trunk/doc/pcre.3

.TH PCRE 3
.SH NAME
PCRE - Perl-compatible regular expressions
.SH INTRODUCTION
.rs
.sp
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
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
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
.\" HREF
\fBpcrematching\fP
.\"
page.
.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
.\" HREF
\fBpcrecpp\fP
.\"
page has details of this interface. Other people's contributions can be found
in the \fIContrib\fR directory at the primary FTP site, which is:
.sp
.\" HTML <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">
.\" </a>
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
.\" HREF
\fBpcrepattern\fR
.\"
and
.\" HREF
\fBpcrecompat\fR
.\"
pages.
.P
Some features of PCRE can be included, excluded, or changed when the library is
built. The
.\" HREF
\fBpcre_config()\fR
.\"
function makes it possible for a client to discover which features are
available. The features themselves are described in the
.\" HREF
\fBpcrebuild\fP
.\"
page. Documentation about building PCRE for various operating systems can be
found in the \fBREADME\fP file in the source distribution.
.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.
.
.
.SH "USER DOCUMENTATION"
.rs
.sp
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:
.sp
  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 \fBpcregrep\fP command
  pcrematching      discussion of the two matching algorithms
  pcrepartial       details of the partial matching facility
.\" JOIN
  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 \fBpcretest\fP testing command
.sp
In addition, in the "man" and HTML formats, there is a short page for each
C library function, listing its arguments and results.
.
.
.SH LIMITATIONS
.rs
.sp
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 \fBREADME\fP file in the source
distribution and the
.\" HREF
\fBpcrebuild\fP
.\"
documentation for details). In these cases the limit is substantially larger.
However, the speed of execution is slower.
.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
There is no limit to the number of parenthesized subpatterns, but there can be
no more than 65535 capturing subpatterns.
.P
The maximum length of name for a named subpattern is 32 characters, and the
maximum number of named subpatterns is 10000.
.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
.\" HREF
\fBpcrestack\fP
.\"
documentation.
.sp
.\" HTML <a name="utf8support"></a>
.
.
.SH "UTF-8 AND UNICODE PROPERTY SUPPORT"
.rs
.sp
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
.\" HREF
\fBpcre_compile()\fP
.\"
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 occasionally, so should not be very big.
.P
If PCRE is built with Unicode character property support (which implies UTF-8
support), the escape sequences \ep{..}, \eP{..}, and \eX 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
.\" HREF
\fBpcrepattern\fP
.\"
documentation. Only the short names for properties are supported. For example,
\ep{L} matches a letter. Its Perl synonym, \ep{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
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. An unbraced hexadecimal escape sequence (such as \exb3) matches a two-byte
UTF-8 character if the value is greater than 127.
.P
3. Octal numbers up to \e777 are recognized, and match two-byte UTF-8
characters for values greater than \e177.
.P
4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
bytes, for example: \ex{100}{3}.
.P
5. The dot metacharacter matches one UTF-8 character instead of a single byte.
.P
6. The escape sequence \eC 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, \fBpcre_dfa_exec()\fP.
.P
7. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW 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 \ep{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. 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.
.
.
.SH AUTHOR
.rs
.sp
.nf
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
.fi
.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.
.
.
.SH REVISION
.rs
.sp
.nf
Last updated: 06 March 2007
Copyright (c) 1997-2007 University of Cambridge.
.fi
1	nigel	41	.TH PCRE 3
2			.SH NAME
3	nigel	63	PCRE - Perl-compatible regular expressions
4	nigel	75	.SH INTRODUCTION
5	nigel	63	.rs
6			.sp
7	nigel	41	The PCRE library is a set of functions that implement regular expression
8	nigel	63	pattern matching using the same syntax and semantics as Perl, with just a few
9	nigel	93	differences. (Certain features that appeared in Python and PCRE before they
10			appeared in Perl are also available using the Python syntax.)
11	nigel	75	.P
12	nigel	93	The current implementation of PCRE (release 7.x) corresponds approximately with
13			Perl 5.10, including support for UTF-8 encoded strings and Unicode general
14			category properties. However, UTF-8 and Unicode support has to be explicitly
15			enabled; it is not the default. The Unicode tables correspond to Unicode
16			release 5.0.0.
17			.P
18			In addition to the Perl-compatible matching function, PCRE contains an
19	nigel	77	alternative matching function that matches the same compiled patterns in a
20			different way. In certain circumstances, the alternative function has some
21			advantages. For a discussion of the two matching algorithms, see the
22			.\" HREF
23			\fBpcrematching\fP
24			.\"
25			page.
26			.P
27	nigel	75	PCRE is written in C and released as a C library. A number of people have
28	nigel	77	written wrappers and interfaces of various kinds. In particular, Google Inc.
29			have provided a comprehensive C++ wrapper. This is now included as part of the
30			PCRE distribution. The
31			.\" HREF
32			\fBpcrecpp\fP
33			.\"
34			page has details of this interface. Other people's contributions can be found
35			in the \fIContrib\fR directory at the primary FTP site, which is:
36	nigel	75	.sp
37	nigel	63	.\" HTML <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">
38			.\" </a>
39			ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
40	nigel	75	.P
41	nigel	63	Details of exactly which Perl regular expression features are and are not
42			supported by PCRE are given in separate documents. See the
43			.\" HREF
44			\fBpcrepattern\fR
45			.\"
46			and
47			.\" HREF
48			\fBpcrecompat\fR
49			.\"
50			pages.
51	nigel	75	.P
52	nigel	63	Some features of PCRE can be included, excluded, or changed when the library is
53			built. The
54			.\" HREF
55			\fBpcre_config()\fR
56			.\"
57			function makes it possible for a client to discover which features are
58	nigel	75	available. The features themselves are described in the
59			.\" HREF
60			\fBpcrebuild\fP
61			.\"
62			page. Documentation about building PCRE for various operating systems can be
63			found in the \fBREADME\fP file in the source distribution.
64	nigel	77	.P
65			The library contains a number of undocumented internal functions and data
66			tables that are used by more than one of the exported external functions, but
67			which are not intended for use by external callers. Their names all begin with
68	nigel	83	"_pcre_", which hopefully will not provoke any name clashes. In some
69			environments, it is possible to control which external symbols are exported
70			when a shared library is built, and in these cases the undocumented symbols are
71			not exported.
72	nigel	75	.
73			.
74			.SH "USER DOCUMENTATION"
75	nigel	63	.rs
76			.sp
77	nigel	75	The user documentation for PCRE comprises a number of different sections. In
78			the "man" format, each of these is a separate "man page". In the HTML format,
79			each is a separate page, linked from the index page. In the plain text format,
80			all the sections are concatenated, for ease of searching. The sections are as
81			follows:
82			.sp
83	nigel	63	pcre this document
84	nigel	77	pcreapi details of PCRE's native C API
85	nigel	63	pcrebuild options for building PCRE
86			pcrecallout details of the callout feature
87			pcrecompat discussion of Perl compatibility
88	nigel	77	pcrecpp details of the C++ wrapper
89	nigel	75	pcregrep description of the \fBpcregrep\fP command
90	nigel	77	pcrematching discussion of the two matching algorithms
91	nigel	75	pcrepartial details of the partial matching facility
92			.\" JOIN
93	nigel	63	pcrepattern syntax and semantics of supported
94			regular expressions
95			pcreperform discussion of performance issues
96	nigel	77	pcreposix the POSIX-compatible C API
97	nigel	75	pcreprecompile details of saving and re-using precompiled patterns
98	nigel	63	pcresample discussion of the sample program
99	nigel	91	pcrestack discussion of stack usage
100	nigel	75	pcretest description of the \fBpcretest\fP testing command
101			.sp
102	nigel	63	In addition, in the "man" and HTML formats, there is a short page for each
103	nigel	77	C library function, listing its arguments and results.
104	nigel	75	.
105			.
106	nigel	41	.SH LIMITATIONS
107	nigel	63	.rs
108			.sp
109	nigel	41	There are some size limitations in PCRE but it is hoped that they will never in
110			practice be relevant.
111	nigel	75	.P
112	nigel	63	The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
113			compiled with the default internal linkage size of 2. If you want to process
114			regular expressions that are truly enormous, you can compile PCRE with an
115	nigel	75	internal linkage size of 3 or 4 (see the \fBREADME\fP file in the source
116	nigel	63	distribution and the
117			.\" HREF
118	nigel	75	\fBpcrebuild\fP
119	nigel	63	.\"
120	nigel	75	documentation for details). In these cases the limit is substantially larger.
121	nigel	93	However, the speed of execution is slower.
122	nigel	75	.P
123	nigel	91	All values in repeating quantifiers must be less than 65536. The maximum
124			compiled length of subpattern with an explicit repeat count is 30000 bytes. The
125			maximum number of capturing subpatterns is 65535.
126	nigel	75	.P
127	nigel	93	There is no limit to the number of parenthesized subpatterns, but there can be
128			no more than 65535 capturing subpatterns.
129	nigel	75	.P
130	nigel	93	The maximum length of name for a named subpattern is 32 characters, and the
131			maximum number of named subpatterns is 10000.
132	nigel	91	.P
133	nigel	41	The maximum length of a subject string is the largest positive number that an
134	nigel	77	integer variable can hold. However, when using the traditional matching
135			function, PCRE uses recursion to handle subpatterns and indefinite repetition.
136			This means that the available stack space may limit the size of a subject
137	nigel	91	string that can be processed by certain patterns. For a discussion of stack
138			issues, see the
139			.\" HREF
140			\fBpcrestack\fP
141			.\"
142			documentation.
143	nigel	75	.sp
144	nigel	63	.\" HTML <a name="utf8support"></a>
145	nigel	75	.
146			.
147			.SH "UTF-8 AND UNICODE PROPERTY SUPPORT"
148	nigel	63	.rs
149			.sp
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			.P
155	nigel	63	In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
156			the code, and, in addition, you must call
157			.\" HREF
158	nigel	75	\fBpcre_compile()\fP
159	nigel	63	.\"
160			with the PCRE_UTF8 option flag. When you do this, both the pattern and any
161			subject strings that are matched against it are treated as UTF-8 strings
162			instead of just strings of bytes.
163	nigel	75	.P
164	nigel	49	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	75	.P
168			If PCRE is built with Unicode character property support (which implies UTF-8
169			support), the escape sequences \ep{..}, \eP{..}, and \eX are supported.
170			The available properties that can be tested are limited to the general
171			category properties such as Lu for an upper case letter or Nd for a decimal
172	nigel	87	number, the Unicode script names such as Arabic or Han, and the derived
173			properties Any and L&. A full list is given in the
174	nigel	75	.\" HREF
175			\fBpcrepattern\fP
176			.\"
177	nigel	87	documentation. Only the short names for properties are supported. For example,
178			\ep{L} matches a letter. Its Perl synonym, \ep{Letter}, is not supported.
179			Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
180			compatibility with Perl 5.6. PCRE does not support this.
181	nigel	75	.P
182	nigel	63	The following comments apply when PCRE is running in UTF-8 mode:
183	nigel	75	.P
184	nigel	71	1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
185			are checked for validity on entry to the relevant functions. If an invalid
186			UTF-8 string is passed, an error return is given. In some situations, you may
187			already know that your strings are valid, and therefore want to skip these
188			checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
189			at compile time or at run time, PCRE assumes that the pattern or subject it
190			is given (respectively) contains only valid UTF-8 codes. In this case, it does
191			not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
192			PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
193			may crash.
194	nigel	75	.P
195	nigel	87	2. An unbraced hexadecimal escape sequence (such as \exb3) matches a two-byte
196			UTF-8 character if the value is greater than 127.
197	nigel	75	.P
198	nigel	91	3. Octal numbers up to \e777 are recognized, and match two-byte UTF-8
199			characters for values greater than \e177.
200			.P
201			4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
202	nigel	75	bytes, for example: \ex{100}{3}.
203			.P
204	nigel	91	5. The dot metacharacter matches one UTF-8 character instead of a single byte.
205	nigel	75	.P
206	nigel	91	6. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
207	nigel	77	but its use can lead to some strange effects. This facility is not available in
208			the alternative matching function, \fBpcre_dfa_exec()\fP.
209	nigel	75	.P
210	nigel	91	7. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
211	nigel	63	test characters of any code value, but the characters that PCRE recognizes as
212			digits, spaces, or word characters remain the same set as before, all with
213	nigel	75	values less than 256. This remains true even when PCRE includes Unicode
214			property support, because to do otherwise would slow down PCRE in many common
215			cases. If you really want to test for a wider sense of, say, "digit", you
216			must use Unicode property tests such as \ep{Nd}.
217			.P
218	nigel	91	8. Similarly, characters that match the POSIX named character classes are all
219	nigel	75	low-valued characters.
220			.P
221	nigel	91	9. Case-insensitive matching applies only to characters whose values are less
222	nigel	75	than 128, unless PCRE is built with Unicode property support. Even when Unicode
223			property support is available, PCRE still uses its own character tables when
224			checking the case of low-valued characters, so as not to degrade performance.
225			The Unicode property information is used only for characters with higher
226	nigel	87	values. Even when Unicode property support is available, PCRE supports
227			case-insensitive matching only when there is a one-to-one mapping between a
228			letter's cases. There are a small number of many-to-one mappings in Unicode;
229			these are not supported by PCRE.
230	nigel	75	.
231	ph10	99	.
232	nigel	41	.SH AUTHOR
233	nigel	63	.rs
234			.sp
235	ph10	99	.nf
236	nigel	77	Philip Hazel
237	ph10	99	University Computing Service
238	nigel	93	Cambridge CB2 3QH, England.
239	ph10	99	.fi
240	nigel	77	.P
241			Putting an actual email address here seems to have been a spam magnet, so I've
242			taken it away. If you want to email me, use my initial and surname, separated
243			by a dot, at the domain ucs.cam.ac.uk.
244	ph10	99	.
245			.
246			.SH REVISION
247			.rs
248	nigel	75	.sp
249	ph10	99	.nf
250			Last updated: 06 March 2007
251			Copyright (c) 1997-2007 University of Cambridge.
252			.fi
Name	Value
svn:eol-style	native
svn:keywords	"Author Date Id Revision Url"