PCRE — Perl-compatible regular expressions
This document describes the optional features of PCRE that
can be selected when the library is compiled. It assumes use
of the configure script, where
the optional features are selected or deselected by providing
options to configure before
running the make command. However, the
same options can be selected in both Unix-like and
non-Unix-like environments using the GUI facility of
CMakeSetup if
you are using CMake instead of
configure to build PCRE.
The complete list of options for configure (which includes the standard ones
such as the selection of the installation directory) can be
obtained by running
./configure --help
The following sections include descriptions of options
whose names begin with --enable or --disable. These settings
specify changes to the defaults for the configure command. Because of the way that
configure works, --enable and
--disable always come in pairs, so the complementary option
always exists as well, but as it specifies the default, it is
not described.
By default, the configure
script will search for a C++ compiler and C++ header files.
If it finds them, it automatically builds the C++ wrapper
library for PCRE. You can disable this by adding
--disable-cpp
to the configure
command.
To build PCRE with support for UTF-8 character strings, add
--enable-utf8
to the configure command. Of
itself, this does not make PCRE treat strings as UTF-8. As
well as compiling PCRE with this option, you also have have
to set the PCRE_UTF8 option when you call the pcre_compile() function.
UTF-8 support allows PCRE to process character values greater than 255 in the strings that it handles. On its own, however, it does not provide any facilities for accessing the properties of such characters. If you want to be able to use the pattern escapes \P, \p, and \X, which refer to Unicode character properties, you must add
--enable-unicode-properties
to the configure command.
This implies UTF-8 support, even if you have not explicitly
requested it.
Including Unicode property support adds around 30K of
tables to the PCRE library. Only the general category
properties such as Lu and Nd are supported. Details are
given in the pcrepattern(3)
documentation.
By default, PCRE interprets character 10 (linefeed, LF) as indicating the end of a line. This is the normal newline character on Unix-like systems. You can compile PCRE to use character 13 (carriage return, CR) instead, by adding
--enable-newline-is-cr
to the configure command.
There is also a --enable-newline-is-lf option, which
explicitly specifies linefeed as the newline character.
Alternatively, you can specify that line endings are to be indicated by the two character sequence CRLF. If you want this, add
--enable-newline-is-crlf
to the configure command.
There is a fourth option, specified by
--enable-newline-is-anycrlf
which causes PCRE to recognize any of the three sequences CR, LF, or CRLF as indicating a line ending. Finally, a fifth option, specified by
--enable-newline-is-any
causes PCRE to recognize any Unicode newline sequence.
Whatever line ending convention is selected when PCRE is built can be overridden when the library functions are called. At build time it is conventional to use the standard for your operating system.
By default, the sequence \R in a pattern matches any Unicode newline sequence, whatever has been selected as the line ending sequence. If you specify
--enable-bsr-anycrlf
the default is changed so that \R matches only CR, LF, or CRLF. Whatever is selected when PCRE is built can be overridden when the library functions are called.
The PCRE building process uses libtool to build both shared and static Unix libraries by default. You can suppress one of these by adding one of
--disable-shared --disable-static
to the configure command, as
required.
When PCRE is called through the POSIX interface (see the
pcreposix(3)
documentation), additional working storage is required for
holding the pointers to capturing substrings, because PCRE
requires three integers per substring, whereas the POSIX
interface provides only two. If the number of expected
substrings is small, the wrapper function uses space on the
stack, because this is faster than using malloc() for each call. The default
threshold above which the stack is no longer used is 10; it
can be changed by adding a setting such as
--with-posix-malloc-threshold=20
to the configure
command.
Within a compiled pattern, offset values are used to point from one part to another (for example, from an opening parenthesis to an alternation metacharacter). By default, two-byte values are used for these offsets, leading to a maximum size for a compiled pattern of around 64K. This is sufficient to handle all but the most gigantic patterns. Nevertheless, some people do want to process enormous patterns, so it is possible to compile PCRE to use three-byte or four-byte offsets by adding a setting such as
--with-link-size=3
to the configure command.
The value given must be 2, 3, or 4. Using longer offsets
slows down the operation of PCRE because it has to load
additional bytes when handling them.
When matching with the pcre_exec() function, PCRE implements
backtracking by making recursive calls to an internal
function called match(). In
environments where the size of the stack is limited, this can
severely limit PCRE's operation. (The Unix environment does
not usually suffer from this problem, but it may sometimes be
necessary to increase the maximum stack size. There is a
discussion in the pcrestack(3)
documentation.) An alternative approach to recursion that
uses memory from the heap to remember data, instead of using
recursive function calls, has been implemented to work round
the problem of limited stack size. If you want to build a
version of PCRE that works this way, add
--disable-stack-for-recursion
to the configure command.
With this configuration, PCRE will use the pcre_stack_malloc and pcre_stack_free variables to call memory
management functions. By default these point to malloc() and free(), but you can replace the pointers so
that your own functions are used.
Separate functions are provided rather than using
pcre_malloc and pcre_free because the usage is very
predictable: the block sizes requested are always the same,
and the blocks are always freed in reverse order. A calling
program might be able to implement optimized functions that
perform better than malloc()
and free(). PCRE runs
noticeably more slowly when built in this way. This option
affects only the pcre_exec()
function; it is not relevant for the the pcre_dfa_exec() function.
Internally, PCRE has a function called match(), which it calls repeatedly
(sometimes recursively) when matching a pattern with the
pcre_exec() function. By
controlling the maximum number of times this function may be
called during a single matching operation, a limit can be
placed on the resources used by a single call to pcre_exec(). The limit can be changed at
run time, as described in the pcreapi(3) documentation.
The default is 10 million, but this can be changed by adding
a setting such as
--with-match-limit=500000
to the configure command.
This setting has no effect on the pcre_dfa_exec() matching function.
In some environments it is desirable to limit the depth of
recursive calls of match() more
strictly than the total number of calls, in order to restrict
the maximum amount of stack (or heap, if
--disable-stack-for-recursion is specified) that is used. A
second limit controls this; it defaults to the value that is
set for --with-match-limit, which imposes no additional
constraints. However, you can set a lower limit by adding,
for example,
--with-match-limit-recursion=10000
to the configure command.
This value can also be overridden at run time.
PCRE uses fixed tables for processing characters whose
code values are less than 256. By default, PCRE is built with
a set of tables that are distributed in the file pcre_chartables.c.dist. These
tables are for ASCII codes only. If you add
--enable-rebuild-chartables
to the configure command,
the distributed tables are no longer used. Instead, a program
called dftables
is compiled and run. This outputs the source for new set of
tables, created in the default locale of your C runtime
system. (This method of replacing the tables does not work if
you are cross compiling, because dftables is run on the
local host. If you need to create alternative tables when
cross compiling, you will have to do so "by hand".)
PCRE assumes by default that it will run in an environment where the character code is ASCII (or Unicode, which is a superset of ASCII). This is the case for most computer operating systems. PCRE can, however, be compiled to run in an EBCDIC environment by adding
--enable-ebcdic
to the configure command.
This setting implies --enable-rebuild-chartables. You should
only use it if you know that you are in an EBCDIC environment
(for example, an IBM mainframe operating system).
By default, pcregrep reads all files as
plain text. You can build it so that it recognizes files
whose names end in .gz or .bz2, and reads them with
libz or libbz2, respectively, by adding one or both
of
--enable-pcregrep-libz --enable-pcregrep-libbz2
to the configure command.
These options naturally require that the relevant libraries
are installed on your system. Configuration will fail if they
are not.
If you add
--enable-pcretest-libreadline
to the configure command,
pcretest is
linked with the libreadline
library, and when its input is from a terminal, it reads it
using the readline() function.
This provides line-editing and history facilities. Note that
libreadline is GPL-licenced, so
if you distribute a binary of pcretest linked in this
way, there may be licensing issues.
Last updated: 18 December 2007 Copyright (c) 1997-2007 University of Cambridge.
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This manual page is taken from the PCRE library, which is distributed under the BSD license. |