.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. There is also some

support for certain .NET and Oniguruma syntax items, and there is an option for

requesting some minor changes that give better JavaScript compatibility.

.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. There is a syntax summary in the

.\" HREF

\fBpcresyntax\fR

.\"

page.

.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

  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

  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

  pcresyntax        quick syntax reference

  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.

.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.

.

.\" 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.

.

.\" HTML <a name="utf8strings"></a>

.

.SS "Validity of UTF-8 strings"

.rs

.sp

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

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

If an invalid UTF-8 string is passed to PCRE, an error return

(PCRE_ERROR_BADUTF8) 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.

.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

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.

.

.SS "General comments about UTF-8 mode"

.rs

.sp

1. An unbraced hexadecimal escape sequence (such as \exb3) matches a two-byte

UTF-8 character if the value is greater than 127.

.P

2. Octal numbers up to \e777 are recognized, and match two-byte UTF-8

characters for values greater than \e177.

.P

3. Repeat quantifiers apply to complete UTF-8 characters, not to individual

bytes, for example: \ex{100}{3}.

.P

4. The dot metacharacter matches one UTF-8 character instead of a single byte.

.P

5. 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

6. 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

7. Similarly, characters that match the POSIX named character classes are all

low-valued characters.

.P

8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes

(\eh, \eH, \ev, and \eV) do match all the appropriate Unicode 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 two initials, followed by the

two digits 10, at the domain cam.ac.uk.

.

.

.SH REVISION

.rs

.sp

.nf

Last updated: 12 April 2008

Copyright (c) 1997-2008 University of Cambridge.

.fi