3
<title>pcre specification</title>
5
<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6
This HTML document has been generated automatically from the original man page.
7
If there is any nonsense in it, please consult the man page, in case the
8
conversion went wrong.<br>
10
<li><a name="TOC1" href="#SEC1">DESCRIPTION</a>
11
<li><a name="TOC2" href="#SEC2">USER DOCUMENTATION</a>
12
<li><a name="TOC3" href="#SEC3">LIMITATIONS</a>
13
<li><a name="TOC4" href="#SEC4">UTF-8 SUPPORT</a>
14
<li><a name="TOC5" href="#SEC5">AUTHOR</a>
16
<br><a name="SEC1" href="#TOC1">DESCRIPTION</a><br>
18
The PCRE library is a set of functions that implement regular expression
19
pattern matching using the same syntax and semantics as Perl, with just a few
20
differences. The current implementation of PCRE (release 4.x) corresponds
21
approximately with Perl 5.8, including support for UTF-8 encoded strings.
22
However, this support has to be explicitly enabled; it is not the default.
25
PCRE is written in C and released as a C library. However, a number of people
26
have written wrappers and interfaces of various kinds. A C++ class is included
27
in these contributions, which can be found in the <i>Contrib</i> directory at
28
the primary FTP site, which is:
30
<a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>
32
Details of exactly which Perl regular expression features are and are not
33
supported by PCRE are given in separate documents. See the
34
<a href="pcrepattern.html"><b>pcrepattern</b></a>
36
<a href="pcrecompat.html"><b>pcrecompat</b></a>
40
Some features of PCRE can be included, excluded, or changed when the library is
42
<a href="pcre_config.html"><b>pcre_config()</b></a>
43
function makes it possible for a client to discover which features are
44
available. Documentation about building PCRE for various operating systems can
45
be found in the <b>README</b> file in the source distribution.
47
<br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
49
The user documentation for PCRE has been split up into a number of different
50
sections. In the "man" format, each of these is a separate "man page". In the
51
HTML format, each is a separate page, linked from the index page. In the plain
52
text format, all the sections are concatenated, for ease of searching. The
53
sections are as follows:
58
pcreapi details of PCRE's native API
59
pcrebuild options for building PCRE
60
pcrecallout details of the callout feature
61
pcrecompat discussion of Perl compatibility
62
pcregrep description of the <b>pcregrep</b> command
63
pcrepattern syntax and semantics of supported
65
pcreperform discussion of performance issues
66
pcreposix the POSIX-compatible API
67
pcresample discussion of the sample program
68
pcretest the <b>pcretest</b> testing command
72
In addition, in the "man" and HTML formats, there is a short page for each
73
library function, listing its arguments and results.
75
<br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
77
There are some size limitations in PCRE but it is hoped that they will never in
81
The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
82
compiled with the default internal linkage size of 2. If you want to process
83
regular expressions that are truly enormous, you can compile PCRE with an
84
internal linkage size of 3 or 4 (see the <b>README</b> file in the source
86
<a href="pcrebuild.html"><b>pcrebuild</b></a>
87
documentation for details). If these cases the limit is substantially larger.
88
However, the speed of execution will be slower.
91
All values in repeating quantifiers must be less than 65536.
92
The maximum number of capturing subpatterns is 65535.
95
There is no limit to the number of non-capturing subpatterns, but the maximum
96
depth of nesting of all kinds of parenthesized subpattern, including capturing
97
subpatterns, assertions, and other types of subpattern, is 200.
100
The maximum length of a subject string is the largest positive number that an
101
integer variable can hold. However, PCRE uses recursion to handle subpatterns
102
and indefinite repetition. This means that the available stack space may limit
103
the size of a subject string that can be processed by certain patterns.
105
<a name="utf8support"></a><br><a name="SEC4" href="#TOC1">UTF-8 SUPPORT</a><br>
107
Starting at release 3.3, PCRE has had some support for character strings
108
encoded in the UTF-8 format. For release 4.0 this has been greatly extended to
109
cover most common requirements.
112
In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
113
the code, and, in addition, you must call
114
<a href="pcre_compile.html"><b>pcre_compile()</b></a>
115
with the PCRE_UTF8 option flag. When you do this, both the pattern and any
116
subject strings that are matched against it are treated as UTF-8 strings
117
instead of just strings of bytes.
120
If you compile PCRE with UTF-8 support, but do not use it at run time, the
121
library will be a bit bigger, but the additional run time overhead is limited
122
to testing the PCRE_UTF8 flag in several places, so should not be very large.
125
The following comments apply when PCRE is running in UTF-8 mode:
128
1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
129
are checked for validity on entry to the relevant functions. If an invalid
130
UTF-8 string is passed, an error return is given. In some situations, you may
131
already know that your strings are valid, and therefore want to skip these
132
checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
133
at compile time or at run time, PCRE assumes that the pattern or subject it
134
is given (respectively) contains only valid UTF-8 codes. In this case, it does
135
not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
136
PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
140
2. In a pattern, the escape sequence \x{...}, where the contents of the braces
141
is a string of hexadecimal digits, is interpreted as a UTF-8 character whose
142
code number is the given hexadecimal number, for example: \x{1234}. If a
143
non-hexadecimal digit appears between the braces, the item is not recognized.
144
This escape sequence can be used either as a literal, or within a character
148
3. The original hexadecimal escape sequence, \xhh, matches a two-byte UTF-8
149
character if the value is greater than 127.
152
4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
153
bytes, for example: \x{100}{3}.
156
5. The dot metacharacter matches one UTF-8 character instead of a single byte.
159
6. The escape sequence \C can be used to match a single byte in UTF-8 mode,
160
but its use can lead to some strange effects.
163
7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
164
test characters of any code value, but the characters that PCRE recognizes as
165
digits, spaces, or word characters remain the same set as before, all with
166
values less than 256.
169
8. Case-insensitive matching applies only to characters whose values are less
170
than 256. PCRE does not support the notion of "case" for higher-valued
174
9. PCRE does not support the use of Unicode tables and properties or the Perl
175
escapes \p, \P, and \X.
177
<br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
179
Philip Hazel <ph10@cam.ac.uk>
181
University Computing Service,
183
Cambridge CB2 3QG, England.
185
Phone: +44 1223 334714
188
Last updated: 20 August 2003
190
Copyright © 1997-2003 University of Cambridge.
added
removed
doc/html/pcre.html
