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. Some features that appeared in Python and PCRE before they
appeared in Perl are also available using the Python syntax, there is some
support for one or two .NET and Oniguruma syntax items, and there is an option
for requesting some minor changes that give better JavaScript compatibility.
</P>
<P>
The current implementation of PCRE corresponds approximately with Perl 5.12,
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 6.0.0.
</P>
<P>
In addition to the Perl-compatible matching function, PCRE contains an
alternative 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. There is a syntax summary in the
<a href="pcresyntax.html"><b>pcresyntax</b></a>
page.
</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> and <b>NON-UNIX-USE</b> files 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, except the <b>pcredemo</b> section, are concatenated, for ease
of searching. The sections are as follows:
<pre>
  pcre              this document
  pcre-config       show PCRE installation configuration information
  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
  pcredemo          a demonstration C program that uses PCRE
  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 pcredemo program
  pcrestack         discussion of stack usage
  pcresyntax        quick syntax reference
  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.
</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, or the pattern must start with the sequence
(*UTF8). When either of these is the case, both the pattern and any subject
strings that are matched against it are treated as UTF-8 strings instead of
strings of 1-byte characters.
</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.
<a name="utf8strings"></a></P>
<br><b>
Validity of UTF-8 strings
</b><br>
<P>
When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
are (by default) checked for validity on entry to the relevant functions. From
release 7.3 of PCRE, the check is according the rules of RFC 3629, which are
themselves derived from the Unicode specification. Earlier releases of PCRE
followed the rules of RFC 2279, which allows the full range of 31-bit values (0
to 0x7FFFFFFF). The current check allows only values in the range U+0 to
U+10FFFF, excluding U+D800 to U+DFFF.
</P>
<P>
The excluded code points are the "Low Surrogate Area" of Unicode, of which the
Unicode Standard says this: "The Low Surrogate Area does not contain any
character assignments, consequently no character code charts or namelists are
provided for this area. Surrogates are reserved for use with UTF-16 and then
must be used in pairs." The code points that are encoded by UTF-16 pairs are
available as independent code points in the UTF-8 encoding. (In other words,
the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up
UTF-8.)
</P>
<P>
If an invalid UTF-8 string is passed to PCRE, an error return is given. At
compile time, the only additional information is the offset to the first byte
of the failing character. The runtime functions (<b>pcre_exec()</b> and
<b>pcre_dfa_exec()</b>), pass back this information as well as a more detailed
reason code if the caller has provided memory in which to do this.
</P>
<P>
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.
</P>
<P>
If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
happens depends on why the string is invalid. If the string conforms to the
"old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
in the range 0 to 0x7FFFFFFF. In other words, apart from the initial validity
test, PCRE (when in UTF-8 mode) handles strings according to the more liberal
rules of RFC 2279. However, if the string does not even conform to RFC 2279,
the result is undefined. Your program may crash.
</P>
<P>
If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
encoded in a UTF-8-like manner as per the old RFC, you can set
PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
situation, you will have to apply your own validity check.
</P>
<br><b>
General comments about UTF-8 mode
</b><br>
<P>
1. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte
UTF-8 character if the value is greater than 127.
</P>
<P>
2. Octal numbers up to \777 are recognized, and match two-byte UTF-8
characters for values greater than \177.
</P>
<P>
3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
bytes, for example: \x{100}{3}.
</P>
<P>
4. The dot metacharacter matches one UTF-8 character instead of a single byte.
</P>
<P>
5. 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>
6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
test characters of any code value, but, by default, 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 is built to
include Unicode property support, because to do otherwise would slow down PCRE
in many common cases. Note in particular that this applies to \b and \B,
because they are defined in terms of \w and \W. If you really want to test
for a wider sense of, say, "digit", you can use explicit Unicode property tests
such as \p{Nd}. Alternatively, if you set the PCRE_UCP option, the way that
the character escapes work is changed so that Unicode properties are used to
determine which characters match. There are more details in the section on
<a href="pcrepattern.html#genericchartypes">generic character types</a>
in the
<a href="pcrepattern.html"><b>pcrepattern</b></a>
documentation.
</P>
<P>
7. Similarly, characters that match the POSIX named character classes are all
low-valued characters, unless the PCRE_UCP option is set.
</P>
<P>
8. However, the horizontal and vertical whitespace matching escapes (\h, \H,
\v, and \V) do match all the appropriate Unicode characters, whether or not
PCRE_UCP is set.
</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. Furthermore, 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 two initials, followed by the
two digits 10, at the domain cam.ac.uk.
</P>
<br><a name="SEC6" href="#TOC1">REVISION</a><br>
<P>
Last updated: 07 May 2011
<br>
Copyright &copy; 1997-2011 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	ph10	461	differences. Some features that appeared in Python and PCRE before they
28			appeared in Perl are also available using the Python syntax, there is some
29			support for one or two .NET and Oniguruma syntax items, and there is an option
30			for requesting some minor changes that give better JavaScript compatibility.
31	nigel	63	</P>
32			<P>
33	ph10	572	The current implementation of PCRE corresponds approximately with Perl 5.12,
34			including support for UTF-8 encoded strings and Unicode general category
35			properties. However, UTF-8 and Unicode support has to be explicitly enabled; it
36	ph10	654	is not the default. The Unicode tables correspond to Unicode release 6.0.0.
37	nigel	93	</P>
38			<P>
39			In addition to the Perl-compatible matching function, PCRE contains an
40	ph10	461	alternative function that matches the same compiled patterns in a different
41			way. In certain circumstances, the alternative function has some advantages.
42			For a discussion of the two matching algorithms, see the
43	nigel	77	<a href="pcrematching.html"><b>pcrematching</b></a>
44			page.
45			</P>
46			<P>
47	nigel	75	PCRE is written in C and released as a C library. A number of people have
48	nigel	77	written wrappers and interfaces of various kinds. In particular, Google Inc.
49			have provided a comprehensive C++ wrapper. This is now included as part of the
50			PCRE distribution. The
51			<a href="pcrecpp.html"><b>pcrecpp</b></a>
52			page has details of this interface. Other people's contributions can be found
53			in the <i>Contrib</i> directory at the primary FTP site, which is:
54	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>
55	nigel	63	</P>
56			<P>
57			Details of exactly which Perl regular expression features are and are not
58			supported by PCRE are given in separate documents. See the
59			<a href="pcrepattern.html"><b>pcrepattern</b></a>
60			and
61			<a href="pcrecompat.html"><b>pcrecompat</b></a>
62	ph10	208	pages. There is a syntax summary in the
63			<a href="pcresyntax.html"><b>pcresyntax</b></a>
64			page.
65	nigel	63	</P>
66			<P>
67			Some features of PCRE can be included, excluded, or changed when the library is
68			built. The
69			<a href="pcre_config.html"><b>pcre_config()</b></a>
70			function makes it possible for a client to discover which features are
71	nigel	75	available. The features themselves are described in the
72			<a href="pcrebuild.html"><b>pcrebuild</b></a>
73			page. Documentation about building PCRE for various operating systems can be
74	ph10	461	found in the <b>README</b> and <b>NON-UNIX-USE</b> files in the source
75			distribution.
76	nigel	63	</P>
77	nigel	77	<P>
78			The library contains a number of undocumented internal functions and data
79			tables that are used by more than one of the exported external functions, but
80			which are not intended for use by external callers. Their names all begin with
81	nigel	83	"_pcre_", which hopefully will not provoke any name clashes. In some
82			environments, it is possible to control which external symbols are exported
83			when a shared library is built, and in these cases the undocumented symbols are
84			not exported.
85	nigel	77	</P>
86	nigel	63	<br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
87			<P>
88	nigel	75	The user documentation for PCRE comprises a number of different sections. In
89			the "man" format, each of these is a separate "man page". In the HTML format,
90			each is a separate page, linked from the index page. In the plain text format,
91	ph10	429	all the sections, except the <b>pcredemo</b> section, are concatenated, for ease
92			of searching. The sections are as follows:
93	nigel	63	<pre>
94			pcre this document
95	ph10	153	pcre-config show PCRE installation configuration information
96	nigel	77	pcreapi details of PCRE's native C API
97	nigel	63	pcrebuild options for building PCRE
98			pcrecallout details of the callout feature
99			pcrecompat discussion of Perl compatibility
100	nigel	77	pcrecpp details of the C++ wrapper
101	ph10	429	pcredemo a demonstration C program that uses PCRE
102	nigel	63	pcregrep description of the <b>pcregrep</b> command
103	nigel	77	pcrematching discussion of the two matching algorithms
104	nigel	75	pcrepartial details of the partial matching facility
105			pcrepattern syntax and semantics of supported regular expressions
106	nigel	63	pcreperform discussion of performance issues
107	nigel	77	pcreposix the POSIX-compatible C API
108	nigel	75	pcreprecompile details of saving and re-using precompiled patterns
109	ph10	429	pcresample discussion of the pcredemo program
110	nigel	91	pcrestack discussion of stack usage
111	ph10	461	pcresyntax quick syntax reference
112	nigel	75	pcretest description of the <b>pcretest</b> testing command
113			</pre>
114	nigel	63	In addition, in the "man" and HTML formats, there is a short page for each
115	nigel	77	C library function, listing its arguments and results.
116	nigel	63	</P>
117			<br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
118			<P>
119			There are some size limitations in PCRE but it is hoped that they will never in
120			practice be relevant.
121			</P>
122			<P>
123			The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
124			compiled with the default internal linkage size of 2. If you want to process
125			regular expressions that are truly enormous, you can compile PCRE with an
126			internal linkage size of 3 or 4 (see the <b>README</b> file in the source
127			distribution and the
128			<a href="pcrebuild.html"><b>pcrebuild</b></a>
129	nigel	75	documentation for details). In these cases the limit is substantially larger.
130	nigel	93	However, the speed of execution is slower.
131	nigel	63	</P>
132			<P>
133	ph10	208	All values in repeating quantifiers must be less than 65536.
134	nigel	63	</P>
135			<P>
136	nigel	93	There is no limit to the number of parenthesized subpatterns, but there can be
137			no more than 65535 capturing subpatterns.
138	nigel	63	</P>
139			<P>
140	nigel	93	The maximum length of name for a named subpattern is 32 characters, and the
141			maximum number of named subpatterns is 10000.
142	nigel	91	</P>
143			<P>
144	nigel	63	The maximum length of a subject string is the largest positive number that an
145	nigel	77	integer variable can hold. However, when using the traditional matching
146			function, PCRE uses recursion to handle subpatterns and indefinite repetition.
147			This means that the available stack space may limit the size of a subject
148	nigel	91	string that can be processed by certain patterns. For a discussion of stack
149			issues, see the
150			<a href="pcrestack.html"><b>pcrestack</b></a>
151			documentation.
152	nigel	75	<a name="utf8support"></a></P>
153			<br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
154	nigel	63	<P>
155	nigel	75	From release 3.3, PCRE has had some support for character strings encoded in
156			the UTF-8 format. For release 4.0 this was greatly extended to cover most
157			common requirements, and in release 5.0 additional support for Unicode general
158			category properties was added.
159	nigel	63	</P>
160			<P>
161			In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
162			the code, and, in addition, you must call
163			<a href="pcre_compile.html"><b>pcre_compile()</b></a>
164	ph10	416	with the PCRE_UTF8 option flag, or the pattern must start with the sequence
165			(*UTF8). When either of these is the case, both the pattern and any subject
166			strings that are matched against it are treated as UTF-8 strings instead of
167	ph10	461	strings of 1-byte characters.
168	nigel	63	</P>
169			<P>
170			If you compile PCRE with UTF-8 support, but do not use it at run time, the
171			library will be a bit bigger, but the additional run time overhead is limited
172	nigel	93	to testing the PCRE_UTF8 flag occasionally, so should not be very big.
173	nigel	63	</P>
174			<P>
175	nigel	75	If PCRE is built with Unicode character property support (which implies UTF-8
176			support), the escape sequences \p{..}, \P{..}, and \X are supported.
177			The available properties that can be tested are limited to the general
178			category properties such as Lu for an upper case letter or Nd for a decimal
179	nigel	87	number, the Unicode script names such as Arabic or Han, and the derived
180			properties Any and L&. A full list is given in the
181	nigel	75	<a href="pcrepattern.html"><b>pcrepattern</b></a>
182	nigel	87	documentation. Only the short names for properties are supported. For example,
183			\p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.
184			Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
185			compatibility with Perl 5.6. PCRE does not support this.
186	ph10	211	<a name="utf8strings"></a></P>
187			<br><b>
188			Validity of UTF-8 strings
189			</b><br>
190			<P>
191			When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
192			are (by default) checked for validity on entry to the relevant functions. From
193			release 7.3 of PCRE, the check is according the rules of RFC 3629, which are
194			themselves derived from the Unicode specification. Earlier releases of PCRE
195			followed the rules of RFC 2279, which allows the full range of 31-bit values (0
196			to 0x7FFFFFFF). The current check allows only values in the range U+0 to
197			U+10FFFF, excluding U+D800 to U+DFFF.
198	nigel	75	</P>
199			<P>
200	ph10	211	The excluded code points are the "Low Surrogate Area" of Unicode, of which the
201			Unicode Standard says this: "The Low Surrogate Area does not contain any
202			character assignments, consequently no character code charts or namelists are
203			provided for this area. Surrogates are reserved for use with UTF-16 and then
204			must be used in pairs." The code points that are encoded by UTF-16 pairs are
205			available as independent code points in the UTF-8 encoding. (In other words,
206			the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up
207			UTF-8.)
208	nigel	63	</P>
209			<P>
210	ph10	654	If an invalid UTF-8 string is passed to PCRE, an error return is given. At
211			compile time, the only additional information is the offset to the first byte
212			of the failing character. The runtime functions (<b>pcre_exec()</b> and
213			<b>pcre_dfa_exec()</b>), pass back this information as well as a more detailed
214			reason code if the caller has provided memory in which to do this.
215	nigel	63	</P>
216			<P>
217	ph10	654	In some situations, you may already know that your strings are valid, and
218			therefore want to skip these checks in order to improve performance. If you set
219			the PCRE_NO_UTF8_CHECK flag at compile time or at run time, PCRE assumes that
220			the pattern or subject it is given (respectively) contains only valid UTF-8
221			codes. In this case, it does not diagnose an invalid UTF-8 string.
222			</P>
223			<P>
224	ph10	211	If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
225			happens depends on why the string is invalid. If the string conforms to the
226			"old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
227			in the range 0 to 0x7FFFFFFF. In other words, apart from the initial validity
228			test, PCRE (when in UTF-8 mode) handles strings according to the more liberal
229			rules of RFC 2279. However, if the string does not even conform to RFC 2279,
230			the result is undefined. Your program may crash.
231			</P>
232			<P>
233			If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
234			encoded in a UTF-8-like manner as per the old RFC, you can set
235			PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
236			situation, you will have to apply your own validity check.
237			</P>
238			<br><b>
239			General comments about UTF-8 mode
240			</b><br>
241			<P>
242			1. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte
243	nigel	87	UTF-8 character if the value is greater than 127.
244	nigel	63	</P>
245			<P>
246	ph10	211	2. Octal numbers up to \777 are recognized, and match two-byte UTF-8
247	nigel	91	characters for values greater than \177.
248			</P>
249			<P>
250	ph10	211	3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
251	nigel	63	bytes, for example: \x{100}{3}.
252			</P>
253			<P>
254	ph10	211	4. The dot metacharacter matches one UTF-8 character instead of a single byte.
255	nigel	63	</P>
256			<P>
257	ph10	211	5. The escape sequence \C can be used to match a single byte in UTF-8 mode,
258	nigel	77	but its use can lead to some strange effects. This facility is not available in
259			the alternative matching function, <b>pcre_dfa_exec()</b>.
260	nigel	63	</P>
261			<P>
262	ph10	211	6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
263	ph10	518	test characters of any code value, but, by default, the characters that PCRE
264			recognizes as digits, spaces, or word characters remain the same set as before,
265			all with values less than 256. This remains true even when PCRE is built to
266			include Unicode property support, because to do otherwise would slow down PCRE
267	ph10	567	in many common cases. Note in particular that this applies to \b and \B,
268			because they are defined in terms of \w and \W. If you really want to test
269			for a wider sense of, say, "digit", you can use explicit Unicode property tests
270			such as \p{Nd}. Alternatively, if you set the PCRE_UCP option, the way that
271			the character escapes work is changed so that Unicode properties are used to
272			determine which characters match. There are more details in the section on
273	ph10	518	<a href="pcrepattern.html#genericchartypes">generic character types</a>
274			in the
275			<a href="pcrepattern.html"><b>pcrepattern</b></a>
276	ph10	535	documentation.
277	nigel	63	</P>
278			<P>
279	ph10	211	7. Similarly, characters that match the POSIX named character classes are all
280	ph10	518	low-valued characters, unless the PCRE_UCP option is set.
281	nigel	63	</P>
282			<P>
283	ph10	572	8. However, the horizontal and vertical whitespace matching escapes (\h, \H,
284			\v, and \V) do match all the appropriate Unicode characters, whether or not
285			PCRE_UCP is set.
286	ph10	182	</P>
287			<P>
288	ph10	211	9. Case-insensitive matching applies only to characters whose values are less
289	nigel	75	than 128, unless PCRE is built with Unicode property support. Even when Unicode
290			property support is available, PCRE still uses its own character tables when
291			checking the case of low-valued characters, so as not to degrade performance.
292			The Unicode property information is used only for characters with higher
293	ph10	572	values. Furthermore, PCRE supports case-insensitive matching only when there is
294			a one-to-one mapping between a letter's cases. There are a small number of
295			many-to-one mappings in Unicode; these are not supported by PCRE.
296	nigel	63	</P>
297			<br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
298			<P>
299	nigel	77	Philip Hazel
300	nigel	63	<br>
301	ph10	99	University Computing Service
302	nigel	63	<br>
303	nigel	93	Cambridge CB2 3QH, England.
304	ph10	99	<br>
305	nigel	77	</P>
306			<P>
307			Putting an actual email address here seems to have been a spam magnet, so I've
308	ph10	153	taken it away. If you want to email me, use my two initials, followed by the
309			two digits 10, at the domain cam.ac.uk.
310	ph10	99	</P>
311			<br><a name="SEC6" href="#TOC1">REVISION</a><br>
312			<P>
313	ph10	654	Last updated: 07 May 2011
314	nigel	63	<br>
315	ph10	654	Copyright © 1997-2011 University of Cambridge.
316	ph10	99	<br>
317	nigel	75	<p>
318			Return to the <a href="index.html">PCRE index page</a>.
319			</p>
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