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1 nigel 41 .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     .B const char **\fIerrptr\fR, int *\fIerroffset\fR,
12     .ti +5n
13     .B const unsigned char *\fItableptr\fR);
14     .PP
15     .br
16     .B pcre_extra *pcre_study(const pcre *\fIcode\fR, int \fIoptions\fR,
17     .ti +5n
18     .B const char **\fIerrptr\fR);
19     .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 \fIstartoffset\fR,
24     .ti +5n
25     .B int \fIoptions\fR, int *\fIovector\fR, int \fIovecsize\fR);
26     .PP
27     .br
28     .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 43 .B int pcre_fullinfo(const pcre *\fIcode\fR, "const pcre_extra *\fIextra\fR,"
51     .ti +5n
52     .B int \fIwhat\fR, void *\fIwhere\fR);
53     .PP
54     .br
55 nigel 41 .B int pcre_info(const pcre *\fIcode\fR, int *\fIoptptr\fR, int
56     .B *\fIfirstcharptr\fR);
57     .PP
58     .br
59     .B char *pcre_version(void);
60     .PP
61     .br
62     .B void *(*pcre_malloc)(size_t);
63     .PP
64     .br
65     .B void (*pcre_free)(void *);
70     The PCRE library is a set of functions that implement regular expression
71     pattern matching using the same syntax and semantics as Perl 5, with just a few
72 nigel 43 differences (see below). The current implementation corresponds to Perl 5.005,
73     with some additional features from the Perl development release.
74 nigel 41
75     PCRE has its own native API, which is described in this document. There is also
76 nigel 43 a set of wrapper functions that correspond to the POSIX regular expression API.
77     These are described in the \fBpcreposix\fR documentation.
78 nigel 41
79     The native API function prototypes are defined in the header file \fBpcre.h\fR,
80     and on Unix systems the library itself is called \fBlibpcre.a\fR, so can be
81     accessed by adding \fB-lpcre\fR to the command for linking an application which
82 nigel 43 calls it. The header file defines the macros PCRE_MAJOR and PCRE_MINOR to
83     contain the major and minor release numbers for the library. Applications can
84     use these to include support for different releases.
85 nigel 41
86     The functions \fBpcre_compile()\fR, \fBpcre_study()\fR, and \fBpcre_exec()\fR
87     are used for compiling and matching regular expressions, while
88     \fBpcre_copy_substring()\fR, \fBpcre_get_substring()\fR, and
89     \fBpcre_get_substring_list()\fR are convenience functions for extracting
90     captured substrings from a matched subject string. The function
91     \fBpcre_maketables()\fR is used (optionally) to build a set of character tables
92     in the current locale for passing to \fBpcre_compile()\fR.
94 nigel 43 The function \fBpcre_fullinfo()\fR is used to find out information about a
95     compiled pattern; \fBpcre_info()\fR is an obsolete version which returns only
96     some of the available information, but is retained for backwards compatibility.
97     The function \fBpcre_version()\fR returns a pointer to a string containing the
98     version of PCRE and its date of release.
99 nigel 41
100     The global variables \fBpcre_malloc\fR and \fBpcre_free\fR initially contain
101     the entry points of the standard \fBmalloc()\fR and \fBfree()\fR functions
102     respectively. PCRE calls the memory management functions via these variables,
103     so a calling program can replace them if it wishes to intercept the calls. This
104     should be done before calling any PCRE functions.
108     The PCRE functions can be used in multi-threading applications, with the
109     proviso that the memory management functions pointed to by \fBpcre_malloc\fR
110     and \fBpcre_free\fR are shared by all threads.
112     The compiled form of a regular expression is not altered during matching, so
113     the same compiled pattern can safely be used by several threads at once.
117     The function \fBpcre_compile()\fR is called to compile a pattern into an
118     internal form. The pattern is a C string terminated by a binary zero, and
119     is passed in the argument \fIpattern\fR. A pointer to a single block of memory
120     that is obtained via \fBpcre_malloc\fR is returned. This contains the
121     compiled code and related data. The \fBpcre\fR type is defined for this for
122     convenience, but in fact \fBpcre\fR is just a typedef for \fBvoid\fR, since the
123     contents of the block are not externally defined. It is up to the caller to
124     free the memory when it is no longer required.
125     .PP
126     The size of a compiled pattern is roughly proportional to the length of the
127     pattern string, except that each character class (other than those containing
128     just a single character, negated or not) requires 33 bytes, and repeat
129     quantifiers with a minimum greater than one or a bounded maximum cause the
130     relevant portions of the compiled pattern to be replicated.
131     .PP
132     The \fIoptions\fR argument contains independent bits that affect the
133     compilation. It should be zero if no options are required. Some of the options,
134     in particular, those that are compatible with Perl, can also be set and unset
135     from within the pattern (see the detailed description of regular expressions
136     below). For these options, the contents of the \fIoptions\fR argument specifies
137     their initial settings at the start of compilation and execution. The
138     PCRE_ANCHORED option can be set at the time of matching as well as at compile
139     time.
140     .PP
141     If \fIerrptr\fR is NULL, \fBpcre_compile()\fR returns NULL immediately.
142     Otherwise, if compilation of a pattern fails, \fBpcre_compile()\fR returns
143     NULL, and sets the variable pointed to by \fIerrptr\fR to point to a textual
144     error message. The offset from the start of the pattern to the character where
145     the error was discovered is placed in the variable pointed to by
146     \fIerroffset\fR, which must not be NULL. If it is, an immediate error is given.
147     .PP
148     If the final argument, \fItableptr\fR, is NULL, PCRE uses a default set of
149     character tables which are built when it is compiled, using the default C
150     locale. Otherwise, \fItableptr\fR must be the result of a call to
151     \fBpcre_maketables()\fR. See the section on locale support below.
152     .PP
153     The following option bits are defined in the header file:
157     If this bit is set, the pattern is forced to be "anchored", that is, it is
158     constrained to match only at the start of the string which is being searched
159     (the "subject string"). This effect can also be achieved by appropriate
160     constructs in the pattern itself, which is the only way to do it in Perl.
164     If this bit is set, letters in the pattern match both upper and lower case
165     letters. It is equivalent to Perl's /i option.
169     If this bit is set, a dollar metacharacter in the pattern matches only at the
170     end of the subject string. Without this option, a dollar also matches
171     immediately before the final character if it is a newline (but not before any
172     other newlines). The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is
173     set. There is no equivalent to this option in Perl.
177     If this bit is set, a dot metacharater in the pattern matches all characters,
178     including newlines. Without it, newlines are excluded. This option is
179     equivalent to Perl's /s option. A negative class such as [^a] always matches a
180     newline character, independent of the setting of this option.
184     If this bit is set, whitespace data characters in the pattern are totally
185     ignored except when escaped or inside a character class, and characters between
186     an unescaped # outside a character class and the next newline character,
187     inclusive, are also ignored. This is equivalent to Perl's /x option, and makes
188     it possible to include comments inside complicated patterns. Note, however,
189     that this applies only to data characters. Whitespace characters may never
190     appear within special character sequences in a pattern, for example within the
191     sequence (?( which introduces a conditional subpattern.
193     PCRE_EXTRA
195 nigel 43 This option was invented in order to turn on additional functionality of PCRE
196     that is incompatible with Perl, but it is currently of very little use. When
197     set, any backslash in a pattern that is followed by a letter that has no
198 nigel 41 special meaning causes an error, thus reserving these combinations for future
199     expansion. By default, as in Perl, a backslash followed by a letter with no
200     special meaning is treated as a literal. There are at present no other features
201 nigel 43 controlled by this option. It can also be set by a (?X) option setting within a
202     pattern.
203 nigel 41
206     By default, PCRE treats the subject string as consisting of a single "line" of
207     characters (even if it actually contains several newlines). The "start of line"
208     metacharacter (^) matches only at the start of the string, while the "end of
209     line" metacharacter ($) matches only at the end of the string, or before a
210     terminating newline (unless PCRE_DOLLAR_ENDONLY is set). This is the same as
211     Perl.
213     When PCRE_MULTILINE it is set, the "start of line" and "end of line" constructs
214     match immediately following or immediately before any newline in the subject
215     string, respectively, as well as at the very start and end. This is equivalent
216     to Perl's /m option. If there are no "\\n" characters in a subject string, or
217     no occurrences of ^ or $ in a pattern, setting PCRE_MULTILINE has no
218     effect.
222     This option inverts the "greediness" of the quantifiers so that they are not
223     greedy by default, but become greedy if followed by "?". It is not compatible
224     with Perl. It can also be set by a (?U) option setting within the pattern.
228     When a pattern is going to be used several times, it is worth spending more
229     time analyzing it in order to speed up the time taken for matching. The
230     function \fBpcre_study()\fR takes a pointer to a compiled pattern as its first
231     argument, and returns a pointer to a \fBpcre_extra\fR block (another \fBvoid\fR
232     typedef) containing additional information about the pattern; this can be
233     passed to \fBpcre_exec()\fR. If no additional information is available, NULL
234     is returned.
236     The second argument contains option bits. At present, no options are defined
237     for \fBpcre_study()\fR, and this argument should always be zero.
239     The third argument for \fBpcre_study()\fR is a pointer to an error message. If
240     studying succeeds (even if no data is returned), the variable it points to is
241     set to NULL. Otherwise it points to a textual error message.
243     At present, studying a pattern is useful only for non-anchored patterns that do
244     not have a single fixed starting character. A bitmap of possible starting
245     characters is created.
249     PCRE handles caseless matching, and determines whether characters are letters,
250     digits, or whatever, by reference to a set of tables. The library contains a
251     default set of tables which is created in the default C locale when PCRE is
252     compiled. This is used when the final argument of \fBpcre_compile()\fR is NULL,
253     and is sufficient for many applications.
255     An alternative set of tables can, however, be supplied. Such tables are built
256     by calling the \fBpcre_maketables()\fR function, which has no arguments, in the
257     relevant locale. The result can then be passed to \fBpcre_compile()\fR as often
258     as necessary. For example, to build and use tables that are appropriate for the
259     French locale (where accented characters with codes greater than 128 are
260     treated as letters), the following code could be used:
262     setlocale(LC_CTYPE, "fr");
263     tables = pcre_maketables();
264     re = pcre_compile(..., tables);
266     The tables are built in memory that is obtained via \fBpcre_malloc\fR. The
267     pointer that is passed to \fBpcre_compile\fR is saved with the compiled
268     pattern, and the same tables are used via this pointer by \fBpcre_study()\fR
269     and \fBpcre_exec()\fR. Thus for any single pattern, compilation, studying and
270     matching all happen in the same locale, but different patterns can be compiled
271     in different locales. It is the caller's responsibility to ensure that the
272     memory containing the tables remains available for as long as it is needed.
276 nigel 43 The \fBpcre_fullinfo()\fR function returns information about a compiled
277     pattern. It replaces the obsolete \fBpcre_info()\fR function, which is
278     nevertheless retained for backwards compability (and is documented below).
279 nigel 41
280 nigel 43 The first argument for \fBpcre_fullinfo()\fR is a pointer to the compiled
281     pattern. The second argument is the result of \fBpcre_study()\fR, or NULL if
282     the pattern was not studied. The third argument specifies which piece of
283     information is required, while the fourth argument is a pointer to a variable
284     to receive the data. The yield of the function is zero for success, or one of
285     the following negative numbers:
287 nigel 41 PCRE_ERROR_NULL the argument \fIcode\fR was NULL
288 nigel 43 the argument \fIwhere\fR was NULL
289 nigel 41 PCRE_ERROR_BADMAGIC the "magic number" was not found
290 nigel 43 PCRE_ERROR_BADOPTION the value of \fIwhat\fR was invalid
291 nigel 41
292 nigel 43 The possible values for the third argument are defined in \fBpcre.h\fR, and are
293     as follows:
297     Return a copy of the options with which the pattern was compiled. The fourth
298     argument should point to au \fBunsigned long int\fR variable. These option bits
299 nigel 41 are those specified in the call to \fBpcre_compile()\fR, modified by any
300     top-level option settings within the pattern itself, and with the PCRE_ANCHORED
301 nigel 43 bit forcibly set if the form of the pattern implies that it can match only at
302     the start of a subject string.
303 nigel 41
304 nigel 43 PCRE_INFO_SIZE
305 nigel 41
306 nigel 43 Return the size of the compiled pattern, that is, the value that was passed as
307     the argument to \fBpcre_malloc()\fR when PCRE was getting memory in which to
308     place the compiled data. The fourth argument should point to a \fBsize_t\fR
309     variable.
313     Return the number of capturing subpatterns in the pattern. The fourth argument
314     should point to an \fbint\fR variable.
318     Return the number of the highest back reference in the pattern. The fourth
319     argument should point to an \fBint\fR variable. Zero is returned if there are
320     no back references.
324     Return information about the first character of any matched string, for a
325     non-anchored pattern. If there is a fixed first character, e.g. from a pattern
326 nigel 47 such as (cat|cow|coyote), it is returned in the integer pointed to by
327 nigel 43 \fIwhere\fR. Otherwise, if either
329 nigel 41 (a) the pattern was compiled with the PCRE_MULTILINE option, and every branch
330     starts with "^", or
332     (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not set
333     (if it were set, the pattern would be anchored),
335 nigel 47 -1 is returned, indicating that the pattern matches only at the start of a
336     subject string or after any "\\n" within the string. Otherwise -2 is returned.
337     For anchored patterns, -2 is returned.
338 nigel 41
340 nigel 41
341 nigel 43 If the pattern was studied, and this resulted in the construction of a 256-bit
342     table indicating a fixed set of characters for the first character in any
343     matching string, a pointer to the table is returned. Otherwise NULL is
344     returned. The fourth argument should point to an \fBunsigned char *\fR
345     variable.
349     For a non-anchored pattern, return the value of the rightmost literal character
350     which must exist in any matched string, other than at its start. The fourth
351     argument should point to an \fBint\fR variable. If there is no such character,
352     or if the pattern is anchored, -1 is returned. For example, for the pattern
353     /a\\d+z\\d+/ the returned value is 'z'.
355     The \fBpcre_info()\fR function is now obsolete because its interface is too
356     restrictive to return all the available data about a compiled pattern. New
357     programs should use \fBpcre_fullinfo()\fR instead. The yield of
358     \fBpcre_info()\fR is the number of capturing subpatterns, or one of the
359     following negative numbers:
361     PCRE_ERROR_NULL the argument \fIcode\fR was NULL
362     PCRE_ERROR_BADMAGIC the "magic number" was not found
364     If the \fIoptptr\fR argument is not NULL, a copy of the options with which the
365     pattern was compiled is placed in the integer it points to (see
366     PCRE_INFO_OPTIONS above).
368     If the pattern is not anchored and the \fIfirstcharptr\fR argument is not NULL,
369     it is used to pass back information about the first character of any matched
370     string (see PCRE_INFO_FIRSTCHAR above).
374     The function \fBpcre_exec()\fR is called to match a subject string against a
375     pre-compiled pattern, which is passed in the \fIcode\fR argument. If the
376     pattern has been studied, the result of the study should be passed in the
377     \fIextra\fR argument. Otherwise this must be NULL.
379     The PCRE_ANCHORED option can be passed in the \fIoptions\fR argument, whose
380     unused bits must be zero. However, if a pattern was compiled with
381     PCRE_ANCHORED, or turned out to be anchored by virtue of its contents, it
382     cannot be made unachored at matching time.
384     There are also three further options that can be set only at matching time:
388     The first character of the string is not the beginning of a line, so the
389     circumflex metacharacter should not match before it. Setting this without
390     PCRE_MULTILINE (at compile time) causes circumflex never to match.
394     The end of the string is not the end of a line, so the dollar metacharacter
395     should not match it nor (except in multiline mode) a newline immediately before
396     it. Setting this without PCRE_MULTILINE (at compile time) causes dollar never
397     to match.
401     An empty string is not considered to be a valid match if this option is set. If
402     there are alternatives in the pattern, they are tried. If all the alternatives
403     match the empty string, the entire match fails. For example, if the pattern
405     a?b?
407     is applied to a string not beginning with "a" or "b", it matches the empty
408     string at the start of the subject. With PCRE_NOTEMPTY set, this match is not
409     valid, so PCRE searches further into the string for occurrences of "a" or "b".
411     Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a special case
412     of a pattern match of the empty string within its \fBsplit()\fR function, and
413     when using the /g modifier. It is possible to emulate Perl's behaviour after
414     matching a null string by first trying the match again at the same offset with
415     PCRE_NOTEMPTY set, and then if that fails by advancing the starting offset (see
416     below) and trying an ordinary match again.
418     The subject string is passed as a pointer in \fIsubject\fR, a length in
419     \fIlength\fR, and a starting offset in \fIstartoffset\fR. Unlike the pattern
420     string, it may contain binary zero characters. When the starting offset is
421     zero, the search for a match starts at the beginning of the subject, and this
422     is by far the most common case.
424     A non-zero starting offset is useful when searching for another match in the
425     same subject by calling \fBpcre_exec()\fR again after a previous success.
426     Setting \fIstartoffset\fR differs from just passing over a shortened string and
427     setting PCRE_NOTBOL in the case of a pattern that begins with any kind of
428     lookbehind. For example, consider the pattern
430     \\Biss\\B
432     which finds occurrences of "iss" in the middle of words. (\\B matches only if
433     the current position in the subject is not a word boundary.) When applied to
434     the string "Mississipi" the first call to \fBpcre_exec()\fR finds the first
435     occurrence. If \fBpcre_exec()\fR is called again with just the remainder of the
436     subject, namely "issipi", it does not match, because \\B is always false at the
437     start of the subject, which is deemed to be a word boundary. However, if
438     \fBpcre_exec()\fR is passed the entire string again, but with \fIstartoffset\fR
439     set to 4, it finds the second occurrence of "iss" because it is able to look
440     behind the starting point to discover that it is preceded by a letter.
442     If a non-zero starting offset is passed when the pattern is anchored, one
443     attempt to match at the given offset is tried. This can only succeed if the
444     pattern does not require the match to be at the start of the subject.
446     In general, a pattern matches a certain portion of the subject, and in
447     addition, further substrings from the subject may be picked out by parts of the
448     pattern. Following the usage in Jeffrey Friedl's book, this is called
449     "capturing" in what follows, and the phrase "capturing subpattern" is used for
450     a fragment of a pattern that picks out a substring. PCRE supports several other
451     kinds of parenthesized subpattern that do not cause substrings to be captured.
453     Captured substrings are returned to the caller via a vector of integer offsets
454     whose address is passed in \fIovector\fR. The number of elements in the vector
455     is passed in \fIovecsize\fR. The first two-thirds of the vector is used to pass
456     back captured substrings, each substring using a pair of integers. The
457     remaining third of the vector is used as workspace by \fBpcre_exec()\fR while
458     matching capturing subpatterns, and is not available for passing back
459     information. The length passed in \fIovecsize\fR should always be a multiple of
460     three. If it is not, it is rounded down.
462     When a match has been successful, information about captured substrings is
463     returned in pairs of integers, starting at the beginning of \fIovector\fR, and
464     continuing up to two-thirds of its length at the most. The first element of a
465     pair is set to the offset of the first character in a substring, and the second
466     is set to the offset of the first character after the end of a substring. The
467     first pair, \fIovector[0]\fR and \fIovector[1]\fR, identify the portion of the
468     subject string matched by the entire pattern. The next pair is used for the
469     first capturing subpattern, and so on. The value returned by \fBpcre_exec()\fR
470     is the number of pairs that have been set. If there are no capturing
471     subpatterns, the return value from a successful match is 1, indicating that
472     just the first pair of offsets has been set.
474     Some convenience functions are provided for extracting the captured substrings
475     as separate strings. These are described in the following section.
477     It is possible for an capturing subpattern number \fIn+1\fR to match some
478     part of the subject when subpattern \fIn\fR has not been used at all. For
479     example, if the string "abc" is matched against the pattern (a|(z))(bc)
480     subpatterns 1 and 3 are matched, but 2 is not. When this happens, both offset
481     values corresponding to the unused subpattern are set to -1.
483     If a capturing subpattern is matched repeatedly, it is the last portion of the
484     string that it matched that gets returned.
486     If the vector is too small to hold all the captured substrings, it is used as
487     far as possible (up to two-thirds of its length), and the function returns a
488     value of zero. In particular, if the substring offsets are not of interest,
489     \fBpcre_exec()\fR may be called with \fIovector\fR passed as NULL and
490     \fIovecsize\fR as zero. However, if the pattern contains back references and
491     the \fIovector\fR isn't big enough to remember the related substrings, PCRE has
492     to get additional memory for use during matching. Thus it is usually advisable
493     to supply an \fIovector\fR.
495     Note that \fBpcre_info()\fR can be used to find out how many capturing
496     subpatterns there are in a compiled pattern. The smallest size for
497     \fIovector\fR that will allow for \fIn\fR captured substrings in addition to
498     the offsets of the substring matched by the whole pattern is (\fIn\fR+1)*3.
500     If \fBpcre_exec()\fR fails, it returns a negative number. The following are
501     defined in the header file:
505     The subject string did not match the pattern.
507     PCRE_ERROR_NULL (-2)
509     Either \fIcode\fR or \fIsubject\fR was passed as NULL, or \fIovector\fR was
510     NULL and \fIovecsize\fR was not zero.
514     An unrecognized bit was set in the \fIoptions\fR argument.
518     PCRE stores a 4-byte "magic number" at the start of the compiled code, to catch
519     the case when it is passed a junk pointer. This is the error it gives when the
520     magic number isn't present.
524     While running the pattern match, an unknown item was encountered in the
525     compiled pattern. This error could be caused by a bug in PCRE or by overwriting
526     of the compiled pattern.
530     If a pattern contains back references, but the \fIovector\fR that is passed to
531     \fBpcre_exec()\fR is not big enough to remember the referenced substrings, PCRE
532     gets a block of memory at the start of matching to use for this purpose. If the
533     call via \fBpcre_malloc()\fR fails, this error is given. The memory is freed at
534     the end of matching.
538     Captured substrings can be accessed directly by using the offsets returned by
539     \fBpcre_exec()\fR in \fIovector\fR. For convenience, the functions
540     \fBpcre_copy_substring()\fR, \fBpcre_get_substring()\fR, and
541     \fBpcre_get_substring_list()\fR are provided for extracting captured substrings
542     as new, separate, zero-terminated strings. A substring that contains a binary
543     zero is correctly extracted and has a further zero added on the end, but the
544     result does not, of course, function as a C string.
546     The first three arguments are the same for all three functions: \fIsubject\fR
547     is the subject string which has just been successfully matched, \fIovector\fR
548     is a pointer to the vector of integer offsets that was passed to
549     \fBpcre_exec()\fR, and \fIstringcount\fR is the number of substrings that
550     were captured by the match, including the substring that matched the entire
551     regular expression. This is the value returned by \fBpcre_exec\fR if it
552     is greater than zero. If \fBpcre_exec()\fR returned zero, indicating that it
553 nigel 47 ran out of space in \fIovector\fR, the value passed as \fIstringcount\fR should
554     be the size of the vector divided by three.
555 nigel 41
556     The functions \fBpcre_copy_substring()\fR and \fBpcre_get_substring()\fR
557     extract a single substring, whose number is given as \fIstringnumber\fR. A
558     value of zero extracts the substring that matched the entire pattern, while
559     higher values extract the captured substrings. For \fBpcre_copy_substring()\fR,
560     the string is placed in \fIbuffer\fR, whose length is given by
561     \fIbuffersize\fR, while for \fBpcre_get_substring()\fR a new block of store is
562     obtained via \fBpcre_malloc\fR, and its address is returned via
563     \fIstringptr\fR. The yield of the function is the length of the string, not
564     including the terminating zero, or one of
568     The buffer was too small for \fBpcre_copy_substring()\fR, or the attempt to get
569     memory failed for \fBpcre_get_substring()\fR.
573     There is no substring whose number is \fIstringnumber\fR.
575     The \fBpcre_get_substring_list()\fR function extracts all available substrings
576     and builds a list of pointers to them. All this is done in a single block of
577     memory which is obtained via \fBpcre_malloc\fR. The address of the memory block
578     is returned via \fIlistptr\fR, which is also the start of the list of string
579     pointers. The end of the list is marked by a NULL pointer. The yield of the
580     function is zero if all went well, or
584     if the attempt to get the memory block failed.
586     When any of these functions encounter a substring that is unset, which can
587     happen when capturing subpattern number \fIn+1\fR matches some part of the
588     subject, but subpattern \fIn\fR has not been used at all, they return an empty
589     string. This can be distinguished from a genuine zero-length substring by
590     inspecting the appropriate offset in \fIovector\fR, which is negative for unset
591     substrings.
596     There are some size limitations in PCRE but it is hoped that they will never in
597     practice be relevant.
598     The maximum length of a compiled pattern is 65539 (sic) bytes.
599     All values in repeating quantifiers must be less than 65536.
600     The maximum number of capturing subpatterns is 99.
601     The maximum number of all parenthesized subpatterns, including capturing
602     subpatterns, assertions, and other types of subpattern, is 200.
604     The maximum length of a subject string is the largest positive number that an
605     integer variable can hold. However, PCRE uses recursion to handle subpatterns
606     and indefinite repetition. This means that the available stack space may limit
607     the size of a subject string that can be processed by certain patterns.
611     The differences described here are with respect to Perl 5.005.
613     1. By default, a whitespace character is any character that the C library
614     function \fBisspace()\fR recognizes, though it is possible to compile PCRE with
615     alternative character type tables. Normally \fBisspace()\fR matches space,
616     formfeed, newline, carriage return, horizontal tab, and vertical tab. Perl 5
617     no longer includes vertical tab in its set of whitespace characters. The \\v
618     escape that was in the Perl documentation for a long time was never in fact
619     recognized. However, the character itself was treated as whitespace at least
620     up to 5.002. In 5.004 and 5.005 it does not match \\s.
622     2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
623     them, but they do not mean what you might think. For example, (?!a){3} does
624     not assert that the next three characters are not "a". It just asserts that the
625     next character is not "a" three times.
627     3. Capturing subpatterns that occur inside negative lookahead assertions are
628     counted, but their entries in the offsets vector are never set. Perl sets its
629     numerical variables from any such patterns that are matched before the
630     assertion fails to match something (thereby succeeding), but only if the
631     negative lookahead assertion contains just one branch.
633     4. Though binary zero characters are supported in the subject string, they are
634     not allowed in a pattern string because it is passed as a normal C string,
635     terminated by zero. The escape sequence "\\0" can be used in the pattern to
636     represent a binary zero.
638     5. The following Perl escape sequences are not supported: \\l, \\u, \\L, \\U,
639     \\E, \\Q. In fact these are implemented by Perl's general string-handling and
640     are not part of its pattern matching engine.
642     6. The Perl \\G assertion is not supported as it is not relevant to single
643     pattern matches.
645 nigel 43 7. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
646     constructions. However, there is some experimental support for recursive
647     patterns using the non-Perl item (?R).
648 nigel 41
649     8. There are at the time of writing some oddities in Perl 5.005_02 concerned
650     with the settings of captured strings when part of a pattern is repeated. For
651     example, matching "aba" against the pattern /^(a(b)?)+$/ sets $2 to the value
652     "b", but matching "aabbaa" against /^(aa(bb)?)+$/ leaves $2 unset. However, if
653 nigel 47 the pattern is changed to /^(aa(b(b))?)+$/ then $2 (and $3) are set.
654 nigel 41
655     In Perl 5.004 $2 is set in both cases, and that is also true of PCRE. If in the
656     future Perl changes to a consistent state that is different, PCRE may change to
657     follow.
659     9. Another as yet unresolved discrepancy is that in Perl 5.005_02 the pattern
660     /^(a)?(?(1)a|b)+$/ matches the string "a", whereas in PCRE it does not.
661     However, in both Perl and PCRE /^(a)?a/ matched against "a" leaves $1 unset.
663     10. PCRE provides some extensions to the Perl regular expression facilities:
665     (a) Although lookbehind assertions must match fixed length strings, each
666     alternative branch of a lookbehind assertion can match a different length of
667     string. Perl 5.005 requires them all to have the same length.
669     (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ meta-
670     character matches only at the very end of the string.
672     (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
673     meaning is faulted.
675     (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
676     inverted, that is, by default they are not greedy, but if followed by a
677     question mark they are.
679     (e) PCRE_ANCHORED can be used to force a pattern to be tried only at the start
680     of the subject.
682     (f) The PCRE_NOTBOL, PCRE_NOTEOL, and PCRE_NOTEMPTY options for
683     \fBpcre_exec()\fR have no Perl equivalents.
685 nigel 43 (g) The (?R) construct allows for recursive pattern matching (Perl 5.6 can do
686     this using the (?p{code}) construct, which PCRE cannot of course support.)
687 nigel 41
688 nigel 43
690     The syntax and semantics of the regular expressions supported by PCRE are
691     described below. Regular expressions are also described in the Perl
692     documentation and in a number of other books, some of which have copious
693     examples. Jeffrey Friedl's "Mastering Regular Expressions", published by
694 nigel 43 O'Reilly (ISBN 1-56592-257), covers them in great detail. The description
695 nigel 41 here is intended as reference documentation.
697     A regular expression is a pattern that is matched against a subject string from
698     left to right. Most characters stand for themselves in a pattern, and match the
699     corresponding characters in the subject. As a trivial example, the pattern
701     The quick brown fox
703     matches a portion of a subject string that is identical to itself. The power of
704     regular expressions comes from the ability to include alternatives and
705     repetitions in the pattern. These are encoded in the pattern by the use of
706     \fImeta-characters\fR, which do not stand for themselves but instead are
707     interpreted in some special way.
709     There are two different sets of meta-characters: those that are recognized
710     anywhere in the pattern except within square brackets, and those that are
711     recognized in square brackets. Outside square brackets, the meta-characters are
712     as follows:
714     \\ general escape character with several uses
715     ^ assert start of subject (or line, in multiline mode)
716     $ assert end of subject (or line, in multiline mode)
717     . match any character except newline (by default)
718     [ start character class definition
719     | start of alternative branch
720     ( start subpattern
721     ) end subpattern
722     ? extends the meaning of (
723     also 0 or 1 quantifier
724     also quantifier minimizer
725     * 0 or more quantifier
726     + 1 or more quantifier
727     { start min/max quantifier
729     Part of a pattern that is in square brackets is called a "character class". In
730     a character class the only meta-characters are:
732     \\ general escape character
733     ^ negate the class, but only if the first character
734     - indicates character range
735     ] terminates the character class
737     The following sections describe the use of each of the meta-characters.
741     The backslash character has several uses. Firstly, if it is followed by a
742     non-alphameric character, it takes away any special meaning that character may
743     have. This use of backslash as an escape character applies both inside and
744     outside character classes.
746     For example, if you want to match a "*" character, you write "\\*" in the
747     pattern. This applies whether or not the following character would otherwise be
748     interpreted as a meta-character, so it is always safe to precede a
749     non-alphameric with "\\" to specify that it stands for itself. In particular,
750     if you want to match a backslash, you write "\\\\".
752     If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
753     pattern (other than in a character class) and characters between a "#" outside
754     a character class and the next newline character are ignored. An escaping
755     backslash can be used to include a whitespace or "#" character as part of the
756     pattern.
758     A second use of backslash provides a way of encoding non-printing characters
759     in patterns in a visible manner. There is no restriction on the appearance of
760     non-printing characters, apart from the binary zero that terminates a pattern,
761     but when a pattern is being prepared by text editing, it is usually easier to
762     use one of the following escape sequences than the binary character it
763     represents:
765     \\a alarm, that is, the BEL character (hex 07)
766     \\cx "control-x", where x is any character
767     \\e escape (hex 1B)
768     \\f formfeed (hex 0C)
769     \\n newline (hex 0A)
770     \\r carriage return (hex 0D)
771     \\t tab (hex 09)
772     \\xhh character with hex code hh
773     \\ddd character with octal code ddd, or backreference
775     The precise effect of "\\cx" is as follows: if "x" is a lower case letter, it
776     is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
777     Thus "\\cz" becomes hex 1A, but "\\c{" becomes hex 3B, while "\\c;" becomes hex
778     7B.
780     After "\\x", up to two hexadecimal digits are read (letters can be in upper or
781     lower case).
783     After "\\0" up to two further octal digits are read. In both cases, if there
784     are fewer than two digits, just those that are present are used. Thus the
785     sequence "\\0\\x\\07" specifies two binary zeros followed by a BEL character.
786     Make sure you supply two digits after the initial zero if the character that
787     follows is itself an octal digit.
789     The handling of a backslash followed by a digit other than 0 is complicated.
790     Outside a character class, PCRE reads it and any following digits as a decimal
791     number. If the number is less than 10, or if there have been at least that many
792     previous capturing left parentheses in the expression, the entire sequence is
793     taken as a \fIback reference\fR. A description of how this works is given
794     later, following the discussion of parenthesized subpatterns.
796     Inside a character class, or if the decimal number is greater than 9 and there
797     have not been that many capturing subpatterns, PCRE re-reads up to three octal
798     digits following the backslash, and generates a single byte from the least
799     significant 8 bits of the value. Any subsequent digits stand for themselves.
800     For example:
802     \\040 is another way of writing a space
803     \\40 is the same, provided there are fewer than 40
804     previous capturing subpatterns
805     \\7 is always a back reference
806     \\11 might be a back reference, or another way of
807     writing a tab
808     \\011 is always a tab
809     \\0113 is a tab followed by the character "3"
810     \\113 is the character with octal code 113 (since there
811     can be no more than 99 back references)
812     \\377 is a byte consisting entirely of 1 bits
813     \\81 is either a back reference, or a binary zero
814     followed by the two characters "8" and "1"
816     Note that octal values of 100 or greater must not be introduced by a leading
817     zero, because no more than three octal digits are ever read.
819     All the sequences that define a single byte value can be used both inside and
820     outside character classes. In addition, inside a character class, the sequence
821     "\\b" is interpreted as the backspace character (hex 08). Outside a character
822     class it has a different meaning (see below).
824     The third use of backslash is for specifying generic character types:
826     \\d any decimal digit
827     \\D any character that is not a decimal digit
828     \\s any whitespace character
829     \\S any character that is not a whitespace character
830     \\w any "word" character
831     \\W any "non-word" character
833     Each pair of escape sequences partitions the complete set of characters into
834     two disjoint sets. Any given character matches one, and only one, of each pair.
836     A "word" character is any letter or digit or the underscore character, that is,
837     any character which can be part of a Perl "word". The definition of letters and
838     digits is controlled by PCRE's character tables, and may vary if locale-
839     specific matching is taking place (see "Locale support" above). For example, in
840     the "fr" (French) locale, some character codes greater than 128 are used for
841     accented letters, and these are matched by \\w.
843     These character type sequences can appear both inside and outside character
844     classes. They each match one character of the appropriate type. If the current
845     matching point is at the end of the subject string, all of them fail, since
846     there is no character to match.
848     The fourth use of backslash is for certain simple assertions. An assertion
849     specifies a condition that has to be met at a particular point in a match,
850     without consuming any characters from the subject string. The use of
851     subpatterns for more complicated assertions is described below. The backslashed
852     assertions are
854     \\b word boundary
855     \\B not a word boundary
856     \\A start of subject (independent of multiline mode)
857     \\Z end of subject or newline at end (independent of multiline mode)
858     \\z end of subject (independent of multiline mode)
860     These assertions may not appear in character classes (but note that "\\b" has a
861     different meaning, namely the backspace character, inside a character class).
863     A word boundary is a position in the subject string where the current character
864     and the previous character do not both match \\w or \\W (i.e. one matches
865     \\w and the other matches \\W), or the start or end of the string if the
866     first or last character matches \\w, respectively.
868     The \\A, \\Z, and \\z assertions differ from the traditional circumflex and
869     dollar (described below) in that they only ever match at the very start and end
870     of the subject string, whatever options are set. They are not affected by the
871     PCRE_NOTBOL or PCRE_NOTEOL options. If the \fIstartoffset\fR argument of
872     \fBpcre_exec()\fR is non-zero, \\A can never match. The difference between \\Z
873     and \\z is that \\Z matches before a newline that is the last character of the
874     string as well as at the end of the string, whereas \\z matches only at the
875     end.
879     Outside a character class, in the default matching mode, the circumflex
880     character is an assertion which is true only if the current matching point is
881     at the start of the subject string. If the \fIstartoffset\fR argument of
882     \fBpcre_exec()\fR is non-zero, circumflex can never match. Inside a character
883     class, circumflex has an entirely different meaning (see below).
885     Circumflex need not be the first character of the pattern if a number of
886     alternatives are involved, but it should be the first thing in each alternative
887     in which it appears if the pattern is ever to match that branch. If all
888     possible alternatives start with a circumflex, that is, if the pattern is
889     constrained to match only at the start of the subject, it is said to be an
890     "anchored" pattern. (There are also other constructs that can cause a pattern
891     to be anchored.)
893     A dollar character is an assertion which is true only if the current matching
894     point is at the end of the subject string, or immediately before a newline
895     character that is the last character in the string (by default). Dollar need
896     not be the last character of the pattern if a number of alternatives are
897     involved, but it should be the last item in any branch in which it appears.
898     Dollar has no special meaning in a character class.
900     The meaning of dollar can be changed so that it matches only at the very end of
901     the string, by setting the PCRE_DOLLAR_ENDONLY option at compile or matching
902     time. This does not affect the \\Z assertion.
904     The meanings of the circumflex and dollar characters are changed if the
905     PCRE_MULTILINE option is set. When this is the case, they match immediately
906     after and immediately before an internal "\\n" character, respectively, in
907     addition to matching at the start and end of the subject string. For example,
908     the pattern /^abc$/ matches the subject string "def\\nabc" in multiline mode,
909     but not otherwise. Consequently, patterns that are anchored in single line mode
910     because all branches start with "^" are not anchored in multiline mode, and a
911     match for circumflex is possible when the \fIstartoffset\fR argument of
912     \fBpcre_exec()\fR is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
913     PCRE_MULTILINE is set.
915     Note that the sequences \\A, \\Z, and \\z can be used to match the start and
916     end of the subject in both modes, and if all branches of a pattern start with
917     \\A is it always anchored, whether PCRE_MULTILINE is set or not.
921     Outside a character class, a dot in the pattern matches any one character in
922     the subject, including a non-printing character, but not (by default) newline.
923 nigel 47 If the PCRE_DOTALL option is set, dots match newlines as well. The handling of
924     dot is entirely independent of the handling of circumflex and dollar, the only
925     relationship being that they both involve newline characters. Dot has no
926     special meaning in a character class.
927 nigel 41
930     An opening square bracket introduces a character class, terminated by a closing
931     square bracket. A closing square bracket on its own is not special. If a
932     closing square bracket is required as a member of the class, it should be the
933     first data character in the class (after an initial circumflex, if present) or
934     escaped with a backslash.
936     A character class matches a single character in the subject; the character must
937     be in the set of characters defined by the class, unless the first character in
938     the class is a circumflex, in which case the subject character must not be in
939     the set defined by the class. If a circumflex is actually required as a member
940     of the class, ensure it is not the first character, or escape it with a
941     backslash.
943     For example, the character class [aeiou] matches any lower case vowel, while
944     [^aeiou] matches any character that is not a lower case vowel. Note that a
945     circumflex is just a convenient notation for specifying the characters which
946     are in the class by enumerating those that are not. It is not an assertion: it
947     still consumes a character from the subject string, and fails if the current
948     pointer is at the end of the string.
950     When caseless matching is set, any letters in a class represent both their
951     upper case and lower case versions, so for example, a caseless [aeiou] matches
952     "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
953     caseful version would.
955     The newline character is never treated in any special way in character classes,
956     whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class
957     such as [^a] will always match a newline.
959     The minus (hyphen) character can be used to specify a range of characters in a
960     character class. For example, [d-m] matches any letter between d and m,
961     inclusive. If a minus character is required in a class, it must be escaped with
962     a backslash or appear in a position where it cannot be interpreted as
963     indicating a range, typically as the first or last character in the class.
965     It is not possible to have the literal character "]" as the end character of a
966     range. A pattern such as [W-]46] is interpreted as a class of two characters
967     ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
968     "-46]". However, if the "]" is escaped with a backslash it is interpreted as
969     the end of range, so [W-\\]46] is interpreted as a single class containing a
970     range followed by two separate characters. The octal or hexadecimal
971     representation of "]" can also be used to end a range.
973     Ranges operate in ASCII collating sequence. They can also be used for
974     characters specified numerically, for example [\\000-\\037]. If a range that
975     includes letters is used when caseless matching is set, it matches the letters
976     in either case. For example, [W-c] is equivalent to [][\\^_`wxyzabc], matched
977     caselessly, and if character tables for the "fr" locale are in use,
978     [\\xc8-\\xcb] matches accented E characters in both cases.
980     The character types \\d, \\D, \\s, \\S, \\w, and \\W may also appear in a
981     character class, and add the characters that they match to the class. For
982     example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can
983     conveniently be used with the upper case character types to specify a more
984     restricted set of characters than the matching lower case type. For example,
985     the class [^\\W_] matches any letter or digit, but not underscore.
987     All non-alphameric characters other than \\, -, ^ (at the start) and the
988     terminating ] are non-special in character classes, but it does no harm if they
989     are escaped.
993     Perl 5.6 (not yet released at the time of writing) is going to support the
994     POSIX notation for character classes, which uses names enclosed by [: and :]
995     within the enclosing square brackets. PCRE supports this notation. For example,
997     [01[:alpha:]%]
999     matches "0", "1", any alphabetic character, or "%". The supported class names
1000     are
1002     alnum letters and digits
1003     alpha letters
1004     ascii character codes 0 - 127
1005     cntrl control characters
1006     digit decimal digits (same as \\d)
1007     graph printing characters, excluding space
1008     lower lower case letters
1009     print printing characters, including space
1010     punct printing characters, excluding letters and digits
1011     space white space (same as \\s)
1012     upper upper case letters
1013     word "word" characters (same as \\w)
1014     xdigit hexadecimal digits
1016     The names "ascii" and "word" are Perl extensions. Another Perl extension is
1017     negation, which is indicated by a ^ character after the colon. For example,
1019     [12[:^digit:]]
1021     matches "1", "2", or any non-digit. PCRE (and Perl) also recogize the POSIX
1022     syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1023     supported, and an error is given if they are encountered.
1026 nigel 41 .SH VERTICAL BAR
1027     Vertical bar characters are used to separate alternative patterns. For example,
1028     the pattern
1030     gilbert|sullivan
1032     matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1033     and an empty alternative is permitted (matching the empty string).
1034     The matching process tries each alternative in turn, from left to right,
1035     and the first one that succeeds is used. If the alternatives are within a
1036     subpattern (defined below), "succeeds" means matching the rest of the main
1037     pattern as well as the alternative in the subpattern.
1042     can be changed from within the pattern by a sequence of Perl option letters
1043     enclosed between "(?" and ")". The option letters are
1045     i for PCRE_CASELESS
1046     m for PCRE_MULTILINE
1047     s for PCRE_DOTALL
1048     x for PCRE_EXTENDED
1050     For example, (?im) sets caseless, multiline matching. It is also possible to
1051     unset these options by preceding the letter with a hyphen, and a combined
1052     setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1053     PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1054     permitted. If a letter appears both before and after the hyphen, the option is
1055     unset.
1057     The scope of these option changes depends on where in the pattern the setting
1058     occurs. For settings that are outside any subpattern (defined below), the
1059     effect is the same as if the options were set or unset at the start of
1060     matching. The following patterns all behave in exactly the same way:
1062     (?i)abc
1063     a(?i)bc
1064     ab(?i)c
1065     abc(?i)
1067     which in turn is the same as compiling the pattern abc with PCRE_CASELESS set.
1068     In other words, such "top level" settings apply to the whole pattern (unless
1069     there are other changes inside subpatterns). If there is more than one setting
1070     of the same option at top level, the rightmost setting is used.
1072     If an option change occurs inside a subpattern, the effect is different. This
1073     is a change of behaviour in Perl 5.005. An option change inside a subpattern
1074     affects only that part of the subpattern that follows it, so
1076     (a(?i)b)c
1078     matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1079     By this means, options can be made to have different settings in different
1080     parts of the pattern. Any changes made in one alternative do carry on
1081     into subsequent branches within the same subpattern. For example,
1083     (a(?i)b|c)
1085     matches "ab", "aB", "c", and "C", even though when matching "C" the first
1086     branch is abandoned before the option setting. This is because the effects of
1087     option settings happen at compile time. There would be some very weird
1088     behaviour otherwise.
1090     The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the
1091     same way as the Perl-compatible options by using the characters U and X
1092     respectively. The (?X) flag setting is special in that it must always occur
1093     earlier in the pattern than any of the additional features it turns on, even
1094     when it is at top level. It is best put at the start.
1098     Subpatterns are delimited by parentheses (round brackets), which can be nested.
1099     Marking part of a pattern as a subpattern does two things:
1101     1. It localizes a set of alternatives. For example, the pattern
1103     cat(aract|erpillar|)
1105     matches one of the words "cat", "cataract", or "caterpillar". Without the
1106     parentheses, it would match "cataract", "erpillar" or the empty string.
1108     2. It sets up the subpattern as a capturing subpattern (as defined above).
1109     When the whole pattern matches, that portion of the subject string that matched
1110     the subpattern is passed back to the caller via the \fIovector\fR argument of
1111     \fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting
1112     from 1) to obtain the numbers of the capturing subpatterns.
1114     For example, if the string "the red king" is matched against the pattern
1116     the ((red|white) (king|queen))
1118     the captured substrings are "red king", "red", and "king", and are numbered 1,
1119     2, and 3.
1121     The fact that plain parentheses fulfil two functions is not always helpful.
1122     There are often times when a grouping subpattern is required without a
1123     capturing requirement. If an opening parenthesis is followed by "?:", the
1124     subpattern does not do any capturing, and is not counted when computing the
1125     number of any subsequent capturing subpatterns. For example, if the string "the
1126     white queen" is matched against the pattern
1128     the ((?:red|white) (king|queen))
1130     the captured substrings are "white queen" and "queen", and are numbered 1 and
1131     2. The maximum number of captured substrings is 99, and the maximum number of
1132     all subpatterns, both capturing and non-capturing, is 200.
1134     As a convenient shorthand, if any option settings are required at the start of
1135     a non-capturing subpattern, the option letters may appear between the "?" and
1136     the ":". Thus the two patterns
1138     (?i:saturday|sunday)
1139     (?:(?i)saturday|sunday)
1141     match exactly the same set of strings. Because alternative branches are tried
1142     from left to right, and options are not reset until the end of the subpattern
1143     is reached, an option setting in one branch does affect subsequent branches, so
1144     the above patterns match "SUNDAY" as well as "Saturday".
1148     Repetition is specified by quantifiers, which can follow any of the following
1149     items:
1151     a single character, possibly escaped
1152     the . metacharacter
1153     a character class
1154     a back reference (see next section)
1155     a parenthesized subpattern (unless it is an assertion - see below)
1157     The general repetition quantifier specifies a minimum and maximum number of
1158     permitted matches, by giving the two numbers in curly brackets (braces),
1159     separated by a comma. The numbers must be less than 65536, and the first must
1160     be less than or equal to the second. For example:
1162     z{2,4}
1164     matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1165     character. If the second number is omitted, but the comma is present, there is
1166     no upper limit; if the second number and the comma are both omitted, the
1167     quantifier specifies an exact number of required matches. Thus
1169     [aeiou]{3,}
1171     matches at least 3 successive vowels, but may match many more, while
1173     \\d{8}
1175     matches exactly 8 digits. An opening curly bracket that appears in a position
1176     where a quantifier is not allowed, or one that does not match the syntax of a
1177     quantifier, is taken as a literal character. For example, {,6} is not a
1178     quantifier, but a literal string of four characters.
1180     The quantifier {0} is permitted, causing the expression to behave as if the
1181     previous item and the quantifier were not present.
1183     For convenience (and historical compatibility) the three most common
1184     quantifiers have single-character abbreviations:
1186     * is equivalent to {0,}
1187     + is equivalent to {1,}
1188     ? is equivalent to {0,1}
1190     It is possible to construct infinite loops by following a subpattern that can
1191     match no characters with a quantifier that has no upper limit, for example:
1193     (a?)*
1195     Earlier versions of Perl and PCRE used to give an error at compile time for
1196     such patterns. However, because there are cases where this can be useful, such
1197     patterns are now accepted, but if any repetition of the subpattern does in fact
1198     match no characters, the loop is forcibly broken.
1200     By default, the quantifiers are "greedy", that is, they match as much as
1201     possible (up to the maximum number of permitted times), without causing the
1202     rest of the pattern to fail. The classic example of where this gives problems
1203     is in trying to match comments in C programs. These appear between the
1204     sequences /* and */ and within the sequence, individual * and / characters may
1205     appear. An attempt to match C comments by applying the pattern
1207     /\\*.*\\*/
1209     to the string
1211     /* first command */ not comment /* second comment */
1213     fails, because it matches the entire string due to the greediness of the .*
1214     item.
1216 nigel 47 However, if a quantifier is followed by a question mark, it ceases to be
1217 nigel 41 greedy, and instead matches the minimum number of times possible, so the
1218     pattern
1220     /\\*.*?\\*/
1222     does the right thing with the C comments. The meaning of the various
1223     quantifiers is not otherwise changed, just the preferred number of matches.
1224     Do not confuse this use of question mark with its use as a quantifier in its
1225     own right. Because it has two uses, it can sometimes appear doubled, as in
1227     \\d??\\d
1229     which matches one digit by preference, but can match two if that is the only
1230     way the rest of the pattern matches.
1232 nigel 47 If the PCRE_UNGREEDY option is set (an option which is not available in Perl),
1233     the quantifiers are not greedy by default, but individual ones can be made
1234 nigel 41 greedy by following them with a question mark. In other words, it inverts the
1235     default behaviour.
1237     When a parenthesized subpattern is quantified with a minimum repeat count that
1238     is greater than 1 or with a limited maximum, more store is required for the
1239     compiled pattern, in proportion to the size of the minimum or maximum.
1241     If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1242 nigel 47 to Perl's /s) is set, thus allowing the . to match newlines, the pattern is
1243     implicitly anchored, because whatever follows will be tried against every
1244 nigel 41 character position in the subject string, so there is no point in retrying the
1245     overall match at any position after the first. PCRE treats such a pattern as
1246     though it were preceded by \\A. In cases where it is known that the subject
1247     string contains no newlines, it is worth setting PCRE_DOTALL when the pattern
1248     begins with .* in order to obtain this optimization, or alternatively using ^
1249     to indicate anchoring explicitly.
1251     When a capturing subpattern is repeated, the value captured is the substring
1252     that matched the final iteration. For example, after
1254     (tweedle[dume]{3}\\s*)+
1256     has matched "tweedledum tweedledee" the value of the captured substring is
1257     "tweedledee". However, if there are nested capturing subpatterns, the
1258     corresponding captured values may have been set in previous iterations. For
1259     example, after
1261     /(a|(b))+/
1263     matches "aba" the value of the second captured substring is "b".
1267     Outside a character class, a backslash followed by a digit greater than 0 (and
1268     possibly further digits) is a back reference to a capturing subpattern earlier
1269     (i.e. to its left) in the pattern, provided there have been that many previous
1270     capturing left parentheses.
1272     However, if the decimal number following the backslash is less than 10, it is
1273     always taken as a back reference, and causes an error only if there are not
1274     that many capturing left parentheses in the entire pattern. In other words, the
1275     parentheses that are referenced need not be to the left of the reference for
1276     numbers less than 10. See the section entitled "Backslash" above for further
1277     details of the handling of digits following a backslash.
1279     A back reference matches whatever actually matched the capturing subpattern in
1280     the current subject string, rather than anything matching the subpattern
1281     itself. So the pattern
1283     (sens|respons)e and \\1ibility
1285     matches "sense and sensibility" and "response and responsibility", but not
1286     "sense and responsibility". If caseful matching is in force at the time of the
1287 nigel 47 back reference, the case of letters is relevant. For example,
1288 nigel 41
1289     ((?i)rah)\\s+\\1
1291     matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1292     capturing subpattern is matched caselessly.
1294     There may be more than one back reference to the same subpattern. If a
1295 nigel 47 subpattern has not actually been used in a particular match, any back
1296 nigel 41 references to it always fail. For example, the pattern
1298     (a|(bc))\\2
1300     always fails if it starts to match "a" rather than "bc". Because there may be
1301     up to 99 back references, all digits following the backslash are taken
1302     as part of a potential back reference number. If the pattern continues with a
1303 nigel 47 digit character, some delimiter must be used to terminate the back reference.
1304     If the PCRE_EXTENDED option is set, this can be whitespace. Otherwise an empty
1305     comment can be used.
1306 nigel 41
1307     A back reference that occurs inside the parentheses to which it refers fails
1308     when the subpattern is first used, so, for example, (a\\1) never matches.
1309     However, such references can be useful inside repeated subpatterns. For
1310     example, the pattern
1312     (a|b\\1)+
1314     matches any number of "a"s and also "aba", "ababaa" etc. At each iteration of
1315     the subpattern, the back reference matches the character string corresponding
1316     to the previous iteration. In order for this to work, the pattern must be such
1317     that the first iteration does not need to match the back reference. This can be
1318     done using alternation, as in the example above, or by a quantifier with a
1319     minimum of zero.
1323     An assertion is a test on the characters following or preceding the current
1324     matching point that does not actually consume any characters. The simple
1325     assertions coded as \\b, \\B, \\A, \\Z, \\z, ^ and $ are described above. More
1326     complicated assertions are coded as subpatterns. There are two kinds: those
1327     that look ahead of the current position in the subject string, and those that
1328     look behind it.
1330     An assertion subpattern is matched in the normal way, except that it does not
1331     cause the current matching position to be changed. Lookahead assertions start
1332     with (?= for positive assertions and (?! for negative assertions. For example,
1334     \\w+(?=;)
1336     matches a word followed by a semicolon, but does not include the semicolon in
1337     the match, and
1339     foo(?!bar)
1341     matches any occurrence of "foo" that is not followed by "bar". Note that the
1342     apparently similar pattern
1344     (?!foo)bar
1346     does not find an occurrence of "bar" that is preceded by something other than
1347     "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1348     (?!foo) is always true when the next three characters are "bar". A
1349     lookbehind assertion is needed to achieve this effect.
1351     Lookbehind assertions start with (?<= for positive assertions and (?<! for
1352     negative assertions. For example,
1354     (?<!foo)bar
1356     does find an occurrence of "bar" that is not preceded by "foo". The contents of
1357     a lookbehind assertion are restricted such that all the strings it matches must
1358     have a fixed length. However, if there are several alternatives, they do not
1359     all have to have the same fixed length. Thus
1361     (?<=bullock|donkey)
1363     is permitted, but
1365     (?<!dogs?|cats?)
1367     causes an error at compile time. Branches that match different length strings
1368     are permitted only at the top level of a lookbehind assertion. This is an
1369     extension compared with Perl 5.005, which requires all branches to match the
1370     same length of string. An assertion such as
1372     (?<=ab(c|de))
1374     is not permitted, because its single top-level branch can match two different
1375     lengths, but it is acceptable if rewritten to use two top-level branches:
1377     (?<=abc|abde)
1379     The implementation of lookbehind assertions is, for each alternative, to
1380     temporarily move the current position back by the fixed width and then try to
1381     match. If there are insufficient characters before the current position, the
1382     match is deemed to fail. Lookbehinds in conjunction with once-only subpatterns
1383     can be particularly useful for matching at the ends of strings; an example is
1384     given at the end of the section on once-only subpatterns.
1386     Several assertions (of any sort) may occur in succession. For example,
1388     (?<=\\d{3})(?<!999)foo
1390     matches "foo" preceded by three digits that are not "999". Notice that each of
1391     the assertions is applied independently at the same point in the subject
1392     string. First there is a check that the previous three characters are all
1393 nigel 47 digits, and then there is a check that the same three characters are not "999".
1394 nigel 41 This pattern does \fInot\fR match "foo" preceded by six characters, the first
1395     of which are digits and the last three of which are not "999". For example, it
1396     doesn't match "123abcfoo". A pattern to do that is
1398     (?<=\\d{3}...)(?<!999)foo
1400     This time the first assertion looks at the preceding six characters, checking
1401     that the first three are digits, and then the second assertion checks that the
1402     preceding three characters are not "999".
1404     Assertions can be nested in any combination. For example,
1406     (?<=(?<!foo)bar)baz
1408     matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1409     preceded by "foo", while
1411     (?<=\\d{3}(?!999)...)foo
1413     is another pattern which matches "foo" preceded by three digits and any three
1414     characters that are not "999".
1416     Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1417     because it makes no sense to assert the same thing several times. If any kind
1418     of assertion contains capturing subpatterns within it, these are counted for
1419     the purposes of numbering the capturing subpatterns in the whole pattern.
1420     However, substring capturing is carried out only for positive assertions,
1421     because it does not make sense for negative assertions.
1423     Assertions count towards the maximum of 200 parenthesized subpatterns.
1427     With both maximizing and minimizing repetition, failure of what follows
1428     normally causes the repeated item to be re-evaluated to see if a different
1429     number of repeats allows the rest of the pattern to match. Sometimes it is
1430     useful to prevent this, either to change the nature of the match, or to cause
1431     it fail earlier than it otherwise might, when the author of the pattern knows
1432     there is no point in carrying on.
1434     Consider, for example, the pattern \\d+foo when applied to the subject line
1436     123456bar
1438     After matching all 6 digits and then failing to match "foo", the normal
1439     action of the matcher is to try again with only 5 digits matching the \\d+
1440     item, and then with 4, and so on, before ultimately failing. Once-only
1441     subpatterns provide the means for specifying that once a portion of the pattern
1442     has matched, it is not to be re-evaluated in this way, so the matcher would
1443     give up immediately on failing to match "foo" the first time. The notation is
1444     another kind of special parenthesis, starting with (?> as in this example:
1446     (?>\\d+)bar
1448     This kind of parenthesis "locks up" the part of the pattern it contains once
1449     it has matched, and a failure further into the pattern is prevented from
1450     backtracking into it. Backtracking past it to previous items, however, works as
1451     normal.
1453     An alternative description is that a subpattern of this type matches the string
1454     of characters that an identical standalone pattern would match, if anchored at
1455     the current point in the subject string.
1457     Once-only subpatterns are not capturing subpatterns. Simple cases such as the
1458     above example can be thought of as a maximizing repeat that must swallow
1459     everything it can. So, while both \\d+ and \\d+? are prepared to adjust the
1460     number of digits they match in order to make the rest of the pattern match,
1461     (?>\\d+) can only match an entire sequence of digits.
1463     This construction can of course contain arbitrarily complicated subpatterns,
1464     and it can be nested.
1466     Once-only subpatterns can be used in conjunction with lookbehind assertions to
1467     specify efficient matching at the end of the subject string. Consider a simple
1468     pattern such as
1470     abcd$
1472 nigel 43 when applied to a long string which does not match. Because matching proceeds
1473     from left to right, PCRE will look for each "a" in the subject and then see if
1474     what follows matches the rest of the pattern. If the pattern is specified as
1475 nigel 41
1476     ^.*abcd$
1478 nigel 47 the initial .* matches the entire string at first, but when this fails (because
1479     there is no following "a"), it backtracks to match all but the last character,
1480     then all but the last two characters, and so on. Once again the search for "a"
1481     covers the entire string, from right to left, so we are no better off. However,
1482     if the pattern is written as
1483 nigel 41
1484     ^(?>.*)(?<=abcd)
1486 nigel 47 there can be no backtracking for the .* item; it can match only the entire
1487 nigel 41 string. The subsequent lookbehind assertion does a single test on the last four
1488     characters. If it fails, the match fails immediately. For long strings, this
1489     approach makes a significant difference to the processing time.
1491 nigel 43 When a pattern contains an unlimited repeat inside a subpattern that can itself
1492     be repeated an unlimited number of times, the use of a once-only subpattern is
1493     the only way to avoid some failing matches taking a very long time indeed.
1494     The pattern
1495 nigel 41
1496 nigel 43 (\\D+|<\\d+>)*[!?]
1498     matches an unlimited number of substrings that either consist of non-digits, or
1499     digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1500     quickly. However, if it is applied to
1502     aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1504     it takes a long time before reporting failure. This is because the string can
1505     be divided between the two repeats in a large number of ways, and all have to
1506     be tried. (The example used [!?] rather than a single character at the end,
1507     because both PCRE and Perl have an optimization that allows for fast failure
1508     when a single character is used. They remember the last single character that
1509     is required for a match, and fail early if it is not present in the string.)
1510     If the pattern is changed to
1512     ((?>\\D+)|<\\d+>)*[!?]
1514     sequences of non-digits cannot be broken, and failure happens quickly.
1518     It is possible to cause the matching process to obey a subpattern
1519     conditionally or to choose between two alternative subpatterns, depending on
1520     the result of an assertion, or whether a previous capturing subpattern matched
1521     or not. The two possible forms of conditional subpattern are
1523     (?(condition)yes-pattern)
1524     (?(condition)yes-pattern|no-pattern)
1526     If the condition is satisfied, the yes-pattern is used; otherwise the
1527     no-pattern (if present) is used. If there are more than two alternatives in the
1528     subpattern, a compile-time error occurs.
1530     There are two kinds of condition. If the text between the parentheses consists
1531 nigel 47 of a sequence of digits, the condition is satisfied if the capturing subpattern
1532     of that number has previously matched. Consider the following pattern, which
1533     contains non-significant white space to make it more readable (assume the
1534     PCRE_EXTENDED option) and to divide it into three parts for ease of discussion:
1535 nigel 41
1536     ( \\( )? [^()]+ (?(1) \\) )
1538     The first part matches an optional opening parenthesis, and if that
1539     character is present, sets it as the first captured substring. The second part
1540     matches one or more characters that are not parentheses. The third part is a
1541     conditional subpattern that tests whether the first set of parentheses matched
1542     or not. If they did, that is, if subject started with an opening parenthesis,
1543     the condition is true, and so the yes-pattern is executed and a closing
1544     parenthesis is required. Otherwise, since no-pattern is not present, the
1545     subpattern matches nothing. In other words, this pattern matches a sequence of
1546     non-parentheses, optionally enclosed in parentheses.
1548     If the condition is not a sequence of digits, it must be an assertion. This may
1549     be a positive or negative lookahead or lookbehind assertion. Consider this
1550     pattern, again containing non-significant white space, and with the two
1551     alternatives on the second line:
1553     (?(?=[^a-z]*[a-z])
1554     \\d{2}-[a-z]{3}-\\d{2} | \\d{2}-\\d{2}-\\d{2} )
1556     The condition is a positive lookahead assertion that matches an optional
1557     sequence of non-letters followed by a letter. In other words, it tests for the
1558     presence of at least one letter in the subject. If a letter is found, the
1559     subject is matched against the first alternative; otherwise it is matched
1560     against the second. This pattern matches strings in one of the two forms
1561     dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1564     .SH COMMENTS
1565     The sequence (?# marks the start of a comment which continues up to the next
1566     closing parenthesis. Nested parentheses are not permitted. The characters
1567     that make up a comment play no part in the pattern matching at all.
1569     If the PCRE_EXTENDED option is set, an unescaped # character outside a
1570     character class introduces a comment that continues up to the next newline
1571     character in the pattern.
1575     Consider the problem of matching a string in parentheses, allowing for
1576     unlimited nested parentheses. Without the use of recursion, the best that can
1577     be done is to use a pattern that matches up to some fixed depth of nesting. It
1578     is not possible to handle an arbitrary nesting depth. Perl 5.6 has provided an
1579     experimental facility that allows regular expressions to recurse (amongst other
1580     things). It does this by interpolating Perl code in the expression at run time,
1581     and the code can refer to the expression itself. A Perl pattern to solve the
1582     parentheses problem can be created like this:
1584     $re = qr{\\( (?: (?>[^()]+) | (?p{$re}) )* \\)}x;
1586     The (?p{...}) item interpolates Perl code at run time, and in this case refers
1587     recursively to the pattern in which it appears. Obviously, PCRE cannot support
1588     the interpolation of Perl code. Instead, the special item (?R) is provided for
1589     the specific case of recursion. This PCRE pattern solves the parentheses
1590     problem (assume the PCRE_EXTENDED option is set so that white space is
1591     ignored):
1593     \\( ( (?>[^()]+) | (?R) )* \\)
1595     First it matches an opening parenthesis. Then it matches any number of
1596     substrings which can either be a sequence of non-parentheses, or a recursive
1597     match of the pattern itself (i.e. a correctly parenthesized substring). Finally
1598     there is a closing parenthesis.
1600     This particular example pattern contains nested unlimited repeats, and so the
1601     use of a once-only subpattern for matching strings of non-parentheses is
1602     important when applying the pattern to strings that do not match. For example,
1603     when it is applied to
1605     (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1607     it yields "no match" quickly. However, if a once-only subpattern is not used,
1608     the match runs for a very long time indeed because there are so many different
1609     ways the + and * repeats can carve up the subject, and all have to be tested
1610     before failure can be reported.
1612     The values set for any capturing subpatterns are those from the outermost level
1613     of the recursion at which the subpattern value is set. If the pattern above is
1614     matched against
1616     (ab(cd)ef)
1618     the value for the capturing parentheses is "ef", which is the last value taken
1619     on at the top level. If additional parentheses are added, giving
1621     \\( ( ( (?>[^()]+) | (?R) )* ) \\)
1622     ^ ^
1623     ^ ^
1624 nigel 47 the string they capture is "ab(cd)ef", the contents of the top level
1625 nigel 43 parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
1626     has to obtain extra memory to store data during a recursion, which it does by
1627     using \fBpcre_malloc\fR, freeing it via \fBpcre_free\fR afterwards. If no
1628     memory can be obtained, it saves data for the first 15 capturing parentheses
1629     only, as there is no way to give an out-of-memory error from within a
1630     recursion.
1633 nigel 41 .SH PERFORMANCE
1634     Certain items that may appear in patterns are more efficient than others. It is
1635     more efficient to use a character class like [aeiou] than a set of alternatives
1636     such as (a|e|i|o|u). In general, the simplest construction that provides the
1637     required behaviour is usually the most efficient. Jeffrey Friedl's book
1638     contains a lot of discussion about optimizing regular expressions for efficient
1639     performance.
1641     When a pattern begins with .* and the PCRE_DOTALL option is set, the pattern is
1642     implicitly anchored by PCRE, since it can match only at the start of a subject
1643     string. However, if PCRE_DOTALL is not set, PCRE cannot make this optimization,
1644     because the . metacharacter does not then match a newline, and if the subject
1645     string contains newlines, the pattern may match from the character immediately
1646     following one of them instead of from the very start. For example, the pattern
1648     (.*) second
1650     matches the subject "first\\nand second" (where \\n stands for a newline
1651     character) with the first captured substring being "and". In order to do this,
1652     PCRE has to retry the match starting after every newline in the subject.
1654     If you are using such a pattern with subject strings that do not contain
1655     newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
1656     the pattern with ^.* to indicate explicit anchoring. That saves PCRE from
1657     having to scan along the subject looking for a newline to restart at.
1659     Beware of patterns that contain nested indefinite repeats. These can take a
1660     long time to run when applied to a string that does not match. Consider the
1661     pattern fragment
1663     (a+)*
1665     This can match "aaaa" in 33 different ways, and this number increases very
1666     rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4
1667     times, and for each of those cases other than 0, the + repeats can match
1668     different numbers of times.) When the remainder of the pattern is such that the
1669     entire match is going to fail, PCRE has in principle to try every possible
1670     variation, and this can take an extremely long time.
1672     An optimization catches some of the more simple cases such as
1674     (a+)*b
1676     where a literal character follows. Before embarking on the standard matching
1677     procedure, PCRE checks that there is a "b" later in the subject string, and if
1678     there is not, it fails the match immediately. However, when there is no
1679     following literal this optimization cannot be used. You can see the difference
1680     by comparing the behaviour of
1682     (a+)*\\d
1684     with the pattern above. The former gives a failure almost instantly when
1685     applied to a whole line of "a" characters, whereas the latter takes an
1686     appreciable time with strings longer than about 20 characters.
1688     .SH AUTHOR
1689     Philip Hazel <ph10@cam.ac.uk>
1690     .br
1691     University Computing Service,
1692     .br
1693     New Museums Site,
1694     .br
1695     Cambridge CB2 3QG, England.
1696     .br
1697     Phone: +44 1223 334714
1699 nigel 43 Last updated: 27 January 2000
1700 nigel 41 .br
1701 nigel 43 Copyright (c) 1997-2000 University of Cambridge.

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