc
887     ab(?i)c
888     abc(?i)
890     which in turn is the same as compiling the pattern abc with PCRE_CASELESS set.
891     In other words, such "top level" settings apply to the whole pattern (unless
892     there are other changes inside subpatterns). If there is more than one setting
893     of the same option at top level, the rightmost setting is used.
895     If an option change occurs inside a subpattern, the effect is different. This
896     is a change of behaviour in Perl 5.005. An option change inside a subpattern
897     affects only that part of the subpattern that follows it, so
899     (a(?i)b)c
901     matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
902     By this means, options can be made to have different settings in different
903     parts of the pattern. Any changes made in one alternative do carry on
904     into subsequent branches within the same subpattern. For example,
906     (a(?i)b|c)
908     matches "ab", "aB", "c", and "C", even though when matching "C" the first
909     branch is abandoned before the option setting. This is because the effects of
910     option settings happen at compile time. There would be some very weird
911     behaviour otherwise.
913     The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the
914     same way as the Perl-compatible options by using the characters U and X
915     respectively. The (?X) flag setting is special in that it must always occur
916     earlier in the pattern than any of the additional features it turns on, even
917     when it is at top level. It is best put at the start.
920 nigel 3 .SH SUBPATTERNS
921     Subpatterns are delimited by parentheses (round brackets), which can be nested.
922     Marking part of a pattern as a subpattern does two things:
924     1. It localizes a set of alternatives. For example, the pattern
926     cat(aract|erpillar|)
928     matches one of the words "cat", "cataract", or "caterpillar". Without the
929     parentheses, it would match "cataract", "erpillar" or the empty string.
931     2. It sets up the subpattern as a capturing subpattern (as defined above).
932     When the whole pattern matches, that portion of the subject string that matched
933     the subpattern is passed back to the caller via the \fIovector\fR argument of
934     \fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting
935     from 1) to obtain the numbers of the capturing subpatterns.
937     For example, if the string "the red king" is matched against the pattern
939     the ((red|white) (king|queen))
941     the captured substrings are "red king", "red", and "king", and are numbered 1,
942     2, and 3.
944     The fact that plain parentheses fulfil two functions is not always helpful.
945     There are often times when a grouping subpattern is required without a
946     capturing requirement. If an opening parenthesis is followed by "?:", the
947     subpattern does not do any capturing, and is not counted when computing the
948     number of any subsequent capturing subpatterns. For example, if the string "the
949     white queen" is matched against the pattern
951     the ((?:red|white) (king|queen))
953     the captured substrings are "white queen" and "queen", and are numbered 1 and
954     2. The maximum number of captured substrings is 99, and the maximum number of
955     all subpatterns, both capturing and non-capturing, is 200.
957 nigel 23 As a convenient shorthand, if any option settings are required at the start of
958     a non-capturing subpattern, the option letters may appear between the "?" and
959     the ":". Thus the two patterns
960 nigel 3
961 nigel 23 (?i:saturday|sunday)
962     (?:(?i)saturday|sunday)
963 nigel 3
964 nigel 23 match exactly the same set of strings. Because alternative branches are tried
965     from left to right, and options are not reset until the end of the subpattern
966     is reached, an option setting in one branch does affect subsequent branches, so
967     the above patterns match "SUNDAY" as well as "Saturday".
968 nigel 3
971     Repetition is specified by quantifiers, which can follow any of the following
972     items:
974     a single character, possibly escaped
975     the . metacharacter
976     a character class
977 nigel 23 a back reference (see next section)
978     a parenthesized subpattern (unless it is an assertion - see below)
979 nigel 3
980     The general repetition quantifier specifies a minimum and maximum number of
981     permitted matches, by giving the two numbers in curly brackets (braces),
982     separated by a comma. The numbers must be less than 65536, and the first must
983     be less than or equal to the second. For example:
985     z{2,4}
987     matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
988     character. If the second number is omitted, but the comma is present, there is
989     no upper limit; if the second number and the comma are both omitted, the
990     quantifier specifies an exact number of required matches. Thus
992     [aeiou]{3,}
994     matches at least 3 successive vowels, but may match many more, while
996     \\d{8}
998     matches exactly 8 digits. An opening curly bracket that appears in a position
999     where a quantifier is not allowed, or one that does not match the syntax of a
1000 nigel 23 quantifier, is taken as a literal character. For example, {,6} is not a
1001 nigel 3 quantifier, but a literal string of four characters.
1003     The quantifier {0} is permitted, causing the expression to behave as if the
1004     previous item and the quantifier were not present.
1006     For convenience (and historical compatibility) the three most common
1007     quantifiers have single-character abbreviations:
1009     * is equivalent to {0,}
1010     + is equivalent to {1,}
1011     ? is equivalent to {0,1}
1013 nigel 23 It is possible to construct infinite loops by following a subpattern that can
1014     match no characters with a quantifier that has no upper limit, for example:
1016     (a?)*
1018     Earlier versions of Perl and PCRE used to give an error at compile time for
1019     such patterns. However, because there are cases where this can be useful, such
1020     patterns are now accepted, but if any repetition of the subpattern does in fact
1021     match no characters, the loop is forcibly broken.
1023 nigel 3 By default, the quantifiers are "greedy", that is, they match as much as
1024     possible (up to the maximum number of permitted times), without causing the
1025     rest of the pattern to fail. The classic example of where this gives problems
1026     is in trying to match comments in C programs. These appear between the
1027     sequences /* and */ and within the sequence, individual * and / characters may
1028     appear. An attempt to match C comments by applying the pattern
1030     /\\*.*\\*/
1032     to the string
1034     /* first command */ not comment /* second comment */
1036     fails, because it matches the entire string due to the greediness of the .*
1037     item.
1039     However, if a quantifier is followed by a question mark, then it ceases to be
1040     greedy, and instead matches the minimum number of times possible, so the
1041     pattern
1043     /\\*.*?\\*/
1045     does the right thing with the C comments. The meaning of the various
1046     quantifiers is not otherwise changed, just the preferred number of matches.
1047     Do not confuse this use of question mark with its use as a quantifier in its
1048     own right. Because it has two uses, it can sometimes appear doubled, as in
1050 nigel 23 \\d??\\d
1051 nigel 3
1052     which matches one digit by preference, but can match two if that is the only
1053     way the rest of the pattern matches.
1055 nigel 19 If the PCRE_UNGREEDY option is set (an option which is not available in Perl)
1056     then the quantifiers are not greedy by default, but individual ones can be made
1057 nigel 23 greedy by following them with a question mark. In other words, it inverts the
1058 nigel 19 default behaviour.
1060 nigel 7 When a parenthesized subpattern is quantified with a minimum repeat count that
1061 nigel 3 is greater than 1 or with a limited maximum, more store is required for the
1062     compiled pattern, in proportion to the size of the minimum or maximum.
1064 nigel 33 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1065     to Perl's /s) is set, thus allowing the . to match newlines, then the pattern
1066     is implicitly anchored, because whatever follows will be tried against every
1067     character position in the subject string, so there is no point in retrying the
1068     overall match at any position after the first. PCRE treats such a pattern as
1069     though it were preceded by \\A. In cases where it is known that the subject
1070     string contains no newlines, it is worth setting PCRE_DOTALL when the pattern
1071     begins with .* in order to obtain this optimization, or alternatively using ^
1072     to indicate anchoring explicitly.
1073 nigel 3
1074     When a capturing subpattern is repeated, the value captured is the substring
1075 nigel 23 that matched the final iteration. For example, after
1076 nigel 3
1077 nigel 23 (tweedle[dume]{3}\\s*)+
1078 nigel 3
1079 nigel 23 has matched "tweedledum tweedledee" the value of the captured substring is
1080     "tweedledee". However, if there are nested capturing subpatterns, the
1081     corresponding captured values may have been set in previous iterations. For
1082     example, after
1083 nigel 3
1084 nigel 23 /(a|(b))+/
1085 nigel 3
1086 nigel 23 matches "aba" the value of the second captured substring is "b".
1090     Outside a character class, a backslash followed by a digit greater than 0 (and
1091     possibly further digits) is a back reference to a capturing subpattern earlier
1092     (i.e. to its left) in the pattern, provided there have been that many previous
1093     capturing left parentheses.
1095     However, if the decimal number following the backslash is less than 10, it is
1096     always taken as a back reference, and causes an error only if there are not
1097     that many capturing left parentheses in the entire pattern. In other words, the
1098     parentheses that are referenced need not be to the left of the reference for
1099     numbers less than 10. See the section entitled "Backslash" above for further
1100     details of the handling of digits following a backslash.
1102     A back reference matches whatever actually matched the capturing subpattern in
1103     the current subject string, rather than anything matching the subpattern
1104     itself. So the pattern
1106     (sens|respons)e and \\1ibility
1108     matches "sense and sensibility" and "response and responsibility", but not
1109     "sense and responsibility". If caseful matching is in force at the time of the
1110     back reference, then the case of letters is relevant. For example,
1112     ((?i)rah)\\s+\\1
1114     matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1115     capturing subpattern is matched caselessly.
1117     There may be more than one back reference to the same subpattern. If a
1118     subpattern has not actually been used in a particular match, then any back
1119     references to it always fail. For example, the pattern
1121     (a|(bc))\\2
1123     always fails if it starts to match "a" rather than "bc". Because there may be
1124     up to 99 back references, all digits following the backslash are taken
1125     as part of a potential back reference number. If the pattern continues with a
1126     digit character, then some delimiter must be used to terminate the back
1127     reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1128     Otherwise an empty comment can be used.
1130     A back reference that occurs inside the parentheses to which it refers fails
1131     when the subpattern is first used, so, for example, (a\\1) never matches.
1132     However, such references can be useful inside repeated subpatterns. For
1133     example, the pattern
1135     (a|b\\1)+
1137     matches any number of "a"s and also "aba", "ababaa" etc. At each iteration of
1138     the subpattern, the back reference matches the character string corresponding
1139     to the previous iteration. In order for this to work, the pattern must be such
1140     that the first iteration does not need to match the back reference. This can be
1141     done using alternation, as in the example above, or by a quantifier with a
1142     minimum of zero.
1145 nigel 3 .SH ASSERTIONS
1146 nigel 23 An assertion is a test on the characters following or preceding the current
1147     matching point that does not actually consume any characters. The simple
1148     assertions coded as \\b, \\B, \\A, \\Z, \\z, ^ and $ are described above. More
1149     complicated assertions are coded as subpatterns. There are two kinds: those
1150     that look ahead of the current position in the subject string, and those that
1151     look behind it.
1152 nigel 3
1153 nigel 23 An assertion subpattern is matched in the normal way, except that it does not
1154     cause the current matching position to be changed. Lookahead assertions start
1155     with (?= for positive assertions and (?! for negative assertions. For example,
1157 nigel 3 \\w+(?=;)
1159     matches a word followed by a semicolon, but does not include the semicolon in
1160     the match, and
1162     foo(?!bar)
1164     matches any occurrence of "foo" that is not followed by "bar". Note that the
1165     apparently similar pattern
1167     (?!foo)bar
1169     does not find an occurrence of "bar" that is preceded by something other than
1170     "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1171 nigel 23 (?!foo) is always true when the next three characters are "bar". A
1172     lookbehind assertion is needed to achieve this effect.
1173 nigel 3
1174 nigel 23 Lookbehind assertions start with (?<= for positive assertions and (?<! for
1175     negative assertions. For example,
1177     (?<!foo)bar
1179     does find an occurrence of "bar" that is not preceded by "foo". The contents of
1180     a lookbehind assertion are restricted such that all the strings it matches must
1181     have a fixed length. However, if there are several alternatives, they do not
1182     all have to have the same fixed length. Thus
1184     (?<=bullock|donkey)
1186     is permitted, but
1188     (?<!dogs?|cats?)
1190     causes an error at compile time. Branches that match different length strings
1191     are permitted only at the top level of a lookbehind assertion. This is an
1192     extension compared with Perl 5.005, which requires all branches to match the
1193     same length of string. An assertion such as
1195     (?<=ab(c|de))
1197 nigel 27 is not permitted, because its single top-level branch can match two different
1198     lengths, but it is acceptable if rewritten to use two top-level branches:
1199 nigel 23
1200     (?<=abc|abde)
1202     The implementation of lookbehind assertions is, for each alternative, to
1203     temporarily move the current position back by the fixed width and then try to
1204     match. If there are insufficient characters before the current position, the
1205 nigel 27 match is deemed to fail. Lookbehinds in conjunction with once-only subpatterns
1206     can be particularly useful for matching at the ends of strings; an example is
1207     given at the end of the section on once-only subpatterns.
1208 nigel 23
1209 nigel 27 Several assertions (of any sort) may occur in succession. For example,
1210 nigel 23
1211 nigel 27 (?<=\\d{3})(?<!999)foo
1213     matches "foo" preceded by three digits that are not "999". Furthermore,
1214     assertions can be nested in any combination. For example,
1216 nigel 23 (?<=(?<!foo)bar)baz
1218     matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1219     preceded by "foo".
1221 nigel 3 Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1222     because it makes no sense to assert the same thing several times. If an
1223     assertion contains capturing subpatterns within it, these are always counted
1224     for the purposes of numbering the capturing subpatterns in the whole pattern.
1225     Substring capturing is carried out for positive assertions, but it does not
1226     make sense for negative assertions.
1228     Assertions count towards the maximum of 200 parenthesized subpatterns.
1232     With both maximizing and minimizing repetition, failure of what follows
1233     normally causes the repeated item to be re-evaluated to see if a different
1234     number of repeats allows the rest of the pattern to match. Sometimes it is
1235     useful to prevent this, either to change the nature of the match, or to cause
1236 nigel 23 it fail earlier than it otherwise might, when the author of the pattern knows
1237 nigel 3 there is no point in carrying on.
1239     Consider, for example, the pattern \\d+foo when applied to the subject line
1241 nigel 23 123456bar
1242 nigel 3
1243     After matching all 6 digits and then failing to match "foo", the normal
1244     action of the matcher is to try again with only 5 digits matching the \\d+
1245     item, and then with 4, and so on, before ultimately failing. Once-only
1246     subpatterns provide the means for specifying that once a portion of the pattern
1247     has matched, it is not to be re-evaluated in this way, so the matcher would
1248     give up immediately on failing to match "foo" the first time. The notation is
1249     another kind of special parenthesis, starting with (?> as in this example:
1251 nigel 23 (?>\\d+)bar
1252 nigel 3
1253     This kind of parenthesis "locks up" the part of the pattern it contains once
1254     it has matched, and a failure further into the pattern is prevented from
1255     backtracking into it. Backtracking past it to previous items, however, works as
1256     normal.
1258 nigel 23 An alternative description is that a subpattern of this type matches the string
1259     of characters that an identical standalone pattern would match, if anchored at
1260     the current point in the subject string.
1261 nigel 3
1262 nigel 23 Once-only subpatterns are not capturing subpatterns. Simple cases such as the
1263 nigel 29 above example can be thought of as a maximizing repeat that must swallow
1264 nigel 23 everything it can. So, while both \\d+ and \\d+? are prepared to adjust the
1265     number of digits they match in order to make the rest of the pattern match,
1266     (?>\\d+) can only match an entire sequence of digits.
1268 nigel 3 This construction can of course contain arbitrarily complicated subpatterns,
1269 nigel 23 and it can be nested.
1270 nigel 3
1271 nigel 27 Once-only subpatterns can be used in conjunction with lookbehind assertions to
1272     specify efficient matching at the end of the subject string. Consider a simple
1273     pattern such as
1274 nigel 3
1275 nigel 27 abcd$
1277     when applied to a long string which does not match it. Because matching
1278     proceeds from left to right, PCRE will look for each "a" in the subject and
1279     then see if what follows matches the rest of the pattern. If the pattern is
1280     specified as
1282 nigel 33 ^.*abcd$
1283 nigel 27
1284     then the initial .* matches the entire string at first, but when this fails, it
1285     backtracks to match all but the last character, then all but the last two
1286     characters, and so on. Once again the search for "a" covers the entire string,
1287     from right to left, so we are no better off. However, if the pattern is written
1288     as
1290 nigel 33 ^(?>.*)(?<=abcd)
1291 nigel 27
1292     then there can be no backtracking for the .* item; it can match only the entire
1293     string. The subsequent lookbehind assertion does a single test on the last four
1294     characters. If it fails, the match fails immediately. For long strings, this
1295     approach makes a significant difference to the processing time.
1299     It is possible to cause the matching process to obey a subpattern
1300     conditionally or to choose between two alternative subpatterns, depending on
1301     the result of an assertion, or whether a previous capturing subpattern matched
1302     or not. The two possible forms of conditional subpattern are
1303 nigel 3
1304 nigel 23 (?(condition)yes-pattern)
1305     (?(condition)yes-pattern|no-pattern)
1306 nigel 3
1307 nigel 23 If the condition is satisfied, the yes-pattern is used; otherwise the
1308     no-pattern (if present) is used. If there are more than two alternatives in the
1309     subpattern, a compile-time error occurs.
1310 nigel 3
1311 nigel 23 There are two kinds of condition. If the text between the parentheses consists
1312     of a sequence of digits, then the condition is satisfied if the capturing
1313     subpattern of that number has previously matched. Consider the following
1314     pattern, which contains non-significant white space to make it more readable
1315     (assume the PCRE_EXTENDED option) and to divide it into three parts for ease
1316     of discussion:
1317 nigel 3
1318 nigel 23 ( \\( )? [^()]+ (?(1) \\) )
1319 nigel 3
1320 nigel 23 The first part matches an optional opening parenthesis, and if that
1321     character is present, sets it as the first captured substring. The second part
1322     matches one or more characters that are not parentheses. The third part is a
1323     conditional subpattern that tests whether the first set of parentheses matched
1324     or not. If they did, that is, if subject started with an opening parenthesis,
1325     the condition is true, and so the yes-pattern is executed and a closing
1326     parenthesis is required. Otherwise, since no-pattern is not present, the
1327     subpattern matches nothing. In other words, this pattern matches a sequence of
1328     non-parentheses, optionally enclosed in parentheses.
1329 nigel 3
1330 nigel 23 If the condition is not a sequence of digits, it must be an assertion. This may
1331     be a positive or negative lookahead or lookbehind assertion. Consider this
1332     pattern, again containing non-significant white space, and with the two
1333     alternatives on the second line:
1334 nigel 3
1335 nigel 23 (?(?=[^a-z]*[a-z])
1336     \\d{2}[a-z]{3}-\\d{2} | \\d{2}-\\d{2}-\\d{2} )
1337 nigel 19
1338 nigel 23 The condition is a positive lookahead assertion that matches an optional
1339     sequence of non-letters followed by a letter. In other words, it tests for the
1340     presence of at least one letter in the subject. If a letter is found, the
1341     subject is matched against the first alternative; otherwise it is matched
1342     against the second. This pattern matches strings in one of the two forms
1343     dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1344 nigel 19
1345 nigel 3
1346 nigel 23 .SH COMMENTS
1347     The sequence (?# marks the start of a comment which continues up to the next
1348     closing parenthesis. Nested parentheses are not permitted. The characters
1349     that make up a comment play no part in the pattern matching at all.
1350 nigel 3
1351 nigel 23 If the PCRE_EXTENDED option is set, an unescaped # character outside a
1352     character class introduces a comment that continues up to the next newline
1353     character in the pattern.
1356 nigel 3 .SH PERFORMANCE
1357     Certain items that may appear in patterns are more efficient than others. It is
1358     more efficient to use a character class like [aeiou] than a set of alternatives
1359     such as (a|e|i|o|u). In general, the simplest construction that provides the
1360     required behaviour is usually the most efficient. Jeffrey Friedl's book
1361     contains a lot of discussion about optimizing regular expressions for efficient
1362     performance.
1364 nigel 33 When a pattern begins with .* and the PCRE_DOTALL option is set, the pattern is
1365     implicitly anchored by PCRE, since it can match only at the start of a subject
1366     string. However, if PCRE_DOTALL is not set, PCRE cannot make this optimization,
1367     because the . metacharacter does not then match a newline, and if the subject
1368     string contains newlines, the pattern may match from the character immediately
1369     following one of them instead of from the very start. For example, the pattern
1370 nigel 3
1371 nigel 33 (.*) second
1373     matches the subject "first\\nand second" (where \\n stands for a newline
1374     character) with the first captured substring being "and". In order to do this,
1375     PCRE has to retry the match starting after every newline in the subject.
1377     If you are using such a pattern with subject strings that do not contain
1378     newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
1379     the pattern with ^.* to indicate explicit anchoring. That saves PCRE from
1380     having to scan along the subject looking for a newline to restart at.
1382 nigel 3 .SH AUTHOR
1383     Philip Hazel <ph10@cam.ac.uk>
1384     .br
1385     University Computing Service,
1386     .br
1387     New Museums Site,
1388     .br
1389     Cambridge CB2 3QG, England.
1390     .br
1391     Phone: +44 1223 334714
1393 nigel 27 Copyright (c) 1997-1999 University of Cambridge.

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