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>
<li><a name="TOC6" href="#SEC6">REVISION</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. (Certain features that appeared in Python and PCRE before they
appeared in Perl are also available using the Python syntax.)
</P>
<P>
The current implementation of PCRE (release 7.x) corresponds approximately with
Perl 5.10, including support for UTF-8 encoded strings and Unicode general
category properties. However, UTF-8 and Unicode support has to be explicitly
enabled; it is not the default. The Unicode tables correspond to Unicode
release 5.0.0.
</P>
<P>
In addition to the Perl-compatible matching function, PCRE contains an
alternative matching function that matches the same compiled patterns in a
different way. In certain circumstances, the alternative function has some
advantages. For a discussion of the two matching algorithms, see the
<a href="pcrematching.html"><b>pcrematching</b></a>
page.
</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. In particular, Google Inc.
have provided a comprehensive C++ wrapper. This is now included as part of the
PCRE distribution. The
<a href="pcrecpp.html"><b>pcrecpp</b></a>
page has details of this interface. Other people's contributions 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>
<P>
The library contains a number of undocumented internal functions and data
tables that are used by more than one of the exported external functions, but
which are not intended for use by external callers. Their names all begin with
"_pcre_", which hopefully will not provoke any name clashes. In some
environments, it is possible to control which external symbols are exported
when a shared library is built, and in these cases the undocumented symbols are
not exported.
</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 C API
  pcrebuild         options for building PCRE
  pcrecallout       details of the callout feature
  pcrecompat        discussion of Perl compatibility
  pcrecpp           details of the C++ wrapper
  pcregrep          description of the <b>pcregrep</b> command
  pcrematching      discussion of the two matching algorithms
  pcrepartial       details of the partial matching facility
  pcrepattern       syntax and semantics of supported regular expressions
  pcreperform       discussion of performance issues
  pcreposix         the POSIX-compatible C API
  pcreprecompile    details of saving and re-using precompiled patterns
  pcresample        discussion of the sample program
  pcrestack         discussion of stack usage
  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
C 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 is slower.
</P>
<P>
All values in repeating quantifiers must be less than 65536. The maximum
compiled length of subpattern with an explicit repeat count is 30000 bytes. The
maximum number of capturing subpatterns is 65535.
</P>
<P>
There is no limit to the number of parenthesized subpatterns, but there can be
no more than 65535 capturing subpatterns.
</P>
<P>
The maximum length of name for a named subpattern is 32 characters, and the
maximum number of named subpatterns is 10000.
</P>
<P>
The maximum length of a subject string is the largest positive number that an
integer variable can hold. However, when using the traditional matching
function, 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. For a discussion of stack
issues, see the
<a href="pcrestack.html"><b>pcrestack</b></a>
documentation.
<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 occasionally, so should not be very big.
</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, the Unicode script names such as Arabic or Han, and the derived
properties Any and L&. A full list is given in the
<a href="pcrepattern.html"><b>pcrepattern</b></a>
documentation. Only the short names for properties are supported. For example,
\p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.
Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
compatibility with Perl 5.6. PCRE does not support this.
</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. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte
UTF-8 character if the value is greater than 127.
</P>
<P>
3. Octal numbers up to \777 are recognized, and match two-byte UTF-8
characters for values greater than \177.
</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. This facility is not available in
the alternative matching function, <b>pcre_dfa_exec()</b>.
</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. Even when Unicode property support is available, PCRE supports
case-insensitive matching only when there is a one-to-one mapping between a
letter's cases. There are a small number of many-to-one mappings in Unicode;
these are not supported by PCRE.
</P>
<br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
<P>
Philip Hazel
<br>
University Computing Service
<br>
Cambridge CB2 3QH, England.
<br>
</P>
<P>
Putting an actual email address here seems to have been a spam magnet, so I've
taken it away. If you want to email me, use my initial and surname, separated
by a dot, at the domain ucs.cam.ac.uk.
</P>
<br><a name="SEC6" href="#TOC1">REVISION</a><br>
<P>
Last updated: 06 March 2007
<br>
Copyright &copy; 1997-2007 University of Cambridge.
<br>
<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	ph10	111	<p>
11	nigel	75	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	ph10	111	<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	ph10	99	<li><a name="TOC6" href="#SEC6">REVISION</a>
22	nigel	63	</ul>
23	nigel	75	<br><a name="SEC1" href="#TOC1">INTRODUCTION</a><br>
24	nigel	63	<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	nigel	93	differences. (Certain features that appeared in Python and PCRE before they
28			appeared in Perl are also available using the Python syntax.)
29	nigel	63	</P>
30			<P>
31	nigel	93	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	nigel	77	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	nigel	75	PCRE is written in C and released as a C library. A number of people have
47	nigel	77	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	nigel	75	<a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>
54	nigel	63	</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	nigel	75	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	nigel	63	</P>
73	nigel	77	<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	nigel	83	"_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	nigel	77	</P>
82	nigel	63	<br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
83			<P>
84	nigel	75	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	nigel	63	<pre>
90			pcre this document
91	nigel	77	pcreapi details of PCRE's native C API
92	nigel	63	pcrebuild options for building PCRE
93			pcrecallout details of the callout feature
94			pcrecompat discussion of Perl compatibility
95	nigel	77	pcrecpp details of the C++ wrapper
96	nigel	63	pcregrep description of the <b>pcregrep</b> command
97	nigel	77	pcrematching discussion of the two matching algorithms
98	nigel	75	pcrepartial details of the partial matching facility
99			pcrepattern syntax and semantics of supported regular expressions
100	nigel	63	pcreperform discussion of performance issues
101	nigel	77	pcreposix the POSIX-compatible C API
102	nigel	75	pcreprecompile details of saving and re-using precompiled patterns
103	nigel	63	pcresample discussion of the sample program
104	nigel	91	pcrestack discussion of stack usage
105	nigel	75	pcretest description of the <b>pcretest</b> testing command
106			</pre>
107	nigel	63	In addition, in the "man" and HTML formats, there is a short page for each
108	nigel	77	C library function, listing its arguments and results.
109	nigel	63	</P>
110			<br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
111			<P>
112			There are some size limitations in PCRE but it is hoped that they will never in
113			practice be relevant.
114			</P>
115			<P>
116			The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
117			compiled with the default internal linkage size of 2. If you want to process
118			regular expressions that are truly enormous, you can compile PCRE with an
119			internal linkage size of 3 or 4 (see the <b>README</b> file in the source
120			distribution and the
121			<a href="pcrebuild.html"><b>pcrebuild</b></a>
122	nigel	75	documentation for details). In these cases the limit is substantially larger.
123	nigel	93	However, the speed of execution is slower.
124	nigel	63	</P>
125			<P>
126	nigel	91	All values in repeating quantifiers must be less than 65536. The maximum
127			compiled length of subpattern with an explicit repeat count is 30000 bytes. The
128			maximum number of capturing subpatterns is 65535.
129	nigel	63	</P>
130			<P>
131	nigel	93	There is no limit to the number of parenthesized subpatterns, but there can be
132			no more than 65535 capturing subpatterns.
133	nigel	63	</P>
134			<P>
135	nigel	93	The maximum length of name for a named subpattern is 32 characters, and the
136			maximum number of named subpatterns is 10000.
137	nigel	91	</P>
138			<P>
139	nigel	63	The maximum length of a subject string is the largest positive number that an
140	nigel	77	integer variable can hold. However, when using the traditional matching
141			function, PCRE uses recursion to handle subpatterns and indefinite repetition.
142			This means that the available stack space may limit the size of a subject
143	nigel	91	string that can be processed by certain patterns. For a discussion of stack
144			issues, see the
145			<a href="pcrestack.html"><b>pcrestack</b></a>
146			documentation.
147	nigel	75	<a name="utf8support"></a></P>
148			<br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
149	nigel	63	<P>
150	nigel	75	From release 3.3, PCRE has had some support for character strings encoded in
151			the UTF-8 format. For release 4.0 this was greatly extended to cover most
152			common requirements, and in release 5.0 additional support for Unicode general
153			category properties was added.
154	nigel	63	</P>
155			<P>
156			In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
157			the code, and, in addition, you must call
158			<a href="pcre_compile.html"><b>pcre_compile()</b></a>
159			with the PCRE_UTF8 option flag. When you do this, both the pattern and any
160			subject strings that are matched against it are treated as UTF-8 strings
161			instead of just strings of bytes.
162			</P>
163			<P>
164			If you compile PCRE with UTF-8 support, but do not use it at run time, the
165			library will be a bit bigger, but the additional run time overhead is limited
166	nigel	93	to testing the PCRE_UTF8 flag occasionally, so should not be very big.
167	nigel	63	</P>
168			<P>
169	nigel	75	If PCRE is built with Unicode character property support (which implies UTF-8
170			support), the escape sequences \p{..}, \P{..}, and \X are supported.
171			The available properties that can be tested are limited to the general
172			category properties such as Lu for an upper case letter or Nd for a decimal
173	nigel	87	number, the Unicode script names such as Arabic or Han, and the derived
174			properties Any and L&. A full list is given in the
175	nigel	75	<a href="pcrepattern.html"><b>pcrepattern</b></a>
176	nigel	87	documentation. Only the short names for properties are supported. For example,
177			\p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.
178			Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
179			compatibility with Perl 5.6. PCRE does not support this.
180	nigel	75	</P>
181			<P>
182	nigel	63	The following comments apply when PCRE is running in UTF-8 mode:
183			</P>
184			<P>
185	nigel	71	1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
186			are checked for validity on entry to the relevant functions. If an invalid
187			UTF-8 string is passed, an error return is given. In some situations, you may
188			already know that your strings are valid, and therefore want to skip these
189			checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
190			at compile time or at run time, PCRE assumes that the pattern or subject it
191			is given (respectively) contains only valid UTF-8 codes. In this case, it does
192			not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
193			PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
194			may crash.
195	nigel	63	</P>
196			<P>
197	nigel	87	2. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte
198			UTF-8 character if the value is greater than 127.
199	nigel	63	</P>
200			<P>
201	nigel	91	3. Octal numbers up to \777 are recognized, and match two-byte UTF-8
202			characters for values greater than \177.
203			</P>
204			<P>
205			4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
206	nigel	63	bytes, for example: \x{100}{3}.
207			</P>
208			<P>
209	nigel	91	5. The dot metacharacter matches one UTF-8 character instead of a single byte.
210	nigel	63	</P>
211			<P>
212	nigel	91	6. The escape sequence \C can be used to match a single byte in UTF-8 mode,
213	nigel	77	but its use can lead to some strange effects. This facility is not available in
214			the alternative matching function, <b>pcre_dfa_exec()</b>.
215	nigel	63	</P>
216			<P>
217	nigel	91	7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
218	nigel	63	test characters of any code value, but the characters that PCRE recognizes as
219			digits, spaces, or word characters remain the same set as before, all with
220	nigel	75	values less than 256. This remains true even when PCRE includes Unicode
221			property support, because to do otherwise would slow down PCRE in many common
222			cases. If you really want to test for a wider sense of, say, "digit", you
223			must use Unicode property tests such as \p{Nd}.
224	nigel	63	</P>
225			<P>
226	nigel	91	8. Similarly, characters that match the POSIX named character classes are all
227	nigel	75	low-valued characters.
228	nigel	63	</P>
229			<P>
230	nigel	91	9. Case-insensitive matching applies only to characters whose values are less
231	nigel	75	than 128, unless PCRE is built with Unicode property support. Even when Unicode
232			property support is available, PCRE still uses its own character tables when
233			checking the case of low-valued characters, so as not to degrade performance.
234			The Unicode property information is used only for characters with higher
235	nigel	87	values. Even when Unicode property support is available, PCRE supports
236			case-insensitive matching only when there is a one-to-one mapping between a
237			letter's cases. There are a small number of many-to-one mappings in Unicode;
238			these are not supported by PCRE.
239	nigel	63	</P>
240			<br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
241			<P>
242	nigel	77	Philip Hazel
243	nigel	63	<br>
244	ph10	99	University Computing Service
245	nigel	63	<br>
246	nigel	93	Cambridge CB2 3QH, England.
247	ph10	99	<br>
248	nigel	77	</P>
249			<P>
250			Putting an actual email address here seems to have been a spam magnet, so I've
251			taken it away. If you want to email me, use my initial and surname, separated
252			by a dot, at the domain ucs.cam.ac.uk.
253	ph10	99	</P>
254			<br><a name="SEC6" href="#TOC1">REVISION</a><br>
255			<P>
256			Last updated: 06 March 2007
257	nigel	63	<br>
258	ph10	99	Copyright © 1997-2007 University of Cambridge.
259			<br>
260	nigel	75	<p>
261			Return to the <a href="index.html">PCRE index page</a>.
262			</p>
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