| 1 |
<html>
|
| 2 |
<head>
|
| 3 |
<title>pcre specification</title>
|
| 4 |
</head>
|
| 5 |
<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
|
| 6 |
<h1>pcre man page</h1>
|
| 7 |
<p>
|
| 8 |
Return to the <a href="index.html">PCRE index page</a>.
|
| 9 |
</p>
|
| 10 |
<p>
|
| 11 |
This page is part of the PCRE HTML documentation. It was generated automatically
|
| 12 |
from the original man page. If there is any nonsense in it, please consult the
|
| 13 |
man page, in case the conversion went wrong.
|
| 14 |
<br>
|
| 15 |
<ul>
|
| 16 |
<li><a name="TOC1" href="#SEC1">INTRODUCTION</a>
|
| 17 |
<li><a name="TOC2" href="#SEC2">USER DOCUMENTATION</a>
|
| 18 |
<li><a name="TOC3" href="#SEC3">LIMITATIONS</a>
|
| 19 |
<li><a name="TOC4" href="#SEC4">UTF-8 AND UNICODE PROPERTY SUPPORT</a>
|
| 20 |
<li><a name="TOC5" href="#SEC5">AUTHOR</a>
|
| 21 |
<li><a name="TOC6" href="#SEC6">REVISION</a>
|
| 22 |
</ul>
|
| 23 |
<br><a name="SEC1" href="#TOC1">INTRODUCTION</a><br>
|
| 24 |
<P>
|
| 25 |
The PCRE library is a set of functions that implement regular expression
|
| 26 |
pattern matching using the same syntax and semantics as Perl, with just a few
|
| 27 |
differences. (Certain features that appeared in Python and PCRE before they
|
| 28 |
appeared in Perl are also available using the Python syntax.)
|
| 29 |
</P>
|
| 30 |
<P>
|
| 31 |
The current implementation of PCRE (release 7.x) corresponds approximately with
|
| 32 |
Perl 5.10, including support for UTF-8 encoded strings and Unicode general
|
| 33 |
category properties. However, UTF-8 and Unicode support has to be explicitly
|
| 34 |
enabled; it is not the default. The Unicode tables correspond to Unicode
|
| 35 |
release 5.0.0.
|
| 36 |
</P>
|
| 37 |
<P>
|
| 38 |
In addition to the Perl-compatible matching function, PCRE contains an
|
| 39 |
alternative matching function that matches the same compiled patterns in a
|
| 40 |
different way. In certain circumstances, the alternative function has some
|
| 41 |
advantages. For a discussion of the two matching algorithms, see the
|
| 42 |
<a href="pcrematching.html"><b>pcrematching</b></a>
|
| 43 |
page.
|
| 44 |
</P>
|
| 45 |
<P>
|
| 46 |
PCRE is written in C and released as a C library. A number of people have
|
| 47 |
written wrappers and interfaces of various kinds. In particular, Google Inc.
|
| 48 |
have provided a comprehensive C++ wrapper. This is now included as part of the
|
| 49 |
PCRE distribution. The
|
| 50 |
<a href="pcrecpp.html"><b>pcrecpp</b></a>
|
| 51 |
page has details of this interface. Other people's contributions can be found
|
| 52 |
in the <i>Contrib</i> directory at the primary FTP site, which is:
|
| 53 |
<a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>
|
| 54 |
</P>
|
| 55 |
<P>
|
| 56 |
Details of exactly which Perl regular expression features are and are not
|
| 57 |
supported by PCRE are given in separate documents. See the
|
| 58 |
<a href="pcrepattern.html"><b>pcrepattern</b></a>
|
| 59 |
and
|
| 60 |
<a href="pcrecompat.html"><b>pcrecompat</b></a>
|
| 61 |
pages.
|
| 62 |
</P>
|
| 63 |
<P>
|
| 64 |
Some features of PCRE can be included, excluded, or changed when the library is
|
| 65 |
built. The
|
| 66 |
<a href="pcre_config.html"><b>pcre_config()</b></a>
|
| 67 |
function makes it possible for a client to discover which features are
|
| 68 |
available. The features themselves are described in the
|
| 69 |
<a href="pcrebuild.html"><b>pcrebuild</b></a>
|
| 70 |
page. Documentation about building PCRE for various operating systems can be
|
| 71 |
found in the <b>README</b> file in the source distribution.
|
| 72 |
</P>
|
| 73 |
<P>
|
| 74 |
The library contains a number of undocumented internal functions and data
|
| 75 |
tables that are used by more than one of the exported external functions, but
|
| 76 |
which are not intended for use by external callers. Their names all begin with
|
| 77 |
"_pcre_", which hopefully will not provoke any name clashes. In some
|
| 78 |
environments, it is possible to control which external symbols are exported
|
| 79 |
when a shared library is built, and in these cases the undocumented symbols are
|
| 80 |
not exported.
|
| 81 |
</P>
|
| 82 |
<br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
|
| 83 |
<P>
|
| 84 |
The user documentation for PCRE comprises a number of different sections. In
|
| 85 |
the "man" format, each of these is a separate "man page". In the HTML format,
|
| 86 |
each is a separate page, linked from the index page. In the plain text format,
|
| 87 |
all the sections are concatenated, for ease of searching. The sections are as
|
| 88 |
follows:
|
| 89 |
<pre>
|
| 90 |
pcre this document
|
| 91 |
pcre-config show PCRE installation configuration information
|
| 92 |
pcreapi details of PCRE's native C API
|
| 93 |
pcrebuild options for building PCRE
|
| 94 |
pcrecallout details of the callout feature
|
| 95 |
pcrecompat discussion of Perl compatibility
|
| 96 |
pcrecpp details of the C++ wrapper
|
| 97 |
pcregrep description of the <b>pcregrep</b> command
|
| 98 |
pcrematching discussion of the two matching algorithms
|
| 99 |
pcrepartial details of the partial matching facility
|
| 100 |
pcrepattern syntax and semantics of supported regular expressions
|
| 101 |
pcreperform discussion of performance issues
|
| 102 |
pcreposix the POSIX-compatible C API
|
| 103 |
pcreprecompile details of saving and re-using precompiled patterns
|
| 104 |
pcresample discussion of the sample program
|
| 105 |
pcrestack discussion of stack usage
|
| 106 |
pcretest description of the <b>pcretest</b> testing command
|
| 107 |
</pre>
|
| 108 |
In addition, in the "man" and HTML formats, there is a short page for each
|
| 109 |
C library function, listing its arguments and results.
|
| 110 |
</P>
|
| 111 |
<br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
|
| 112 |
<P>
|
| 113 |
There are some size limitations in PCRE but it is hoped that they will never in
|
| 114 |
practice be relevant.
|
| 115 |
</P>
|
| 116 |
<P>
|
| 117 |
The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
|
| 118 |
compiled with the default internal linkage size of 2. If you want to process
|
| 119 |
regular expressions that are truly enormous, you can compile PCRE with an
|
| 120 |
internal linkage size of 3 or 4 (see the <b>README</b> file in the source
|
| 121 |
distribution and the
|
| 122 |
<a href="pcrebuild.html"><b>pcrebuild</b></a>
|
| 123 |
documentation for details). In these cases the limit is substantially larger.
|
| 124 |
However, the speed of execution is slower.
|
| 125 |
</P>
|
| 126 |
<P>
|
| 127 |
All values in repeating quantifiers must be less than 65536. The maximum
|
| 128 |
compiled length of subpattern with an explicit repeat count is 30000 bytes. The
|
| 129 |
maximum number of capturing subpatterns is 65535.
|
| 130 |
</P>
|
| 131 |
<P>
|
| 132 |
There is no limit to the number of parenthesized subpatterns, but there can be
|
| 133 |
no more than 65535 capturing subpatterns.
|
| 134 |
</P>
|
| 135 |
<P>
|
| 136 |
If a non-capturing subpattern with an unlimited repetition quantifier can match
|
| 137 |
an empty string, there is a limit of 1000 on the number of times it can be
|
| 138 |
repeated while not matching an empty string - if it does match an empty
|
| 139 |
string, the loop is immediately broken.
|
| 140 |
</P>
|
| 141 |
<P>
|
| 142 |
The maximum length of name for a named subpattern is 32 characters, and the
|
| 143 |
maximum number of named subpatterns is 10000.
|
| 144 |
</P>
|
| 145 |
<P>
|
| 146 |
The maximum length of a subject string is the largest positive number that an
|
| 147 |
integer variable can hold. However, when using the traditional matching
|
| 148 |
function, PCRE uses recursion to handle subpatterns and indefinite repetition.
|
| 149 |
This means that the available stack space may limit the size of a subject
|
| 150 |
string that can be processed by certain patterns. For a discussion of stack
|
| 151 |
issues, see the
|
| 152 |
<a href="pcrestack.html"><b>pcrestack</b></a>
|
| 153 |
documentation.
|
| 154 |
<a name="utf8support"></a></P>
|
| 155 |
<br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
|
| 156 |
<P>
|
| 157 |
From release 3.3, PCRE has had some support for character strings encoded in
|
| 158 |
the UTF-8 format. For release 4.0 this was greatly extended to cover most
|
| 159 |
common requirements, and in release 5.0 additional support for Unicode general
|
| 160 |
category properties was added.
|
| 161 |
</P>
|
| 162 |
<P>
|
| 163 |
In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
|
| 164 |
the code, and, in addition, you must call
|
| 165 |
<a href="pcre_compile.html"><b>pcre_compile()</b></a>
|
| 166 |
with the PCRE_UTF8 option flag. When you do this, both the pattern and any
|
| 167 |
subject strings that are matched against it are treated as UTF-8 strings
|
| 168 |
instead of just strings of bytes.
|
| 169 |
</P>
|
| 170 |
<P>
|
| 171 |
If you compile PCRE with UTF-8 support, but do not use it at run time, the
|
| 172 |
library will be a bit bigger, but the additional run time overhead is limited
|
| 173 |
to testing the PCRE_UTF8 flag occasionally, so should not be very big.
|
| 174 |
</P>
|
| 175 |
<P>
|
| 176 |
If PCRE is built with Unicode character property support (which implies UTF-8
|
| 177 |
support), the escape sequences \p{..}, \P{..}, and \X are supported.
|
| 178 |
The available properties that can be tested are limited to the general
|
| 179 |
category properties such as Lu for an upper case letter or Nd for a decimal
|
| 180 |
number, the Unicode script names such as Arabic or Han, and the derived
|
| 181 |
properties Any and L&. A full list is given in the
|
| 182 |
<a href="pcrepattern.html"><b>pcrepattern</b></a>
|
| 183 |
documentation. Only the short names for properties are supported. For example,
|
| 184 |
\p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.
|
| 185 |
Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
|
| 186 |
compatibility with Perl 5.6. PCRE does not support this.
|
| 187 |
</P>
|
| 188 |
<P>
|
| 189 |
The following comments apply when PCRE is running in UTF-8 mode:
|
| 190 |
</P>
|
| 191 |
<P>
|
| 192 |
1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
|
| 193 |
are checked for validity on entry to the relevant functions. If an invalid
|
| 194 |
UTF-8 string is passed, an error return is given. In some situations, you may
|
| 195 |
already know that your strings are valid, and therefore want to skip these
|
| 196 |
checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
|
| 197 |
at compile time or at run time, PCRE assumes that the pattern or subject it
|
| 198 |
is given (respectively) contains only valid UTF-8 codes. In this case, it does
|
| 199 |
not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
|
| 200 |
PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
|
| 201 |
may crash.
|
| 202 |
</P>
|
| 203 |
<P>
|
| 204 |
2. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte
|
| 205 |
UTF-8 character if the value is greater than 127.
|
| 206 |
</P>
|
| 207 |
<P>
|
| 208 |
3. Octal numbers up to \777 are recognized, and match two-byte UTF-8
|
| 209 |
characters for values greater than \177.
|
| 210 |
</P>
|
| 211 |
<P>
|
| 212 |
4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
|
| 213 |
bytes, for example: \x{100}{3}.
|
| 214 |
</P>
|
| 215 |
<P>
|
| 216 |
5. The dot metacharacter matches one UTF-8 character instead of a single byte.
|
| 217 |
</P>
|
| 218 |
<P>
|
| 219 |
6. The escape sequence \C can be used to match a single byte in UTF-8 mode,
|
| 220 |
but its use can lead to some strange effects. This facility is not available in
|
| 221 |
the alternative matching function, <b>pcre_dfa_exec()</b>.
|
| 222 |
</P>
|
| 223 |
<P>
|
| 224 |
7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
|
| 225 |
test characters of any code value, but the characters that PCRE recognizes as
|
| 226 |
digits, spaces, or word characters remain the same set as before, all with
|
| 227 |
values less than 256. This remains true even when PCRE includes Unicode
|
| 228 |
property support, because to do otherwise would slow down PCRE in many common
|
| 229 |
cases. If you really want to test for a wider sense of, say, "digit", you
|
| 230 |
must use Unicode property tests such as \p{Nd}.
|
| 231 |
</P>
|
| 232 |
<P>
|
| 233 |
8. Similarly, characters that match the POSIX named character classes are all
|
| 234 |
low-valued characters.
|
| 235 |
</P>
|
| 236 |
<P>
|
| 237 |
9. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
|
| 238 |
(\h, \H, \v, and \V) do match all the appropriate Unicode characters.
|
| 239 |
</P>
|
| 240 |
<P>
|
| 241 |
10. Case-insensitive matching applies only to characters whose values are less
|
| 242 |
than 128, unless PCRE is built with Unicode property support. Even when Unicode
|
| 243 |
property support is available, PCRE still uses its own character tables when
|
| 244 |
checking the case of low-valued characters, so as not to degrade performance.
|
| 245 |
The Unicode property information is used only for characters with higher
|
| 246 |
values. Even when Unicode property support is available, PCRE supports
|
| 247 |
case-insensitive matching only when there is a one-to-one mapping between a
|
| 248 |
letter's cases. There are a small number of many-to-one mappings in Unicode;
|
| 249 |
these are not supported by PCRE.
|
| 250 |
</P>
|
| 251 |
<br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
|
| 252 |
<P>
|
| 253 |
Philip Hazel
|
| 254 |
<br>
|
| 255 |
University Computing Service
|
| 256 |
<br>
|
| 257 |
Cambridge CB2 3QH, England.
|
| 258 |
<br>
|
| 259 |
</P>
|
| 260 |
<P>
|
| 261 |
Putting an actual email address here seems to have been a spam magnet, so I've
|
| 262 |
taken it away. If you want to email me, use my two initials, followed by the
|
| 263 |
two digits 10, at the domain cam.ac.uk.
|
| 264 |
</P>
|
| 265 |
<br><a name="SEC6" href="#TOC1">REVISION</a><br>
|
| 266 |
<P>
|
| 267 |
Last updated: 30 July 2007
|
| 268 |
<br>
|
| 269 |
Copyright © 1997-2007 University of Cambridge.
|
| 270 |
<br>
|
| 271 |
<p>
|
| 272 |
Return to the <a href="index.html">PCRE index page</a>.
|
| 273 |
</p>
|
Parent Directory
| 
Revision Log