3
pcre - Perl-compatible regular expressions.
9
.B pcre *pcre_compile(const char *\fIpattern\fR, int \fIoptions\fR,
11
.B const char **\fIerrptr\fR, int *\fIerroffset\fR,
13
.B const unsigned char *\fItableptr\fR);
16
.B pcre_extra *pcre_study(const pcre *\fIcode\fR, int \fIoptions\fR,
18
.B const char **\fIerrptr\fR);
21
.B int pcre_exec(const pcre *\fIcode\fR, "const pcre_extra *\fIextra\fR,"
23
.B "const char *\fIsubject\fR," int \fIlength\fR, int \fIstartoffset\fR,
25
.B int \fIoptions\fR, int *\fIovector\fR, int \fIovecsize\fR);
28
.B int pcre_copy_substring(const char *\fIsubject\fR, int *\fIovector\fR,
30
.B int \fIstringcount\fR, int \fIstringnumber\fR, char *\fIbuffer\fR,
32
.B int \fIbuffersize\fR);
35
.B int pcre_get_substring(const char *\fIsubject\fR, int *\fIovector\fR,
37
.B int \fIstringcount\fR, int \fIstringnumber\fR,
39
.B const char **\fIstringptr\fR);
42
.B int pcre_get_substring_list(const char *\fIsubject\fR,
44
.B int *\fIovector\fR, int \fIstringcount\fR, "const char ***\fIlistptr\fR);"
47
.B void pcre_free_substring(const char *\fIstringptr\fR);
50
.B void pcre_free_substring_list(const char **\fIstringptr\fR);
53
.B const unsigned char *pcre_maketables(void);
56
.B int pcre_fullinfo(const pcre *\fIcode\fR, "const pcre_extra *\fIextra\fR,"
58
.B int \fIwhat\fR, void *\fIwhere\fR);
61
.B int pcre_info(const pcre *\fIcode\fR, int *\fIoptptr\fR, int
62
.B *\fIfirstcharptr\fR);
65
.B char *pcre_version(void);
68
.B void *(*pcre_malloc)(size_t);
71
.B void (*pcre_free)(void *);
76
The PCRE library is a set of functions that implement regular expression
77
pattern matching using the same syntax and semantics as Perl 5, with just a few
78
differences (see below). The current implementation corresponds to Perl 5.005,
79
with some additional features from later versions. This includes some
80
experimental, incomplete support for UTF-8 encoded strings. Details of exactly
81
what is and what is not supported are given below.
83
PCRE has its own native API, which is described in this document. There is also
84
a set of wrapper functions that correspond to the POSIX regular expression API.
85
These are described in the \fBpcreposix\fR documentation.
87
The native API function prototypes are defined in the header file \fBpcre.h\fR,
88
and on Unix systems the library itself is called \fBlibpcre.a\fR, so can be
89
accessed by adding \fB-lpcre\fR to the command for linking an application which
90
calls it. The header file defines the macros PCRE_MAJOR and PCRE_MINOR to
91
contain the major and minor release numbers for the library. Applications can
92
use these to include support for different releases.
94
The functions \fBpcre_compile()\fR, \fBpcre_study()\fR, and \fBpcre_exec()\fR
95
are used for compiling and matching regular expressions. A sample program that
96
demonstrates the simplest way of using them is given in the file
97
\fIpcredemo.c\fR. The last section of this man page describes how to run it.
99
The functions \fBpcre_copy_substring()\fR, \fBpcre_get_substring()\fR, and
100
\fBpcre_get_substring_list()\fR are convenience functions for extracting
101
captured substrings from a matched subject string; \fBpcre_free_substring()\fR
102
and \fBpcre_free_substring_list()\fR are also provided, to free the memory used
103
for extracted strings.
105
The function \fBpcre_maketables()\fR is used (optionally) to build a set of
106
character tables in the current locale for passing to \fBpcre_compile()\fR.
108
The function \fBpcre_fullinfo()\fR is used to find out information about a
109
compiled pattern; \fBpcre_info()\fR is an obsolete version which returns only
110
some of the available information, but is retained for backwards compatibility.
111
The function \fBpcre_version()\fR returns a pointer to a string containing the
112
version of PCRE and its date of release.
114
The global variables \fBpcre_malloc\fR and \fBpcre_free\fR initially contain
115
the entry points of the standard \fBmalloc()\fR and \fBfree()\fR functions
116
respectively. PCRE calls the memory management functions via these variables,
117
so a calling program can replace them if it wishes to intercept the calls. This
118
should be done before calling any PCRE functions.
122
The PCRE functions can be used in multi-threading applications, with the
123
proviso that the memory management functions pointed to by \fBpcre_malloc\fR
124
and \fBpcre_free\fR are shared by all threads.
126
The compiled form of a regular expression is not altered during matching, so
127
the same compiled pattern can safely be used by several threads at once.
130
.SH COMPILING A PATTERN
131
The function \fBpcre_compile()\fR is called to compile a pattern into an
132
internal form. The pattern is a C string terminated by a binary zero, and
133
is passed in the argument \fIpattern\fR. A pointer to a single block of memory
134
that is obtained via \fBpcre_malloc\fR is returned. This contains the compiled
135
code and related data. The \fBpcre\fR type is defined for the returned block;
136
this is a typedef for a structure whose contents are not externally defined. It
137
is up to the caller to free the memory when it is no longer required.
139
Although the compiled code of a PCRE regex is relocatable, that is, it does not
140
depend on memory location, the complete \fBpcre\fR data block is not
141
fully relocatable, because it contains a copy of the \fItableptr\fR argument,
142
which is an address (see below).
144
The size of a compiled pattern is roughly proportional to the length of the
145
pattern string, except that each character class (other than those containing
146
just a single character, negated or not) requires 33 bytes, and repeat
147
quantifiers with a minimum greater than one or a bounded maximum cause the
148
relevant portions of the compiled pattern to be replicated.
150
The \fIoptions\fR argument contains independent bits that affect the
151
compilation. It should be zero if no options are required. Some of the options,
152
in particular, those that are compatible with Perl, can also be set and unset
153
from within the pattern (see the detailed description of regular expressions
154
below). For these options, the contents of the \fIoptions\fR argument specifies
155
their initial settings at the start of compilation and execution. The
156
PCRE_ANCHORED option can be set at the time of matching as well as at compile
159
If \fIerrptr\fR is NULL, \fBpcre_compile()\fR returns NULL immediately.
160
Otherwise, if compilation of a pattern fails, \fBpcre_compile()\fR returns
161
NULL, and sets the variable pointed to by \fIerrptr\fR to point to a textual
162
error message. The offset from the start of the pattern to the character where
163
the error was discovered is placed in the variable pointed to by
164
\fIerroffset\fR, which must not be NULL. If it is, an immediate error is given.
166
If the final argument, \fItableptr\fR, is NULL, PCRE uses a default set of
167
character tables which are built when it is compiled, using the default C
168
locale. Otherwise, \fItableptr\fR must be the result of a call to
169
\fBpcre_maketables()\fR. See the section on locale support below.
171
This code fragment shows a typical straightforward call to \fBpcre_compile()\fR:
177
"^A.*Z", /* the pattern */
178
0, /* default options */
179
&error, /* for error message */
180
&erroffset, /* for error offset */
181
NULL); /* use default character tables */
183
The following option bits are defined in the header file:
187
If this bit is set, the pattern is forced to be "anchored", that is, it is
188
constrained to match only at the start of the string which is being searched
189
(the "subject string"). This effect can also be achieved by appropriate
190
constructs in the pattern itself, which is the only way to do it in Perl.
194
If this bit is set, letters in the pattern match both upper and lower case
195
letters. It is equivalent to Perl's /i option.
199
If this bit is set, a dollar metacharacter in the pattern matches only at the
200
end of the subject string. Without this option, a dollar also matches
201
immediately before the final character if it is a newline (but not before any
202
other newlines). The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is
203
set. There is no equivalent to this option in Perl.
207
If this bit is set, a dot metacharater in the pattern matches all characters,
208
including newlines. Without it, newlines are excluded. This option is
209
equivalent to Perl's /s option. A negative class such as [^a] always matches a
210
newline character, independent of the setting of this option.
214
If this bit is set, whitespace data characters in the pattern are totally
215
ignored except when escaped or inside a character class, and characters between
216
an unescaped # outside a character class and the next newline character,
217
inclusive, are also ignored. This is equivalent to Perl's /x option, and makes
218
it possible to include comments inside complicated patterns. Note, however,
219
that this applies only to data characters. Whitespace characters may never
220
appear within special character sequences in a pattern, for example within the
221
sequence (?( which introduces a conditional subpattern.
225
This option was invented in order to turn on additional functionality of PCRE
226
that is incompatible with Perl, but it is currently of very little use. When
227
set, any backslash in a pattern that is followed by a letter that has no
228
special meaning causes an error, thus reserving these combinations for future
229
expansion. By default, as in Perl, a backslash followed by a letter with no
230
special meaning is treated as a literal. There are at present no other features
231
controlled by this option. It can also be set by a (?X) option setting within a
236
By default, PCRE treats the subject string as consisting of a single "line" of
237
characters (even if it actually contains several newlines). The "start of line"
238
metacharacter (^) matches only at the start of the string, while the "end of
239
line" metacharacter ($) matches only at the end of the string, or before a
240
terminating newline (unless PCRE_DOLLAR_ENDONLY is set). This is the same as
243
When PCRE_MULTILINE it is set, the "start of line" and "end of line" constructs
244
match immediately following or immediately before any newline in the subject
245
string, respectively, as well as at the very start and end. This is equivalent
246
to Perl's /m option. If there are no "\\n" characters in a subject string, or
247
no occurrences of ^ or $ in a pattern, setting PCRE_MULTILINE has no
252
This option inverts the "greediness" of the quantifiers so that they are not
253
greedy by default, but become greedy if followed by "?". It is not compatible
254
with Perl. It can also be set by a (?U) option setting within the pattern.
258
This option causes PCRE to regard both the pattern and the subject as strings
259
of UTF-8 characters instead of just byte strings. However, it is available only
260
if PCRE has been built to include UTF-8 support. If not, the use of this option
261
provokes an error. Support for UTF-8 is new, experimental, and incomplete.
262
Details of exactly what it entails are given below.
265
.SH STUDYING A PATTERN
266
When a pattern is going to be used several times, it is worth spending more
267
time analyzing it in order to speed up the time taken for matching. The
268
function \fBpcre_study()\fR takes a pointer to a compiled pattern as its first
269
argument, and returns a pointer to a \fBpcre_extra\fR block (another typedef
270
for a structure with hidden contents) containing additional information about
271
the pattern; this can be passed to \fBpcre_exec()\fR. If no additional
272
information is available, NULL is returned.
274
The second argument contains option bits. At present, no options are defined
275
for \fBpcre_study()\fR, and this argument should always be zero.
277
The third argument for \fBpcre_study()\fR is a pointer to an error message. If
278
studying succeeds (even if no data is returned), the variable it points to is
279
set to NULL. Otherwise it points to a textual error message.
281
This is a typical call to \fBpcre_study\fR():
285
re, /* result of pcre_compile() */
286
0, /* no options exist */
287
&error); /* set to NULL or points to a message */
289
At present, studying a pattern is useful only for non-anchored patterns that do
290
not have a single fixed starting character. A bitmap of possible starting
291
characters is created.
295
PCRE handles caseless matching, and determines whether characters are letters,
296
digits, or whatever, by reference to a set of tables. The library contains a
297
default set of tables which is created in the default C locale when PCRE is
298
compiled. This is used when the final argument of \fBpcre_compile()\fR is NULL,
299
and is sufficient for many applications.
301
An alternative set of tables can, however, be supplied. Such tables are built
302
by calling the \fBpcre_maketables()\fR function, which has no arguments, in the
303
relevant locale. The result can then be passed to \fBpcre_compile()\fR as often
304
as necessary. For example, to build and use tables that are appropriate for the
305
French locale (where accented characters with codes greater than 128 are
306
treated as letters), the following code could be used:
308
setlocale(LC_CTYPE, "fr");
309
tables = pcre_maketables();
310
re = pcre_compile(..., tables);
312
The tables are built in memory that is obtained via \fBpcre_malloc\fR. The
313
pointer that is passed to \fBpcre_compile\fR is saved with the compiled
314
pattern, and the same tables are used via this pointer by \fBpcre_study()\fR
315
and \fBpcre_exec()\fR. Thus for any single pattern, compilation, studying and
316
matching all happen in the same locale, but different patterns can be compiled
317
in different locales. It is the caller's responsibility to ensure that the
318
memory containing the tables remains available for as long as it is needed.
321
.SH INFORMATION ABOUT A PATTERN
322
The \fBpcre_fullinfo()\fR function returns information about a compiled
323
pattern. It replaces the obsolete \fBpcre_info()\fR function, which is
324
nevertheless retained for backwards compability (and is documented below).
326
The first argument for \fBpcre_fullinfo()\fR is a pointer to the compiled
327
pattern. The second argument is the result of \fBpcre_study()\fR, or NULL if
328
the pattern was not studied. The third argument specifies which piece of
329
information is required, while the fourth argument is a pointer to a variable
330
to receive the data. The yield of the function is zero for success, or one of
331
the following negative numbers:
333
PCRE_ERROR_NULL the argument \fIcode\fR was NULL
334
the argument \fIwhere\fR was NULL
335
PCRE_ERROR_BADMAGIC the "magic number" was not found
336
PCRE_ERROR_BADOPTION the value of \fIwhat\fR was invalid
338
Here is a typical call of \fBpcre_fullinfo()\fR, to obtain the length of the
342
unsigned long int length;
344
re, /* result of pcre_compile() */
345
pe, /* result of pcre_study(), or NULL */
346
PCRE_INFO_SIZE, /* what is required */
347
&length); /* where to put the data */
349
The possible values for the third argument are defined in \fBpcre.h\fR, and are
354
Return a copy of the options with which the pattern was compiled. The fourth
355
argument should point to an \fBunsigned long int\fR variable. These option bits
356
are those specified in the call to \fBpcre_compile()\fR, modified by any
357
top-level option settings within the pattern itself, and with the PCRE_ANCHORED
358
bit forcibly set if the form of the pattern implies that it can match only at
359
the start of a subject string.
363
Return the size of the compiled pattern, that is, the value that was passed as
364
the argument to \fBpcre_malloc()\fR when PCRE was getting memory in which to
365
place the compiled data. The fourth argument should point to a \fBsize_t\fR
368
PCRE_INFO_CAPTURECOUNT
370
Return the number of capturing subpatterns in the pattern. The fourth argument
371
should point to an \fbint\fR variable.
375
Return the number of the highest back reference in the pattern. The fourth
376
argument should point to an \fBint\fR variable. Zero is returned if there are
381
Return information about the first character of any matched string, for a
382
non-anchored pattern. If there is a fixed first character, e.g. from a pattern
383
such as (cat|cow|coyote), it is returned in the integer pointed to by
384
\fIwhere\fR. Otherwise, if either
386
(a) the pattern was compiled with the PCRE_MULTILINE option, and every branch
389
(b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not set
390
(if it were set, the pattern would be anchored),
392
-1 is returned, indicating that the pattern matches only at the start of a
393
subject string or after any "\\n" within the string. Otherwise -2 is returned.
394
For anchored patterns, -2 is returned.
398
If the pattern was studied, and this resulted in the construction of a 256-bit
399
table indicating a fixed set of characters for the first character in any
400
matching string, a pointer to the table is returned. Otherwise NULL is
401
returned. The fourth argument should point to an \fBunsigned char *\fR
404
PCRE_INFO_LASTLITERAL
406
For a non-anchored pattern, return the value of the rightmost literal character
407
which must exist in any matched string, other than at its start. The fourth
408
argument should point to an \fBint\fR variable. If there is no such character,
409
or if the pattern is anchored, -1 is returned. For example, for the pattern
410
/a\\d+z\\d+/ the returned value is 'z'.
412
The \fBpcre_info()\fR function is now obsolete because its interface is too
413
restrictive to return all the available data about a compiled pattern. New
414
programs should use \fBpcre_fullinfo()\fR instead. The yield of
415
\fBpcre_info()\fR is the number of capturing subpatterns, or one of the
416
following negative numbers:
418
PCRE_ERROR_NULL the argument \fIcode\fR was NULL
419
PCRE_ERROR_BADMAGIC the "magic number" was not found
421
If the \fIoptptr\fR argument is not NULL, a copy of the options with which the
422
pattern was compiled is placed in the integer it points to (see
423
PCRE_INFO_OPTIONS above).
425
If the pattern is not anchored and the \fIfirstcharptr\fR argument is not NULL,
426
it is used to pass back information about the first character of any matched
427
string (see PCRE_INFO_FIRSTCHAR above).
430
.SH MATCHING A PATTERN
431
The function \fBpcre_exec()\fR is called to match a subject string against a
432
pre-compiled pattern, which is passed in the \fIcode\fR argument. If the
433
pattern has been studied, the result of the study should be passed in the
434
\fIextra\fR argument. Otherwise this must be NULL.
436
Here is an example of a simple call to \fBpcre_exec()\fR:
441
re, /* result of pcre_compile() */
442
NULL, /* we didn't study the pattern */
443
"some string", /* the subject string */
444
11, /* the length of the subject string */
445
0, /* start at offset 0 in the subject */
446
0, /* default options */
447
ovector, /* vector for substring information */
448
30); /* number of elements in the vector */
450
The PCRE_ANCHORED option can be passed in the \fIoptions\fR argument, whose
451
unused bits must be zero. However, if a pattern was compiled with
452
PCRE_ANCHORED, or turned out to be anchored by virtue of its contents, it
453
cannot be made unachored at matching time.
455
There are also three further options that can be set only at matching time:
459
The first character of the string is not the beginning of a line, so the
460
circumflex metacharacter should not match before it. Setting this without
461
PCRE_MULTILINE (at compile time) causes circumflex never to match.
465
The end of the string is not the end of a line, so the dollar metacharacter
466
should not match it nor (except in multiline mode) a newline immediately before
467
it. Setting this without PCRE_MULTILINE (at compile time) causes dollar never
472
An empty string is not considered to be a valid match if this option is set. If
473
there are alternatives in the pattern, they are tried. If all the alternatives
474
match the empty string, the entire match fails. For example, if the pattern
478
is applied to a string not beginning with "a" or "b", it matches the empty
479
string at the start of the subject. With PCRE_NOTEMPTY set, this match is not
480
valid, so PCRE searches further into the string for occurrences of "a" or "b".
482
Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a special case
483
of a pattern match of the empty string within its \fBsplit()\fR function, and
484
when using the /g modifier. It is possible to emulate Perl's behaviour after
485
matching a null string by first trying the match again at the same offset with
486
PCRE_NOTEMPTY set, and then if that fails by advancing the starting offset (see
487
below) and trying an ordinary match again.
489
The subject string is passed as a pointer in \fIsubject\fR, a length in
490
\fIlength\fR, and a starting offset in \fIstartoffset\fR. Unlike the pattern
491
string, the subject may contain binary zero characters. When the starting
492
offset is zero, the search for a match starts at the beginning of the subject,
493
and this is by far the most common case.
495
A non-zero starting offset is useful when searching for another match in the
496
same subject by calling \fBpcre_exec()\fR again after a previous success.
497
Setting \fIstartoffset\fR differs from just passing over a shortened string and
498
setting PCRE_NOTBOL in the case of a pattern that begins with any kind of
499
lookbehind. For example, consider the pattern
503
which finds occurrences of "iss" in the middle of words. (\\B matches only if
504
the current position in the subject is not a word boundary.) When applied to
505
the string "Mississipi" the first call to \fBpcre_exec()\fR finds the first
506
occurrence. If \fBpcre_exec()\fR is called again with just the remainder of the
507
subject, namely "issipi", it does not match, because \\B is always false at the
508
start of the subject, which is deemed to be a word boundary. However, if
509
\fBpcre_exec()\fR is passed the entire string again, but with \fIstartoffset\fR
510
set to 4, it finds the second occurrence of "iss" because it is able to look
511
behind the starting point to discover that it is preceded by a letter.
513
If a non-zero starting offset is passed when the pattern is anchored, one
514
attempt to match at the given offset is tried. This can only succeed if the
515
pattern does not require the match to be at the start of the subject.
517
In general, a pattern matches a certain portion of the subject, and in
518
addition, further substrings from the subject may be picked out by parts of the
519
pattern. Following the usage in Jeffrey Friedl's book, this is called
520
"capturing" in what follows, and the phrase "capturing subpattern" is used for
521
a fragment of a pattern that picks out a substring. PCRE supports several other
522
kinds of parenthesized subpattern that do not cause substrings to be captured.
524
Captured substrings are returned to the caller via a vector of integer offsets
525
whose address is passed in \fIovector\fR. The number of elements in the vector
526
is passed in \fIovecsize\fR. The first two-thirds of the vector is used to pass
527
back captured substrings, each substring using a pair of integers. The
528
remaining third of the vector is used as workspace by \fBpcre_exec()\fR while
529
matching capturing subpatterns, and is not available for passing back
530
information. The length passed in \fIovecsize\fR should always be a multiple of
531
three. If it is not, it is rounded down.
533
When a match has been successful, information about captured substrings is
534
returned in pairs of integers, starting at the beginning of \fIovector\fR, and
535
continuing up to two-thirds of its length at the most. The first element of a
536
pair is set to the offset of the first character in a substring, and the second
537
is set to the offset of the first character after the end of a substring. The
538
first pair, \fIovector[0]\fR and \fIovector[1]\fR, identify the portion of the
539
subject string matched by the entire pattern. The next pair is used for the
540
first capturing subpattern, and so on. The value returned by \fBpcre_exec()\fR
541
is the number of pairs that have been set. If there are no capturing
542
subpatterns, the return value from a successful match is 1, indicating that
543
just the first pair of offsets has been set.
545
Some convenience functions are provided for extracting the captured substrings
546
as separate strings. These are described in the following section.
548
It is possible for an capturing subpattern number \fIn+1\fR to match some
549
part of the subject when subpattern \fIn\fR has not been used at all. For
550
example, if the string "abc" is matched against the pattern (a|(z))(bc)
551
subpatterns 1 and 3 are matched, but 2 is not. When this happens, both offset
552
values corresponding to the unused subpattern are set to -1.
554
If a capturing subpattern is matched repeatedly, it is the last portion of the
555
string that it matched that gets returned.
557
If the vector is too small to hold all the captured substrings, it is used as
558
far as possible (up to two-thirds of its length), and the function returns a
559
value of zero. In particular, if the substring offsets are not of interest,
560
\fBpcre_exec()\fR may be called with \fIovector\fR passed as NULL and
561
\fIovecsize\fR as zero. However, if the pattern contains back references and
562
the \fIovector\fR isn't big enough to remember the related substrings, PCRE has
563
to get additional memory for use during matching. Thus it is usually advisable
564
to supply an \fIovector\fR.
566
Note that \fBpcre_info()\fR can be used to find out how many capturing
567
subpatterns there are in a compiled pattern. The smallest size for
568
\fIovector\fR that will allow for \fIn\fR captured substrings in addition to
569
the offsets of the substring matched by the whole pattern is (\fIn\fR+1)*3.
571
If \fBpcre_exec()\fR fails, it returns a negative number. The following are
572
defined in the header file:
574
PCRE_ERROR_NOMATCH (-1)
576
The subject string did not match the pattern.
580
Either \fIcode\fR or \fIsubject\fR was passed as NULL, or \fIovector\fR was
581
NULL and \fIovecsize\fR was not zero.
583
PCRE_ERROR_BADOPTION (-3)
585
An unrecognized bit was set in the \fIoptions\fR argument.
587
PCRE_ERROR_BADMAGIC (-4)
589
PCRE stores a 4-byte "magic number" at the start of the compiled code, to catch
590
the case when it is passed a junk pointer. This is the error it gives when the
591
magic number isn't present.
593
PCRE_ERROR_UNKNOWN_NODE (-5)
595
While running the pattern match, an unknown item was encountered in the
596
compiled pattern. This error could be caused by a bug in PCRE or by overwriting
597
of the compiled pattern.
599
PCRE_ERROR_NOMEMORY (-6)
601
If a pattern contains back references, but the \fIovector\fR that is passed to
602
\fBpcre_exec()\fR is not big enough to remember the referenced substrings, PCRE
603
gets a block of memory at the start of matching to use for this purpose. If the
604
call via \fBpcre_malloc()\fR fails, this error is given. The memory is freed at
608
.SH EXTRACTING CAPTURED SUBSTRINGS
609
Captured substrings can be accessed directly by using the offsets returned by
610
\fBpcre_exec()\fR in \fIovector\fR. For convenience, the functions
611
\fBpcre_copy_substring()\fR, \fBpcre_get_substring()\fR, and
612
\fBpcre_get_substring_list()\fR are provided for extracting captured substrings
613
as new, separate, zero-terminated strings. A substring that contains a binary
614
zero is correctly extracted and has a further zero added on the end, but the
615
result does not, of course, function as a C string.
617
The first three arguments are the same for all three functions: \fIsubject\fR
618
is the subject string which has just been successfully matched, \fIovector\fR
619
is a pointer to the vector of integer offsets that was passed to
620
\fBpcre_exec()\fR, and \fIstringcount\fR is the number of substrings that
621
were captured by the match, including the substring that matched the entire
622
regular expression. This is the value returned by \fBpcre_exec\fR if it
623
is greater than zero. If \fBpcre_exec()\fR returned zero, indicating that it
624
ran out of space in \fIovector\fR, the value passed as \fIstringcount\fR should
625
be the size of the vector divided by three.
627
The functions \fBpcre_copy_substring()\fR and \fBpcre_get_substring()\fR
628
extract a single substring, whose number is given as \fIstringnumber\fR. A
629
value of zero extracts the substring that matched the entire pattern, while
630
higher values extract the captured substrings. For \fBpcre_copy_substring()\fR,
631
the string is placed in \fIbuffer\fR, whose length is given by
632
\fIbuffersize\fR, while for \fBpcre_get_substring()\fR a new block of memory is
633
obtained via \fBpcre_malloc\fR, and its address is returned via
634
\fIstringptr\fR. The yield of the function is the length of the string, not
635
including the terminating zero, or one of
637
PCRE_ERROR_NOMEMORY (-6)
639
The buffer was too small for \fBpcre_copy_substring()\fR, or the attempt to get
640
memory failed for \fBpcre_get_substring()\fR.
642
PCRE_ERROR_NOSUBSTRING (-7)
644
There is no substring whose number is \fIstringnumber\fR.
646
The \fBpcre_get_substring_list()\fR function extracts all available substrings
647
and builds a list of pointers to them. All this is done in a single block of
648
memory which is obtained via \fBpcre_malloc\fR. The address of the memory block
649
is returned via \fIlistptr\fR, which is also the start of the list of string
650
pointers. The end of the list is marked by a NULL pointer. The yield of the
651
function is zero if all went well, or
653
PCRE_ERROR_NOMEMORY (-6)
655
if the attempt to get the memory block failed.
657
When any of these functions encounter a substring that is unset, which can
658
happen when capturing subpattern number \fIn+1\fR matches some part of the
659
subject, but subpattern \fIn\fR has not been used at all, they return an empty
660
string. This can be distinguished from a genuine zero-length substring by
661
inspecting the appropriate offset in \fIovector\fR, which is negative for unset
664
The two convenience functions \fBpcre_free_substring()\fR and
665
\fBpcre_free_substring_list()\fR can be used to free the memory returned by
666
a previous call of \fBpcre_get_substring()\fR or
667
\fBpcre_get_substring_list()\fR, respectively. They do nothing more than call
668
the function pointed to by \fBpcre_free\fR, which of course could be called
669
directly from a C program. However, PCRE is used in some situations where it is
670
linked via a special interface to another programming language which cannot use
671
\fBpcre_free\fR directly; it is for these cases that the functions are
676
There are some size limitations in PCRE but it is hoped that they will never in
677
practice be relevant.
678
The maximum length of a compiled pattern is 65539 (sic) bytes.
679
All values in repeating quantifiers must be less than 65536.
680
There maximum number of capturing subpatterns is 65535.
681
There is no limit to the number of non-capturing subpatterns, but the maximum
682
depth of nesting of all kinds of parenthesized subpattern, including capturing
683
subpatterns, assertions, and other types of subpattern, is 200.
685
The maximum length of a subject string is the largest positive number that an
686
integer variable can hold. However, PCRE uses recursion to handle subpatterns
687
and indefinite repetition. This means that the available stack space may limit
688
the size of a subject string that can be processed by certain patterns.
691
.SH DIFFERENCES FROM PERL
692
The differences described here are with respect to Perl 5.005.
694
1. By default, a whitespace character is any character that the C library
695
function \fBisspace()\fR recognizes, though it is possible to compile PCRE with
696
alternative character type tables. Normally \fBisspace()\fR matches space,
697
formfeed, newline, carriage return, horizontal tab, and vertical tab. Perl 5
698
no longer includes vertical tab in its set of whitespace characters. The \\v
699
escape that was in the Perl documentation for a long time was never in fact
700
recognized. However, the character itself was treated as whitespace at least
701
up to 5.002. In 5.004 and 5.005 it does not match \\s.
703
2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
704
them, but they do not mean what you might think. For example, (?!a){3} does
705
not assert that the next three characters are not "a". It just asserts that the
706
next character is not "a" three times.
708
3. Capturing subpatterns that occur inside negative lookahead assertions are
709
counted, but their entries in the offsets vector are never set. Perl sets its
710
numerical variables from any such patterns that are matched before the
711
assertion fails to match something (thereby succeeding), but only if the
712
negative lookahead assertion contains just one branch.
714
4. Though binary zero characters are supported in the subject string, they are
715
not allowed in a pattern string because it is passed as a normal C string,
716
terminated by zero. The escape sequence "\\0" can be used in the pattern to
717
represent a binary zero.
719
5. The following Perl escape sequences are not supported: \\l, \\u, \\L, \\U,
720
\\E, \\Q. In fact these are implemented by Perl's general string-handling and
721
are not part of its pattern matching engine.
723
6. The Perl \\G assertion is not supported as it is not relevant to single
726
7. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
727
constructions. However, there is some experimental support for recursive
728
patterns using the non-Perl item (?R).
730
8. There are at the time of writing some oddities in Perl 5.005_02 concerned
731
with the settings of captured strings when part of a pattern is repeated. For
732
example, matching "aba" against the pattern /^(a(b)?)+$/ sets $2 to the value
733
"b", but matching "aabbaa" against /^(aa(bb)?)+$/ leaves $2 unset. However, if
734
the pattern is changed to /^(aa(b(b))?)+$/ then $2 (and $3) are set.
736
In Perl 5.004 $2 is set in both cases, and that is also true of PCRE. If in the
737
future Perl changes to a consistent state that is different, PCRE may change to
740
9. Another as yet unresolved discrepancy is that in Perl 5.005_02 the pattern
741
/^(a)?(?(1)a|b)+$/ matches the string "a", whereas in PCRE it does not.
742
However, in both Perl and PCRE /^(a)?a/ matched against "a" leaves $1 unset.
744
10. PCRE provides some extensions to the Perl regular expression facilities:
746
(a) Although lookbehind assertions must match fixed length strings, each
747
alternative branch of a lookbehind assertion can match a different length of
748
string. Perl 5.005 requires them all to have the same length.
750
(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ meta-
751
character matches only at the very end of the string.
753
(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
756
(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
757
inverted, that is, by default they are not greedy, but if followed by a
758
question mark they are.
760
(e) PCRE_ANCHORED can be used to force a pattern to be tried only at the start
763
(f) The PCRE_NOTBOL, PCRE_NOTEOL, and PCRE_NOTEMPTY options for
764
\fBpcre_exec()\fR have no Perl equivalents.
766
(g) The (?R) construct allows for recursive pattern matching (Perl 5.6 can do
767
this using the (?p{code}) construct, which PCRE cannot of course support.)
770
.SH REGULAR EXPRESSION DETAILS
771
The syntax and semantics of the regular expressions supported by PCRE are
772
described below. Regular expressions are also described in the Perl
773
documentation and in a number of other books, some of which have copious
774
examples. Jeffrey Friedl's "Mastering Regular Expressions", published by
775
O'Reilly (ISBN 1-56592-257), covers them in great detail.
777
The description here is intended as reference documentation. The basic
778
operation of PCRE is on strings of bytes. However, there is the beginnings of
779
some support for UTF-8 character strings. To use this support you must
780
configure PCRE to include it, and then call \fBpcre_compile()\fR with the
781
PCRE_UTF8 option. How this affects the pattern matching is described in the
782
final section of this document.
784
A regular expression is a pattern that is matched against a subject string from
785
left to right. Most characters stand for themselves in a pattern, and match the
786
corresponding characters in the subject. As a trivial example, the pattern
790
matches a portion of a subject string that is identical to itself. The power of
791
regular expressions comes from the ability to include alternatives and
792
repetitions in the pattern. These are encoded in the pattern by the use of
793
\fImeta-characters\fR, which do not stand for themselves but instead are
794
interpreted in some special way.
796
There are two different sets of meta-characters: those that are recognized
797
anywhere in the pattern except within square brackets, and those that are
798
recognized in square brackets. Outside square brackets, the meta-characters are
801
\\ general escape character with several uses
802
^ assert start of subject (or line, in multiline mode)
803
$ assert end of subject (or line, in multiline mode)
804
. match any character except newline (by default)
805
[ start character class definition
806
| start of alternative branch
809
? extends the meaning of (
810
also 0 or 1 quantifier
811
also quantifier minimizer
812
* 0 or more quantifier
813
+ 1 or more quantifier
814
{ start min/max quantifier
816
Part of a pattern that is in square brackets is called a "character class". In
817
a character class the only meta-characters are:
819
\\ general escape character
820
^ negate the class, but only if the first character
821
- indicates character range
822
] terminates the character class
824
The following sections describe the use of each of the meta-characters.
828
The backslash character has several uses. Firstly, if it is followed by a
829
non-alphameric character, it takes away any special meaning that character may
830
have. This use of backslash as an escape character applies both inside and
831
outside character classes.
833
For example, if you want to match a "*" character, you write "\\*" in the
834
pattern. This applies whether or not the following character would otherwise be
835
interpreted as a meta-character, so it is always safe to precede a
836
non-alphameric with "\\" to specify that it stands for itself. In particular,
837
if you want to match a backslash, you write "\\\\".
839
If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
840
pattern (other than in a character class) and characters between a "#" outside
841
a character class and the next newline character are ignored. An escaping
842
backslash can be used to include a whitespace or "#" character as part of the
845
A second use of backslash provides a way of encoding non-printing characters
846
in patterns in a visible manner. There is no restriction on the appearance of
847
non-printing characters, apart from the binary zero that terminates a pattern,
848
but when a pattern is being prepared by text editing, it is usually easier to
849
use one of the following escape sequences than the binary character it
852
\\a alarm, that is, the BEL character (hex 07)
853
\\cx "control-x", where x is any character
855
\\f formfeed (hex 0C)
857
\\r carriage return (hex 0D)
859
\\xhh character with hex code hh
860
\\ddd character with octal code ddd, or backreference
862
The precise effect of "\\cx" is as follows: if "x" is a lower case letter, it
863
is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
864
Thus "\\cz" becomes hex 1A, but "\\c{" becomes hex 3B, while "\\c;" becomes hex
867
After "\\x", up to two hexadecimal digits are read (letters can be in upper or
870
After "\\0" up to two further octal digits are read. In both cases, if there
871
are fewer than two digits, just those that are present are used. Thus the
872
sequence "\\0\\x\\07" specifies two binary zeros followed by a BEL character.
873
Make sure you supply two digits after the initial zero if the character that
874
follows is itself an octal digit.
876
The handling of a backslash followed by a digit other than 0 is complicated.
877
Outside a character class, PCRE reads it and any following digits as a decimal
878
number. If the number is less than 10, or if there have been at least that many
879
previous capturing left parentheses in the expression, the entire sequence is
880
taken as a \fIback reference\fR. A description of how this works is given
881
later, following the discussion of parenthesized subpatterns.
883
Inside a character class, or if the decimal number is greater than 9 and there
884
have not been that many capturing subpatterns, PCRE re-reads up to three octal
885
digits following the backslash, and generates a single byte from the least
886
significant 8 bits of the value. Any subsequent digits stand for themselves.
889
\\040 is another way of writing a space
890
\\40 is the same, provided there are fewer than 40
891
previous capturing subpatterns
892
\\7 is always a back reference
893
\\11 might be a back reference, or another way of
895
\\011 is always a tab
896
\\0113 is a tab followed by the character "3"
897
\\113 is the character with octal code 113 (since there
898
can be no more than 99 back references)
899
\\377 is a byte consisting entirely of 1 bits
900
\\81 is either a back reference, or a binary zero
901
followed by the two characters "8" and "1"
903
Note that octal values of 100 or greater must not be introduced by a leading
904
zero, because no more than three octal digits are ever read.
906
All the sequences that define a single byte value can be used both inside and
907
outside character classes. In addition, inside a character class, the sequence
908
"\\b" is interpreted as the backspace character (hex 08). Outside a character
909
class it has a different meaning (see below).
911
The third use of backslash is for specifying generic character types:
913
\\d any decimal digit
914
\\D any character that is not a decimal digit
915
\\s any whitespace character
916
\\S any character that is not a whitespace character
917
\\w any "word" character
918
\\W any "non-word" character
920
Each pair of escape sequences partitions the complete set of characters into
921
two disjoint sets. Any given character matches one, and only one, of each pair.
923
A "word" character is any letter or digit or the underscore character, that is,
924
any character which can be part of a Perl "word". The definition of letters and
925
digits is controlled by PCRE's character tables, and may vary if locale-
926
specific matching is taking place (see "Locale support" above). For example, in
927
the "fr" (French) locale, some character codes greater than 128 are used for
928
accented letters, and these are matched by \\w.
930
These character type sequences can appear both inside and outside character
931
classes. They each match one character of the appropriate type. If the current
932
matching point is at the end of the subject string, all of them fail, since
933
there is no character to match.
935
The fourth use of backslash is for certain simple assertions. An assertion
936
specifies a condition that has to be met at a particular point in a match,
937
without consuming any characters from the subject string. The use of
938
subpatterns for more complicated assertions is described below. The backslashed
942
\\B not a word boundary
943
\\A start of subject (independent of multiline mode)
944
\\Z end of subject or newline at end (independent of multiline mode)
945
\\z end of subject (independent of multiline mode)
947
These assertions may not appear in character classes (but note that "\\b" has a
948
different meaning, namely the backspace character, inside a character class).
950
A word boundary is a position in the subject string where the current character
951
and the previous character do not both match \\w or \\W (i.e. one matches
952
\\w and the other matches \\W), or the start or end of the string if the
953
first or last character matches \\w, respectively.
955
The \\A, \\Z, and \\z assertions differ from the traditional circumflex and
956
dollar (described below) in that they only ever match at the very start and end
957
of the subject string, whatever options are set. They are not affected by the
958
PCRE_NOTBOL or PCRE_NOTEOL options. If the \fIstartoffset\fR argument of
959
\fBpcre_exec()\fR is non-zero, \\A can never match. The difference between \\Z
960
and \\z is that \\Z matches before a newline that is the last character of the
961
string as well as at the end of the string, whereas \\z matches only at the
965
.SH CIRCUMFLEX AND DOLLAR
966
Outside a character class, in the default matching mode, the circumflex
967
character is an assertion which is true only if the current matching point is
968
at the start of the subject string. If the \fIstartoffset\fR argument of
969
\fBpcre_exec()\fR is non-zero, circumflex can never match. Inside a character
970
class, circumflex has an entirely different meaning (see below).
972
Circumflex need not be the first character of the pattern if a number of
973
alternatives are involved, but it should be the first thing in each alternative
974
in which it appears if the pattern is ever to match that branch. If all
975
possible alternatives start with a circumflex, that is, if the pattern is
976
constrained to match only at the start of the subject, it is said to be an
977
"anchored" pattern. (There are also other constructs that can cause a pattern
980
A dollar character is an assertion which is true only if the current matching
981
point is at the end of the subject string, or immediately before a newline
982
character that is the last character in the string (by default). Dollar need
983
not be the last character of the pattern if a number of alternatives are
984
involved, but it should be the last item in any branch in which it appears.
985
Dollar has no special meaning in a character class.
987
The meaning of dollar can be changed so that it matches only at the very end of
988
the string, by setting the PCRE_DOLLAR_ENDONLY option at compile or matching
989
time. This does not affect the \\Z assertion.
991
The meanings of the circumflex and dollar characters are changed if the
992
PCRE_MULTILINE option is set. When this is the case, they match immediately
993
after and immediately before an internal "\\n" character, respectively, in
994
addition to matching at the start and end of the subject string. For example,
995
the pattern /^abc$/ matches the subject string "def\\nabc" in multiline mode,
996
but not otherwise. Consequently, patterns that are anchored in single line mode
997
because all branches start with "^" are not anchored in multiline mode, and a
998
match for circumflex is possible when the \fIstartoffset\fR argument of
999
\fBpcre_exec()\fR is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
1000
PCRE_MULTILINE is set.
1002
Note that the sequences \\A, \\Z, and \\z can be used to match the start and
1003
end of the subject in both modes, and if all branches of a pattern start with
1004
\\A it is always anchored, whether PCRE_MULTILINE is set or not.
1007
.SH FULL STOP (PERIOD, DOT)
1008
Outside a character class, a dot in the pattern matches any one character in
1009
the subject, including a non-printing character, but not (by default) newline.
1010
If the PCRE_DOTALL option is set, dots match newlines as well. The handling of
1011
dot is entirely independent of the handling of circumflex and dollar, the only
1012
relationship being that they both involve newline characters. Dot has no
1013
special meaning in a character class.
1017
An opening square bracket introduces a character class, terminated by a closing
1018
square bracket. A closing square bracket on its own is not special. If a
1019
closing square bracket is required as a member of the class, it should be the
1020
first data character in the class (after an initial circumflex, if present) or
1021
escaped with a backslash.
1023
A character class matches a single character in the subject; the character must
1024
be in the set of characters defined by the class, unless the first character in
1025
the class is a circumflex, in which case the subject character must not be in
1026
the set defined by the class. If a circumflex is actually required as a member
1027
of the class, ensure it is not the first character, or escape it with a
1030
For example, the character class [aeiou] matches any lower case vowel, while
1031
[^aeiou] matches any character that is not a lower case vowel. Note that a
1032
circumflex is just a convenient notation for specifying the characters which
1033
are in the class by enumerating those that are not. It is not an assertion: it
1034
still consumes a character from the subject string, and fails if the current
1035
pointer is at the end of the string.
1037
When caseless matching is set, any letters in a class represent both their
1038
upper case and lower case versions, so for example, a caseless [aeiou] matches
1039
"A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1040
caseful version would.
1042
The newline character is never treated in any special way in character classes,
1043
whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class
1044
such as [^a] will always match a newline.
1046
The minus (hyphen) character can be used to specify a range of characters in a
1047
character class. For example, [d-m] matches any letter between d and m,
1048
inclusive. If a minus character is required in a class, it must be escaped with
1049
a backslash or appear in a position where it cannot be interpreted as
1050
indicating a range, typically as the first or last character in the class.
1052
It is not possible to have the literal character "]" as the end character of a
1053
range. A pattern such as [W-]46] is interpreted as a class of two characters
1054
("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1055
"-46]". However, if the "]" is escaped with a backslash it is interpreted as
1056
the end of range, so [W-\\]46] is interpreted as a single class containing a
1057
range followed by two separate characters. The octal or hexadecimal
1058
representation of "]" can also be used to end a range.
1060
Ranges operate in ASCII collating sequence. They can also be used for
1061
characters specified numerically, for example [\\000-\\037]. If a range that
1062
includes letters is used when caseless matching is set, it matches the letters
1063
in either case. For example, [W-c] is equivalent to [][\\^_`wxyzabc], matched
1064
caselessly, and if character tables for the "fr" locale are in use,
1065
[\\xc8-\\xcb] matches accented E characters in both cases.
1067
The character types \\d, \\D, \\s, \\S, \\w, and \\W may also appear in a
1068
character class, and add the characters that they match to the class. For
1069
example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can
1070
conveniently be used with the upper case character types to specify a more
1071
restricted set of characters than the matching lower case type. For example,
1072
the class [^\\W_] matches any letter or digit, but not underscore.
1074
All non-alphameric characters other than \\, -, ^ (at the start) and the
1075
terminating ] are non-special in character classes, but it does no harm if they
1079
.SH POSIX CHARACTER CLASSES
1080
Perl 5.6 (not yet released at the time of writing) is going to support the
1081
POSIX notation for character classes, which uses names enclosed by [: and :]
1082
within the enclosing square brackets. PCRE supports this notation. For example,
1086
matches "0", "1", any alphabetic character, or "%". The supported class names
1089
alnum letters and digits
1091
ascii character codes 0 - 127
1092
cntrl control characters
1093
digit decimal digits (same as \\d)
1094
graph printing characters, excluding space
1095
lower lower case letters
1096
print printing characters, including space
1097
punct printing characters, excluding letters and digits
1098
space white space (same as \\s)
1099
upper upper case letters
1100
word "word" characters (same as \\w)
1101
xdigit hexadecimal digits
1103
The names "ascii" and "word" are Perl extensions. Another Perl extension is
1104
negation, which is indicated by a ^ character after the colon. For example,
1108
matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1109
syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1110
supported, and an error is given if they are encountered.
1114
Vertical bar characters are used to separate alternative patterns. For example,
1119
matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1120
and an empty alternative is permitted (matching the empty string).
1121
The matching process tries each alternative in turn, from left to right,
1122
and the first one that succeeds is used. If the alternatives are within a
1123
subpattern (defined below), "succeeds" means matching the rest of the main
1124
pattern as well as the alternative in the subpattern.
1127
.SH INTERNAL OPTION SETTING
1128
The settings of PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and PCRE_EXTENDED
1129
can be changed from within the pattern by a sequence of Perl option letters
1130
enclosed between "(?" and ")". The option letters are
1133
m for PCRE_MULTILINE
1137
For example, (?im) sets caseless, multiline matching. It is also possible to
1138
unset these options by preceding the letter with a hyphen, and a combined
1139
setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1140
PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1141
permitted. If a letter appears both before and after the hyphen, the option is
1144
The scope of these option changes depends on where in the pattern the setting
1145
occurs. For settings that are outside any subpattern (defined below), the
1146
effect is the same as if the options were set or unset at the start of
1147
matching. The following patterns all behave in exactly the same way:
1154
which in turn is the same as compiling the pattern abc with PCRE_CASELESS set.
1155
In other words, such "top level" settings apply to the whole pattern (unless
1156
there are other changes inside subpatterns). If there is more than one setting
1157
of the same option at top level, the rightmost setting is used.
1159
If an option change occurs inside a subpattern, the effect is different. This
1160
is a change of behaviour in Perl 5.005. An option change inside a subpattern
1161
affects only that part of the subpattern that follows it, so
1165
matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1166
By this means, options can be made to have different settings in different
1167
parts of the pattern. Any changes made in one alternative do carry on
1168
into subsequent branches within the same subpattern. For example,
1172
matches "ab", "aB", "c", and "C", even though when matching "C" the first
1173
branch is abandoned before the option setting. This is because the effects of
1174
option settings happen at compile time. There would be some very weird
1175
behaviour otherwise.
1177
The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the
1178
same way as the Perl-compatible options by using the characters U and X
1179
respectively. The (?X) flag setting is special in that it must always occur
1180
earlier in the pattern than any of the additional features it turns on, even
1181
when it is at top level. It is best put at the start.
1185
Subpatterns are delimited by parentheses (round brackets), which can be nested.
1186
Marking part of a pattern as a subpattern does two things:
1188
1. It localizes a set of alternatives. For example, the pattern
1190
cat(aract|erpillar|)
1192
matches one of the words "cat", "cataract", or "caterpillar". Without the
1193
parentheses, it would match "cataract", "erpillar" or the empty string.
1195
2. It sets up the subpattern as a capturing subpattern (as defined above).
1196
When the whole pattern matches, that portion of the subject string that matched
1197
the subpattern is passed back to the caller via the \fIovector\fR argument of
1198
\fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting
1199
from 1) to obtain the numbers of the capturing subpatterns.
1201
For example, if the string "the red king" is matched against the pattern
1203
the ((red|white) (king|queen))
1205
the captured substrings are "red king", "red", and "king", and are numbered 1,
1206
2, and 3, respectively.
1208
The fact that plain parentheses fulfil two functions is not always helpful.
1209
There are often times when a grouping subpattern is required without a
1210
capturing requirement. If an opening parenthesis is followed by "?:", the
1211
subpattern does not do any capturing, and is not counted when computing the
1212
number of any subsequent capturing subpatterns. For example, if the string "the
1213
white queen" is matched against the pattern
1215
the ((?:red|white) (king|queen))
1217
the captured substrings are "white queen" and "queen", and are numbered 1 and
1218
2. The maximum number of captured substrings is 99, and the maximum number of
1219
all subpatterns, both capturing and non-capturing, is 200.
1221
As a convenient shorthand, if any option settings are required at the start of
1222
a non-capturing subpattern, the option letters may appear between the "?" and
1223
the ":". Thus the two patterns
1225
(?i:saturday|sunday)
1226
(?:(?i)saturday|sunday)
1228
match exactly the same set of strings. Because alternative branches are tried
1229
from left to right, and options are not reset until the end of the subpattern
1230
is reached, an option setting in one branch does affect subsequent branches, so
1231
the above patterns match "SUNDAY" as well as "Saturday".
1235
Repetition is specified by quantifiers, which can follow any of the following
1238
a single character, possibly escaped
1241
a back reference (see next section)
1242
a parenthesized subpattern (unless it is an assertion - see below)
1244
The general repetition quantifier specifies a minimum and maximum number of
1245
permitted matches, by giving the two numbers in curly brackets (braces),
1246
separated by a comma. The numbers must be less than 65536, and the first must
1247
be less than or equal to the second. For example:
1251
matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1252
character. If the second number is omitted, but the comma is present, there is
1253
no upper limit; if the second number and the comma are both omitted, the
1254
quantifier specifies an exact number of required matches. Thus
1258
matches at least 3 successive vowels, but may match many more, while
1262
matches exactly 8 digits. An opening curly bracket that appears in a position
1263
where a quantifier is not allowed, or one that does not match the syntax of a
1264
quantifier, is taken as a literal character. For example, {,6} is not a
1265
quantifier, but a literal string of four characters.
1267
The quantifier {0} is permitted, causing the expression to behave as if the
1268
previous item and the quantifier were not present.
1270
For convenience (and historical compatibility) the three most common
1271
quantifiers have single-character abbreviations:
1273
* is equivalent to {0,}
1274
+ is equivalent to {1,}
1275
? is equivalent to {0,1}
1277
It is possible to construct infinite loops by following a subpattern that can
1278
match no characters with a quantifier that has no upper limit, for example:
1282
Earlier versions of Perl and PCRE used to give an error at compile time for
1283
such patterns. However, because there are cases where this can be useful, such
1284
patterns are now accepted, but if any repetition of the subpattern does in fact
1285
match no characters, the loop is forcibly broken.
1287
By default, the quantifiers are "greedy", that is, they match as much as
1288
possible (up to the maximum number of permitted times), without causing the
1289
rest of the pattern to fail. The classic example of where this gives problems
1290
is in trying to match comments in C programs. These appear between the
1291
sequences /* and */ and within the sequence, individual * and / characters may
1292
appear. An attempt to match C comments by applying the pattern
1298
/* first command */ not comment /* second comment */
1300
fails, because it matches the entire string owing to the greediness of the .*
1303
However, if a quantifier is followed by a question mark, it ceases to be
1304
greedy, and instead matches the minimum number of times possible, so the
1309
does the right thing with the C comments. The meaning of the various
1310
quantifiers is not otherwise changed, just the preferred number of matches.
1311
Do not confuse this use of question mark with its use as a quantifier in its
1312
own right. Because it has two uses, it can sometimes appear doubled, as in
1316
which matches one digit by preference, but can match two if that is the only
1317
way the rest of the pattern matches.
1319
If the PCRE_UNGREEDY option is set (an option which is not available in Perl),
1320
the quantifiers are not greedy by default, but individual ones can be made
1321
greedy by following them with a question mark. In other words, it inverts the
1324
When a parenthesized subpattern is quantified with a minimum repeat count that
1325
is greater than 1 or with a limited maximum, more store is required for the
1326
compiled pattern, in proportion to the size of the minimum or maximum.
1328
If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1329
to Perl's /s) is set, thus allowing the . to match newlines, the pattern is
1330
implicitly anchored, because whatever follows will be tried against every
1331
character position in the subject string, so there is no point in retrying the
1332
overall match at any position after the first. PCRE treats such a pattern as
1333
though it were preceded by \\A. In cases where it is known that the subject
1334
string contains no newlines, it is worth setting PCRE_DOTALL when the pattern
1335
begins with .* in order to obtain this optimization, or alternatively using ^
1336
to indicate anchoring explicitly.
1338
When a capturing subpattern is repeated, the value captured is the substring
1339
that matched the final iteration. For example, after
1341
(tweedle[dume]{3}\\s*)+
1343
has matched "tweedledum tweedledee" the value of the captured substring is
1344
"tweedledee". However, if there are nested capturing subpatterns, the
1345
corresponding captured values may have been set in previous iterations. For
1350
matches "aba" the value of the second captured substring is "b".
1354
Outside a character class, a backslash followed by a digit greater than 0 (and
1355
possibly further digits) is a back reference to a capturing subpattern earlier
1356
(i.e. to its left) in the pattern, provided there have been that many previous
1357
capturing left parentheses.
1359
However, if the decimal number following the backslash is less than 10, it is
1360
always taken as a back reference, and causes an error only if there are not
1361
that many capturing left parentheses in the entire pattern. In other words, the
1362
parentheses that are referenced need not be to the left of the reference for
1363
numbers less than 10. See the section entitled "Backslash" above for further
1364
details of the handling of digits following a backslash.
1366
A back reference matches whatever actually matched the capturing subpattern in
1367
the current subject string, rather than anything matching the subpattern
1368
itself. So the pattern
1370
(sens|respons)e and \\1ibility
1372
matches "sense and sensibility" and "response and responsibility", but not
1373
"sense and responsibility". If caseful matching is in force at the time of the
1374
back reference, the case of letters is relevant. For example,
1378
matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1379
capturing subpattern is matched caselessly.
1381
There may be more than one back reference to the same subpattern. If a
1382
subpattern has not actually been used in a particular match, any back
1383
references to it always fail. For example, the pattern
1387
always fails if it starts to match "a" rather than "bc". Because there may be
1388
up to 99 back references, all digits following the backslash are taken
1389
as part of a potential back reference number. If the pattern continues with a
1390
digit character, some delimiter must be used to terminate the back reference.
1391
If the PCRE_EXTENDED option is set, this can be whitespace. Otherwise an empty
1392
comment can be used.
1394
A back reference that occurs inside the parentheses to which it refers fails
1395
when the subpattern is first used, so, for example, (a\\1) never matches.
1396
However, such references can be useful inside repeated subpatterns. For
1397
example, the pattern
1401
matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1402
the subpattern, the back reference matches the character string corresponding
1403
to the previous iteration. In order for this to work, the pattern must be such
1404
that the first iteration does not need to match the back reference. This can be
1405
done using alternation, as in the example above, or by a quantifier with a
1410
An assertion is a test on the characters following or preceding the current
1411
matching point that does not actually consume any characters. The simple
1412
assertions coded as \\b, \\B, \\A, \\Z, \\z, ^ and $ are described above. More
1413
complicated assertions are coded as subpatterns. There are two kinds: those
1414
that look ahead of the current position in the subject string, and those that
1417
An assertion subpattern is matched in the normal way, except that it does not
1418
cause the current matching position to be changed. Lookahead assertions start
1419
with (?= for positive assertions and (?! for negative assertions. For example,
1423
matches a word followed by a semicolon, but does not include the semicolon in
1428
matches any occurrence of "foo" that is not followed by "bar". Note that the
1429
apparently similar pattern
1433
does not find an occurrence of "bar" that is preceded by something other than
1434
"foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1435
(?!foo) is always true when the next three characters are "bar". A
1436
lookbehind assertion is needed to achieve this effect.
1438
Lookbehind assertions start with (?<= for positive assertions and (?<! for
1439
negative assertions. For example,
1443
does find an occurrence of "bar" that is not preceded by "foo". The contents of
1444
a lookbehind assertion are restricted such that all the strings it matches must
1445
have a fixed length. However, if there are several alternatives, they do not
1446
all have to have the same fixed length. Thus
1454
causes an error at compile time. Branches that match different length strings
1455
are permitted only at the top level of a lookbehind assertion. This is an
1456
extension compared with Perl 5.005, which requires all branches to match the
1457
same length of string. An assertion such as
1461
is not permitted, because its single top-level branch can match two different
1462
lengths, but it is acceptable if rewritten to use two top-level branches:
1466
The implementation of lookbehind assertions is, for each alternative, to
1467
temporarily move the current position back by the fixed width and then try to
1468
match. If there are insufficient characters before the current position, the
1469
match is deemed to fail. Lookbehinds in conjunction with once-only subpatterns
1470
can be particularly useful for matching at the ends of strings; an example is
1471
given at the end of the section on once-only subpatterns.
1473
Several assertions (of any sort) may occur in succession. For example,
1475
(?<=\\d{3})(?<!999)foo
1477
matches "foo" preceded by three digits that are not "999". Notice that each of
1478
the assertions is applied independently at the same point in the subject
1479
string. First there is a check that the previous three characters are all
1480
digits, and then there is a check that the same three characters are not "999".
1481
This pattern does \fInot\fR match "foo" preceded by six characters, the first
1482
of which are digits and the last three of which are not "999". For example, it
1483
doesn't match "123abcfoo". A pattern to do that is
1485
(?<=\\d{3}...)(?<!999)foo
1487
This time the first assertion looks at the preceding six characters, checking
1488
that the first three are digits, and then the second assertion checks that the
1489
preceding three characters are not "999".
1491
Assertions can be nested in any combination. For example,
1495
matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1496
preceded by "foo", while
1498
(?<=\\d{3}(?!999)...)foo
1500
is another pattern which matches "foo" preceded by three digits and any three
1501
characters that are not "999".
1503
Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1504
because it makes no sense to assert the same thing several times. If any kind
1505
of assertion contains capturing subpatterns within it, these are counted for
1506
the purposes of numbering the capturing subpatterns in the whole pattern.
1507
However, substring capturing is carried out only for positive assertions,
1508
because it does not make sense for negative assertions.
1510
Assertions count towards the maximum of 200 parenthesized subpatterns.
1513
.SH ONCE-ONLY SUBPATTERNS
1514
With both maximizing and minimizing repetition, failure of what follows
1515
normally causes the repeated item to be re-evaluated to see if a different
1516
number of repeats allows the rest of the pattern to match. Sometimes it is
1517
useful to prevent this, either to change the nature of the match, or to cause
1518
it fail earlier than it otherwise might, when the author of the pattern knows
1519
there is no point in carrying on.
1521
Consider, for example, the pattern \\d+foo when applied to the subject line
1525
After matching all 6 digits and then failing to match "foo", the normal
1526
action of the matcher is to try again with only 5 digits matching the \\d+
1527
item, and then with 4, and so on, before ultimately failing. Once-only
1528
subpatterns provide the means for specifying that once a portion of the pattern
1529
has matched, it is not to be re-evaluated in this way, so the matcher would
1530
give up immediately on failing to match "foo" the first time. The notation is
1531
another kind of special parenthesis, starting with (?> as in this example:
1535
This kind of parenthesis "locks up" the part of the pattern it contains once
1536
it has matched, and a failure further into the pattern is prevented from
1537
backtracking into it. Backtracking past it to previous items, however, works as
1540
An alternative description is that a subpattern of this type matches the string
1541
of characters that an identical standalone pattern would match, if anchored at
1542
the current point in the subject string.
1544
Once-only subpatterns are not capturing subpatterns. Simple cases such as the
1545
above example can be thought of as a maximizing repeat that must swallow
1546
everything it can. So, while both \\d+ and \\d+? are prepared to adjust the
1547
number of digits they match in order to make the rest of the pattern match,
1548
(?>\\d+) can only match an entire sequence of digits.
1550
This construction can of course contain arbitrarily complicated subpatterns,
1551
and it can be nested.
1553
Once-only subpatterns can be used in conjunction with lookbehind assertions to
1554
specify efficient matching at the end of the subject string. Consider a simple
1559
when applied to a long string which does not match. Because matching proceeds
1560
from left to right, PCRE will look for each "a" in the subject and then see if
1561
what follows matches the rest of the pattern. If the pattern is specified as
1565
the initial .* matches the entire string at first, but when this fails (because
1566
there is no following "a"), it backtracks to match all but the last character,
1567
then all but the last two characters, and so on. Once again the search for "a"
1568
covers the entire string, from right to left, so we are no better off. However,
1569
if the pattern is written as
1573
there can be no backtracking for the .* item; it can match only the entire
1574
string. The subsequent lookbehind assertion does a single test on the last four
1575
characters. If it fails, the match fails immediately. For long strings, this
1576
approach makes a significant difference to the processing time.
1578
When a pattern contains an unlimited repeat inside a subpattern that can itself
1579
be repeated an unlimited number of times, the use of a once-only subpattern is
1580
the only way to avoid some failing matches taking a very long time indeed.
1585
matches an unlimited number of substrings that either consist of non-digits, or
1586
digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1587
quickly. However, if it is applied to
1589
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1591
it takes a long time before reporting failure. This is because the string can
1592
be divided between the two repeats in a large number of ways, and all have to
1593
be tried. (The example used [!?] rather than a single character at the end,
1594
because both PCRE and Perl have an optimization that allows for fast failure
1595
when a single character is used. They remember the last single character that
1596
is required for a match, and fail early if it is not present in the string.)
1597
If the pattern is changed to
1599
((?>\\D+)|<\\d+>)*[!?]
1601
sequences of non-digits cannot be broken, and failure happens quickly.
1604
.SH CONDITIONAL SUBPATTERNS
1605
It is possible to cause the matching process to obey a subpattern
1606
conditionally or to choose between two alternative subpatterns, depending on
1607
the result of an assertion, or whether a previous capturing subpattern matched
1608
or not. The two possible forms of conditional subpattern are
1610
(?(condition)yes-pattern)
1611
(?(condition)yes-pattern|no-pattern)
1613
If the condition is satisfied, the yes-pattern is used; otherwise the
1614
no-pattern (if present) is used. If there are more than two alternatives in the
1615
subpattern, a compile-time error occurs.
1617
There are two kinds of condition. If the text between the parentheses consists
1618
of a sequence of digits, the condition is satisfied if the capturing subpattern
1619
of that number has previously matched. The number must be greater than zero.
1620
Consider the following pattern, which contains non-significant white space to
1621
make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1622
three parts for ease of discussion:
1624
( \\( )? [^()]+ (?(1) \\) )
1626
The first part matches an optional opening parenthesis, and if that
1627
character is present, sets it as the first captured substring. The second part
1628
matches one or more characters that are not parentheses. The third part is a
1629
conditional subpattern that tests whether the first set of parentheses matched
1630
or not. If they did, that is, if subject started with an opening parenthesis,
1631
the condition is true, and so the yes-pattern is executed and a closing
1632
parenthesis is required. Otherwise, since no-pattern is not present, the
1633
subpattern matches nothing. In other words, this pattern matches a sequence of
1634
non-parentheses, optionally enclosed in parentheses.
1636
If the condition is not a sequence of digits, it must be an assertion. This may
1637
be a positive or negative lookahead or lookbehind assertion. Consider this
1638
pattern, again containing non-significant white space, and with the two
1639
alternatives on the second line:
1642
\\d{2}-[a-z]{3}-\\d{2} | \\d{2}-\\d{2}-\\d{2} )
1644
The condition is a positive lookahead assertion that matches an optional
1645
sequence of non-letters followed by a letter. In other words, it tests for the
1646
presence of at least one letter in the subject. If a letter is found, the
1647
subject is matched against the first alternative; otherwise it is matched
1648
against the second. This pattern matches strings in one of the two forms
1649
dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1653
The sequence (?# marks the start of a comment which continues up to the next
1654
closing parenthesis. Nested parentheses are not permitted. The characters
1655
that make up a comment play no part in the pattern matching at all.
1657
If the PCRE_EXTENDED option is set, an unescaped # character outside a
1658
character class introduces a comment that continues up to the next newline
1659
character in the pattern.
1662
.SH RECURSIVE PATTERNS
1663
Consider the problem of matching a string in parentheses, allowing for
1664
unlimited nested parentheses. Without the use of recursion, the best that can
1665
be done is to use a pattern that matches up to some fixed depth of nesting. It
1666
is not possible to handle an arbitrary nesting depth. Perl 5.6 has provided an
1667
experimental facility that allows regular expressions to recurse (amongst other
1668
things). It does this by interpolating Perl code in the expression at run time,
1669
and the code can refer to the expression itself. A Perl pattern to solve the
1670
parentheses problem can be created like this:
1672
$re = qr{\\( (?: (?>[^()]+) | (?p{$re}) )* \\)}x;
1674
The (?p{...}) item interpolates Perl code at run time, and in this case refers
1675
recursively to the pattern in which it appears. Obviously, PCRE cannot support
1676
the interpolation of Perl code. Instead, the special item (?R) is provided for
1677
the specific case of recursion. This PCRE pattern solves the parentheses
1678
problem (assume the PCRE_EXTENDED option is set so that white space is
1681
\\( ( (?>[^()]+) | (?R) )* \\)
1683
First it matches an opening parenthesis. Then it matches any number of
1684
substrings which can either be a sequence of non-parentheses, or a recursive
1685
match of the pattern itself (i.e. a correctly parenthesized substring). Finally
1686
there is a closing parenthesis.
1688
This particular example pattern contains nested unlimited repeats, and so the
1689
use of a once-only subpattern for matching strings of non-parentheses is
1690
important when applying the pattern to strings that do not match. For example,
1691
when it is applied to
1693
(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1695
it yields "no match" quickly. However, if a once-only subpattern is not used,
1696
the match runs for a very long time indeed because there are so many different
1697
ways the + and * repeats can carve up the subject, and all have to be tested
1698
before failure can be reported.
1700
The values set for any capturing subpatterns are those from the outermost level
1701
of the recursion at which the subpattern value is set. If the pattern above is
1706
the value for the capturing parentheses is "ef", which is the last value taken
1707
on at the top level. If additional parentheses are added, giving
1709
\\( ( ( (?>[^()]+) | (?R) )* ) \\)
1712
the string they capture is "ab(cd)ef", the contents of the top level
1713
parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
1714
has to obtain extra memory to store data during a recursion, which it does by
1715
using \fBpcre_malloc\fR, freeing it via \fBpcre_free\fR afterwards. If no
1716
memory can be obtained, it saves data for the first 15 capturing parentheses
1717
only, as there is no way to give an out-of-memory error from within a
1722
Certain items that may appear in patterns are more efficient than others. It is
1723
more efficient to use a character class like [aeiou] than a set of alternatives
1724
such as (a|e|i|o|u). In general, the simplest construction that provides the
1725
required behaviour is usually the most efficient. Jeffrey Friedl's book
1726
contains a lot of discussion about optimizing regular expressions for efficient
1729
When a pattern begins with .* and the PCRE_DOTALL option is set, the pattern is
1730
implicitly anchored by PCRE, since it can match only at the start of a subject
1731
string. However, if PCRE_DOTALL is not set, PCRE cannot make this optimization,
1732
because the . metacharacter does not then match a newline, and if the subject
1733
string contains newlines, the pattern may match from the character immediately
1734
following one of them instead of from the very start. For example, the pattern
1738
matches the subject "first\\nand second" (where \\n stands for a newline
1739
character) with the first captured substring being "and". In order to do this,
1740
PCRE has to retry the match starting after every newline in the subject.
1742
If you are using such a pattern with subject strings that do not contain
1743
newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
1744
the pattern with ^.* to indicate explicit anchoring. That saves PCRE from
1745
having to scan along the subject looking for a newline to restart at.
1747
Beware of patterns that contain nested indefinite repeats. These can take a
1748
long time to run when applied to a string that does not match. Consider the
1753
This can match "aaaa" in 33 different ways, and this number increases very
1754
rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4
1755
times, and for each of those cases other than 0, the + repeats can match
1756
different numbers of times.) When the remainder of the pattern is such that the
1757
entire match is going to fail, PCRE has in principle to try every possible
1758
variation, and this can take an extremely long time.
1760
An optimization catches some of the more simple cases such as
1764
where a literal character follows. Before embarking on the standard matching
1765
procedure, PCRE checks that there is a "b" later in the subject string, and if
1766
there is not, it fails the match immediately. However, when there is no
1767
following literal this optimization cannot be used. You can see the difference
1768
by comparing the behaviour of
1772
with the pattern above. The former gives a failure almost instantly when
1773
applied to a whole line of "a" characters, whereas the latter takes an
1774
appreciable time with strings longer than about 20 characters.
1778
Starting at release 3.3, PCRE has some support for character strings encoded
1779
in the UTF-8 format. This is incomplete, and is regarded as experimental. In
1780
order to use it, you must configure PCRE to include UTF-8 support in the code,
1781
and, in addition, you must call \fBpcre_compile()\fR with the PCRE_UTF8 option
1782
flag. When you do this, both the pattern and any subject strings that are
1783
matched against it are treated as UTF-8 strings instead of just strings of
1784
bytes, but only in the cases that are mentioned below.
1786
If you compile PCRE with UTF-8 support, but do not use it at run time, the
1787
library will be a bit bigger, but the additional run time overhead is limited
1788
to testing the PCRE_UTF8 flag in several places, so should not be very large.
1790
PCRE assumes that the strings it is given contain valid UTF-8 codes. It does
1791
not diagnose invalid UTF-8 strings. If you pass invalid UTF-8 strings to PCRE,
1792
the results are undefined.
1794
Running with PCRE_UTF8 set causes these changes in the way PCRE works:
1796
1. In a pattern, the escape sequence \\x{...}, where the contents of the braces
1797
is a string of hexadecimal digits, is interpreted as a UTF-8 character whose
1798
code number is the given hexadecimal number, for example: \\x{1234}. This
1799
inserts from one to six literal bytes into the pattern, using the UTF-8
1800
encoding. If a non-hexadecimal digit appears between the braces, the item is
1803
2. The original hexadecimal escape sequence, \\xhh, generates a two-byte UTF-8
1804
character if its value is greater than 127.
1806
3. Repeat quantifiers are NOT correctly handled if they follow a multibyte
1807
character. For example, \\x{100}* and \\xc3+ do not work. If you want to
1808
repeat such characters, you must enclose them in non-capturing parentheses,
1809
for example (?:\\x{100}), at present.
1811
4. The dot metacharacter matches one UTF-8 character instead of a single byte.
1813
5. Unlike literal UTF-8 characters, the dot metacharacter followed by a
1814
repeat quantifier does operate correctly on UTF-8 characters instead of
1817
4. Although the \\x{...} escape is permitted in a character class, characters
1818
whose values are greater than 255 cannot be included in a class.
1820
5. A class is matched against a UTF-8 character instead of just a single byte,
1821
but it can match only characters whose values are less than 256. Characters
1822
with greater values always fail to match a class.
1824
6. Repeated classes work correctly on multiple characters.
1826
7. Classes containing just a single character whose value is greater than 127
1827
(but less than 256), for example, [\\x80] or [^\\x{93}], do not work because
1828
these are optimized into single byte matches. In the first case, of course,
1829
the class brackets are just redundant.
1831
8. Lookbehind assertions move backwards in the subject by a fixed number of
1832
characters instead of a fixed number of bytes. Simple cases have been tested
1833
to work correctly, but there may be hidden gotchas herein.
1835
9. The character types such as \\d and \\w do not work correctly with UTF-8
1836
characters. They continue to test a single byte.
1838
10. Anything not explicitly mentioned here continues to work in bytes rather
1841
The following UTF-8 features of Perl 5.6 are not implemented:
1843
1. The escape sequence \\C to match a single byte.
1845
2. The use of Unicode tables and properties and escapes \\p, \\P, and \\X.
1849
The code below is a simple, complete demonstration program, to get you started
1850
with using PCRE. This code is also supplied in the file \fIpcredemo.c\fR in the
1853
The program compiles the regular expression that is its first argument, and
1854
matches it against the subject string in its second argument. No options are
1855
set, and default character tables are used. If matching succeeds, the program
1856
outputs the portion of the subject that matched, together with the contents of
1857
any captured substrings.
1859
On a Unix system that has PCRE installed in \fI/usr/local\fR, you can compile
1860
the demonstration program using a command like this:
1862
gcc -o pcredemo pcredemo.c -I/usr/local/include -L/usr/local/lib -lpcre
1864
Then you can run simple tests like this:
1866
./pcredemo 'cat|dog' 'the cat sat on the mat'
1868
Note that there is a much more comprehensive test program, called
1869
\fBpcretest\fR, which supports many more facilities for testing regular
1870
expressions. The \fBpcredemo\fR program is provided as a simple coding example.
1872
On some operating systems (e.g. Solaris) you may get an error like this when
1873
you try to run \fBpcredemo\fR:
1875
ld.so.1: a.out: fatal: libpcre.so.0: open failed: No such file or directory
1877
This is caused by the way shared library support works on those systems. You
1882
to the compile command to get round this problem. Here's the code:
1888
#define OVECCOUNT 30 /* should be a multiple of 3 */
1890
int main(int argc, char **argv)
1895
int ovector[OVECCOUNT];
1900
printf("Two arguments required: a regex and a "
1901
"subject string\\n");
1905
/* Compile the regular expression in the first argument */
1908
argv[1], /* the pattern */
1909
0, /* default options */
1910
&error, /* for error message */
1911
&erroffset, /* for error offset */
1912
NULL); /* use default character tables */
1914
/* Compilation failed: print the error message and exit */
1918
printf("PCRE compilation failed at offset %d: %s\\n",
1923
/* Compilation succeeded: match the subject in the second
1927
re, /* the compiled pattern */
1928
NULL, /* we didn't study the pattern */
1929
argv[2], /* the subject string */
1930
(int)strlen(argv[2]), /* the length of the subject */
1931
0, /* start at offset 0 in the subject */
1932
0, /* default options */
1933
ovector, /* vector for substring information */
1934
OVECCOUNT); /* number of elements in the vector */
1936
/* Matching failed: handle error cases */
1942
case PCRE_ERROR_NOMATCH: printf("No match\\n"); break;
1944
Handle other special cases if you like
1946
default: printf("Matching error %d\\n", rc); break;
1951
/* Match succeded */
1953
printf("Match succeeded\\n");
1955
/* The output vector wasn't big enough */
1960
printf("ovector only has room for %d captured "
1961
substrings\\n", rc - 1);
1964
/* Show substrings stored in the output vector */
1966
for (i = 0; i < rc; i++)
1968
char *substring_start = argv[2] + ovector[2*i];
1969
int substring_length = ovector[2*i+1] - ovector[2*i];
1970
printf("%2d: %.*s\\n", i, substring_length,
1979
Philip Hazel <ph10@cam.ac.uk>
1981
University Computing Service,
1985
Cambridge CB2 3QG, England.
1987
Phone: +44 1223 334714
1989
Last updated: 15 August 2001
1991
Copyright (c) 1997-2001 University of Cambridge.
added
removed
regexp/doc/pcre.3
