1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
|
<!-- manual page source format generated by PolyglotMan v3.2, -->
<!-- available at http://polyglotman.sourceforge.net/ -->
<html>
<head>
<title>PCRE(3) manual page</title>
</head>
<body bgcolor='white'>
<a href='#toc'>Table of Contents</a><p>
<h2><a name='sect0' href='#toc0'>Name</a></h2>
PCRE - Perl-compatible regular expressions
<h2><a name='sect1' href='#toc1'>Introduction</a></h2>
<p>
The PCRE library
is a set of functions that implement regular expression pattern matching
using the same syntax and semantics as Perl, with just a few differences.
The current implementation of PCRE (release 5.x) corresponds approximately
with Perl 5.8, including support for UTF-8 encoded strings and Unicode general
category properties. However, this support has to be explicitly enabled;
it is not the default. <p>
PCRE is written in C and released as a C library.
A number of people have written wrappers and interfaces of various kinds.
A C++ class is included in these contributions, which can be found in the
<i>Contrib</i> directory at the primary FTP site, which is: <p>
ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
<p>
Details of exactly which Perl regular expression features are and are not
supported by PCRE are given in separate documents. See the <b>pcrepattern</b>
and <b>pcrecompat</b> pages. <p>
Some features of PCRE can be included, excluded,
or changed when the library is built. The <b>pcre_config()</b> function makes
it possible for a client to discover which features are available. The features
themselves are described in the <b>pcrebuild</b> page. Documentation about building
PCRE for various operating systems can be found in the <b>README</b> file in the
source distribution.
<h2><a name='sect2' href='#toc2'>User Documentation</a></h2>
<p>
The user documentation for PCRE
comprises a number of different sections. In the "man" format, each of these
is a separate "man page". In the HTML format, each is a separate page, linked
from the index page. In the plain text format, all the sections are concatenated,
for ease of searching. The sections are as follows: <p>
pcre
this document<br>
pcreapi details of PCRE’s native API<br>
pcrebuild options for building PCRE<br>
pcrecallout details of the callout feature<br>
pcrecompat discussion of Perl compatibility<br>
pcregrep description of the <b>pcregrep</b> command<br>
pcrepartial details of the partial matching facility<br>
pcrepattern syntax and semantics of supported<br>
regular expressions<br>
pcreperform discussion of performance issues<br>
pcreposix the POSIX-compatible API<br>
pcreprecompile details of saving and re-using precompiled patterns<br>
pcresample discussion of the sample program<br>
pcretest description of the <b>pcretest</b> testing command<br>
<p>
In addition, in the "man" and HTML formats, there is a short page for
each library function, listing its arguments and results.
<h2><a name='sect3' href='#toc3'>Limitations</a></h2>
<p>
There are some size limitations in PCRE but it is hoped that they will
never in practice be relevant. <p>
The maximum length of a compiled pattern
is 65539 (sic) bytes if PCRE is compiled with the default internal linkage
size of 2. If you want to process regular expressions that are truly enormous,
you can compile PCRE with an internal linkage size of 3 or 4 (see the <b>README</b>
file in the source distribution and the <b>pcrebuild</b> documentation for details).
In these cases the limit is substantially larger. However, the speed of
execution will be slower. <p>
All values in repeating quantifiers must be less
than 65536. The maximum number of capturing subpatterns is 65535. <p>
There is
no limit to the number of non-capturing subpatterns, but the maximum depth
of nesting of all kinds of parenthesized subpattern, including capturing
subpatterns, assertions, and other types of subpattern, is 200. <p>
The maximum
length of a subject string is the largest positive number that an integer
variable can hold. However, PCRE uses recursion to handle subpatterns and
indefinite repetition. This means that the available stack space may limit
the size of a subject string that can be processed by certain patterns.
<p>
<h2><a name='sect4' href='#toc4'>Utf-8 and Unicode Property Support</a></h2>
<p>
From release 3.3, PCRE has had some
support for character strings encoded in the UTF-8 format. For release 4.0
this was greatly extended to cover most common requirements, and in release
5.0 additional support for Unicode general category properties was added.
<p>
In order process UTF-8 strings, you must build PCRE to include UTF-8 support
in the code, and, in addition, you must call <b>pcre_compile()</b> with the
PCRE_UTF8 option flag. When you do this, both the pattern and any subject
strings that are matched against it are treated as UTF-8 strings instead
of just strings of bytes. <p>
If you compile PCRE with UTF-8 support, but do
not use it at run time, the library will be a bit bigger, but the additional
run time overhead is limited to testing the PCRE_UTF8 flag in several places,
so should not be very large. <p>
If PCRE is built with Unicode character property
support (which implies UTF-8 support), the escape sequences \p{..}, \P{..}, and
\X are supported. The available properties that can be tested are limited
to the general category properties such as Lu for an upper case letter
or Nd for a decimal number. A full list is given in the <b>pcrepattern</b> documentation.
The PCRE library is increased in size by about 90K when Unicode property
support is included. <p>
The following comments apply when PCRE is running in
UTF-8 mode: <p>
1. When you set the PCRE_UTF8 flag, the strings passed as patterns
and subjects are checked for validity on entry to the relevant functions.
If an invalid UTF-8 string is passed, an error return is given. In some situations,
you may already know that your strings are valid, and therefore want to
skip these checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK
flag at compile time or at run time, PCRE assumes that the pattern or subject
it is given (respectively) contains only valid UTF-8 codes. In this case,
it does not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8
string to PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined.
Your program may crash. <p>
2. In a pattern, the escape sequence \x{...}, where the
contents of the braces is a string of hexadecimal digits, is interpreted
as a UTF-8 character whose code number is the given hexadecimal number,
for example: \x{1234}. If a non-hexadecimal digit appears between the braces,
the item is not recognized. This escape sequence can be used either as a
literal, or within a character class. <p>
3. The original hexadecimal escape
sequence, \xhh, matches a two-byte UTF-8 character if the value is greater
than 127. <p>
4. Repeat quantifiers apply to complete UTF-8 characters, not to
individual bytes, for example: \x{100}{3}. <p>
5. The dot metacharacter matches
one UTF-8 character instead of a single byte. <p>
6. The escape sequence \C can
be used to match a single byte in UTF-8 mode, but its use can lead to some
strange effects. <p>
7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
test characters of any code value, but the characters that PCRE recognizes
as digits, spaces, or word characters remain the same set as before, all
with values less than 256. This remains true even when PCRE includes Unicode
property support, because to do otherwise would slow down PCRE in many
common cases. If you really want to test for a wider sense of, say, "digit",
you must use Unicode property tests such as \p{Nd}. <p>
8. Similarly, characters
that match the POSIX named character classes are all low-valued characters.
<p>
9. Case-insensitive matching applies only to characters whose values are
less than 128, unless PCRE is built with Unicode property support. Even
when Unicode property support is available, PCRE still uses its own character
tables when checking the case of low-valued characters, so as not to degrade
performance. The Unicode property information is used only for characters
with higher values.
<h2><a name='sect5' href='#toc5'>Author</a></h2>
<p>
Philip Hazel <ph10@cam.ac.uk> <br>
University Computing Service, <br>
Cambridge CB2 3QG, England. <br>
Phone: +44 1223 334714 <p>
Last updated: 09 September 2004 <br>
Copyright (c) 1997-2004 University of Cambridge. <p>
<hr><p>
<a name='toc'><b>Table of Contents</b></a><p>
<ul>
<li><a name='toc0' href='#sect0'>Name</a></li>
<li><a name='toc1' href='#sect1'>Introduction</a></li>
<li><a name='toc2' href='#sect2'>User Documentation</a></li>
<li><a name='toc3' href='#sect3'>Limitations</a></li>
<li><a name='toc4' href='#sect4'>Utf-8 and Unicode Property Support</a></li>
<li><a name='toc5' href='#sect5'>Author</a></li>
</ul>
</body>
</html>
|
