doc/html/pcre.html

<html>
<head>
<title>pcre specification</title>
</head>
<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
<h1>pcre man page</h1>
<p>
Return to the <a href="index.html">PCRE index page</a>.
</p>
<p>
This page is part of the PCRE HTML documentation. It was generated automatically
from the original man page. If there is any nonsense in it, please consult the
man page, in case the conversion went wrong.
<br>
<ul>
<li><a name="TOC1" href="#SEC1">INTRODUCTION</a>
<li><a name="TOC2" href="#SEC2">USER DOCUMENTATION</a>
<li><a name="TOC3" href="#SEC3">LIMITATIONS</a>
<li><a name="TOC4" href="#SEC4">UTF-8 AND UNICODE PROPERTY SUPPORT</a>
<li><a name="TOC5" href="#SEC5">AUTHOR</a>
</ul>
<br><a name="SEC1" href="#TOC1">INTRODUCTION</a><br>
<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>
<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:
<a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>
</P>
<P>
Details of exactly which Perl regular expression features are and are not
supported by PCRE are given in separate documents. See the
<a href="pcrepattern.html"><b>pcrepattern</b></a>
and
<a href="pcrecompat.html"><b>pcrecompat</b></a>
pages.
</P>
<P>
Some features of PCRE can be included, excluded, or changed when the library is
built. The
<a href="pcre_config.html"><b>pcre_config()</b></a>
function makes it possible for a client to discover which features are
available. The features themselves are described in the
<a href="pcrebuild.html"><b>pcrebuild</b></a>
page. Documentation about building PCRE for various operating systems can be
found in the <b>README</b> file in the source distribution.
</P>
<br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
<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:
<pre>
  pcre              this document
  pcreapi           details of PCRE's native API
  pcrebuild         options for building PCRE
  pcrecallout       details of the callout feature
  pcrecompat        discussion of Perl compatibility
  pcregrep          description of the <b>pcregrep</b> command
  pcrepartial       details of the partial matching facility
  pcrepattern       syntax and semantics of supported regular expressions
  pcreperform       discussion of performance issues
  pcreposix         the POSIX-compatible API
  pcreprecompile    details of saving and re-using precompiled patterns
  pcresample        discussion of the sample program
  pcretest          description of the <b>pcretest</b> testing command
</pre>
In addition, in the "man" and HTML formats, there is a short page for each
library function, listing its arguments and results.
</P>
<br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
<P>
There are some size limitations in PCRE but it is hoped that they will never in
practice be relevant.
</P>
<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
<a href="pcrebuild.html"><b>pcrebuild</b></a>
documentation for details). In these cases the limit is substantially larger.
However, the speed of execution will be slower.
</P>
<P>
All values in repeating quantifiers must be less than 65536.
The maximum number of capturing subpatterns is 65535.
</P>
<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>
<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.
<a name="utf8support"></a></P>
<br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
<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>
<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
<a href="pcre_compile.html"><b>pcre_compile()</b></a>
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>
<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>
<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
<a href="pcrepattern.html"><b>pcrepattern</b></a>
documentation. The PCRE library is increased in size by about 90K when Unicode
property support is included.
</P>
<P>
The following comments apply when PCRE is running in UTF-8 mode:
</P>
<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>
<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>
<P>
3. The original hexadecimal escape sequence, \xhh, matches a two-byte UTF-8
character if the value is greater than 127.
</P>
<P>
4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
bytes, for example: \x{100}{3}.
</P>
<P>
5. The dot metacharacter matches one UTF-8 character instead of a single byte.
</P>
<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>
<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>
<P>
8. Similarly, characters that match the POSIX named character classes are all
low-valued characters.
</P>
<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.
</P>
<br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
<P>
Philip Hazel &#60;ph10@cam.ac.uk&#62;
<br>
University Computing Service,
<br>
Cambridge CB2 3QG, England.
<br>
Phone: +44 1223 334714
Last updated: 09 September 2004
<br>
Copyright &copy; 1997-2004 University of Cambridge.
<p>
Return to the <a href="index.html">PCRE index page</a>.
</p>
1	nigel	63	<html>
2			<head>
3			<title>pcre specification</title>
4			</head>
5			<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6	nigel	75	<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	nigel	63	<ul>
16	nigel	75	<li><a name="TOC1" href="#SEC1">INTRODUCTION</a>
17	nigel	63	<li><a name="TOC2" href="#SEC2">USER DOCUMENTATION</a>
18			<li><a name="TOC3" href="#SEC3">LIMITATIONS</a>
19	nigel	75	<li><a name="TOC4" href="#SEC4">UTF-8 AND UNICODE PROPERTY SUPPORT</a>
20	nigel	63	<li><a name="TOC5" href="#SEC5">AUTHOR</a>
21			</ul>
22	nigel	75	<br><a name="SEC1" href="#TOC1">INTRODUCTION</a><br>
23	nigel	63	<P>
24			The PCRE library is a set of functions that implement regular expression
25			pattern matching using the same syntax and semantics as Perl, with just a few
26	nigel	75	differences. The current implementation of PCRE (release 5.x) corresponds
27			approximately with Perl 5.8, including support for UTF-8 encoded strings and
28			Unicode general category properties. However, this support has to be explicitly
29			enabled; it is not the default.
30	nigel	63	</P>
31			<P>
32	nigel	75	PCRE is written in C and released as a C library. A number of people have
33			written wrappers and interfaces of various kinds. A C++ class is included in
34			these contributions, which can be found in the <i>Contrib</i> directory at the
35			primary FTP site, which is:
36			<a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>
37	nigel	63	</P>
38			<P>
39			Details of exactly which Perl regular expression features are and are not
40			supported by PCRE are given in separate documents. See the
41			<a href="pcrepattern.html"><b>pcrepattern</b></a>
42			and
43			<a href="pcrecompat.html"><b>pcrecompat</b></a>
44			pages.
45			</P>
46			<P>
47			Some features of PCRE can be included, excluded, or changed when the library is
48			built. The
49			<a href="pcre_config.html"><b>pcre_config()</b></a>
50			function makes it possible for a client to discover which features are
51	nigel	75	available. The features themselves are described in the
52			<a href="pcrebuild.html"><b>pcrebuild</b></a>
53			page. Documentation about building PCRE for various operating systems can be
54			found in the <b>README</b> file in the source distribution.
55	nigel	63	</P>
56			<br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
57			<P>
58	nigel	75	The user documentation for PCRE comprises a number of different sections. In
59			the "man" format, each of these is a separate "man page". In the HTML format,
60			each is a separate page, linked from the index page. In the plain text format,
61			all the sections are concatenated, for ease of searching. The sections are as
62			follows:
63	nigel	63	<pre>
64			pcre this document
65			pcreapi details of PCRE's native API
66			pcrebuild options for building PCRE
67			pcrecallout details of the callout feature
68			pcrecompat discussion of Perl compatibility
69			pcregrep description of the <b>pcregrep</b> command
70	nigel	75	pcrepartial details of the partial matching facility
71			pcrepattern syntax and semantics of supported regular expressions
72	nigel	63	pcreperform discussion of performance issues
73			pcreposix the POSIX-compatible API
74	nigel	75	pcreprecompile details of saving and re-using precompiled patterns
75	nigel	63	pcresample discussion of the sample program
76	nigel	75	pcretest description of the <b>pcretest</b> testing command
77			</pre>
78	nigel	63	In addition, in the "man" and HTML formats, there is a short page for each
79			library function, listing its arguments and results.
80			</P>
81			<br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
82			<P>
83			There are some size limitations in PCRE but it is hoped that they will never in
84			practice be relevant.
85			</P>
86			<P>
87			The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
88			compiled with the default internal linkage size of 2. If you want to process
89			regular expressions that are truly enormous, you can compile PCRE with an
90			internal linkage size of 3 or 4 (see the <b>README</b> file in the source
91			distribution and the
92			<a href="pcrebuild.html"><b>pcrebuild</b></a>
93	nigel	75	documentation for details). In these cases the limit is substantially larger.
94	nigel	63	However, the speed of execution will be slower.
95			</P>
96			<P>
97			All values in repeating quantifiers must be less than 65536.
98			The maximum number of capturing subpatterns is 65535.
99			</P>
100			<P>
101			There is no limit to the number of non-capturing subpatterns, but the maximum
102			depth of nesting of all kinds of parenthesized subpattern, including capturing
103			subpatterns, assertions, and other types of subpattern, is 200.
104			</P>
105			<P>
106			The maximum length of a subject string is the largest positive number that an
107			integer variable can hold. However, PCRE uses recursion to handle subpatterns
108			and indefinite repetition. This means that the available stack space may limit
109			the size of a subject string that can be processed by certain patterns.
110	nigel	75	<a name="utf8support"></a></P>
111			<br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
112	nigel	63	<P>
113	nigel	75	From release 3.3, PCRE has had some support for character strings encoded in
114			the UTF-8 format. For release 4.0 this was greatly extended to cover most
115			common requirements, and in release 5.0 additional support for Unicode general
116			category properties was added.
117	nigel	63	</P>
118			<P>
119			In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
120			the code, and, in addition, you must call
121			<a href="pcre_compile.html"><b>pcre_compile()</b></a>
122			with the PCRE_UTF8 option flag. When you do this, both the pattern and any
123			subject strings that are matched against it are treated as UTF-8 strings
124			instead of just strings of bytes.
125			</P>
126			<P>
127			If you compile PCRE with UTF-8 support, but do not use it at run time, the
128			library will be a bit bigger, but the additional run time overhead is limited
129			to testing the PCRE_UTF8 flag in several places, so should not be very large.
130			</P>
131			<P>
132	nigel	75	If PCRE is built with Unicode character property support (which implies UTF-8
133			support), the escape sequences \p{..}, \P{..}, and \X are supported.
134			The available properties that can be tested are limited to the general
135			category properties such as Lu for an upper case letter or Nd for a decimal
136			number. A full list is given in the
137			<a href="pcrepattern.html"><b>pcrepattern</b></a>
138			documentation. The PCRE library is increased in size by about 90K when Unicode
139			property support is included.
140			</P>
141			<P>
142	nigel	63	The following comments apply when PCRE is running in UTF-8 mode:
143			</P>
144			<P>
145	nigel	71	1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
146			are checked for validity on entry to the relevant functions. If an invalid
147			UTF-8 string is passed, an error return is given. In some situations, you may
148			already know that your strings are valid, and therefore want to skip these
149			checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
150			at compile time or at run time, PCRE assumes that the pattern or subject it
151			is given (respectively) contains only valid UTF-8 codes. In this case, it does
152			not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
153			PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
154			may crash.
155	nigel	63	</P>
156			<P>
157			2. In a pattern, the escape sequence \x{...}, where the contents of the braces
158			is a string of hexadecimal digits, is interpreted as a UTF-8 character whose
159			code number is the given hexadecimal number, for example: \x{1234}. If a
160			non-hexadecimal digit appears between the braces, the item is not recognized.
161			This escape sequence can be used either as a literal, or within a character
162			class.
163			</P>
164			<P>
165			3. The original hexadecimal escape sequence, \xhh, matches a two-byte UTF-8
166			character if the value is greater than 127.
167			</P>
168			<P>
169			4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
170			bytes, for example: \x{100}{3}.
171			</P>
172			<P>
173			5. The dot metacharacter matches one UTF-8 character instead of a single byte.
174			</P>
175			<P>
176			6. The escape sequence \C can be used to match a single byte in UTF-8 mode,
177			but its use can lead to some strange effects.
178			</P>
179			<P>
180			7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
181			test characters of any code value, but the characters that PCRE recognizes as
182			digits, spaces, or word characters remain the same set as before, all with
183	nigel	75	values less than 256. This remains true even when PCRE includes Unicode
184			property support, because to do otherwise would slow down PCRE in many common
185			cases. If you really want to test for a wider sense of, say, "digit", you
186			must use Unicode property tests such as \p{Nd}.
187	nigel	63	</P>
188			<P>
189	nigel	75	8. Similarly, characters that match the POSIX named character classes are all
190			low-valued characters.
191	nigel	63	</P>
192			<P>
193	nigel	75	9. Case-insensitive matching applies only to characters whose values are less
194			than 128, unless PCRE is built with Unicode property support. Even when Unicode
195			property support is available, PCRE still uses its own character tables when
196			checking the case of low-valued characters, so as not to degrade performance.
197			The Unicode property information is used only for characters with higher
198			values.
199	nigel	63	</P>
200			<br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
201			<P>
202			Philip Hazel <ph10@cam.ac.uk>
203			<br>
204			University Computing Service,
205			<br>
206			Cambridge CB2 3QG, England.
207			<br>
208			Phone: +44 1223 334714
209	nigel	75	Last updated: 09 September 2004
210	nigel	63	<br>
211	nigel	75	Copyright © 1997-2004 University of Cambridge.
212			<p>
213			Return to the <a href="index.html">PCRE index page</a>.
214			</p>