PCRE - Perl-compatible regular expressions
PCRE BUILD-TIME OPTIONS
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 pro‐
viding 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 cmake-gui if you are using CMake
instead of configure to build PCRE.
There is a lot more information about building PCRE in non-Unix-like
environments in the file called NON_UNIX_USE, which is part of the PCRE
distribution. You should consult this file as well as the README file
if you are building in a non-Unix-like environment.
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
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 complemen‐
tary 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
to the configure command.
To build PCRE with support for UTF-8 Unicode character strings, add
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()
or pcre_compile2() functions.
If you set --enable-utf8 when compiling in an EBCDIC environment, PCRE
expects its input to be either ASCII or UTF-8 (depending on the runtime
option). It is not possible to support both EBCDIC and UTF-8 codes in
the same version of the library. Consequently, --enable-utf8 and
--enable-ebcdic are mutually exclusive.
UNICODE CHARACTER PROPERTY SUPPORT
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 pro‐
vide 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
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 documentation.
CODE VALUE OF NEWLINE
By default, PCRE interprets the linefeed (LF) character as indicating
the end of a line. This is the normal newline character on Unix-like
systems. You can compile PCRE to use carriage return (CR) instead, by
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
to the configure command. There is a fourth option, specified by
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
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.
WHAT \R MATCHES
By default, the sequence \R in a pattern matches any Unicode newline
sequence, whatever has been selected as the line ending sequence. If
the default is changed so that \R matches only CR, LF, or CRLF. What‐
ever is selected when PCRE is built can be overridden when the library
functions are called.
BUILDING SHARED AND STATIC LIBRARIES
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
to the configure command, as required.
POSIX MALLOC USAGE
When PCRE is called through the POSIX interface (see the pcreposix doc‐
umentation), 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
to the configure command.
HANDLING VERY LARGE PATTERNS
Within a compiled pattern, offset values are used to point from one
part to another (for example, from an opening parenthesis to an alter‐
nation 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 truyl enormous patterns,
so it is possible to compile PCRE to use three-byte or four-byte off‐
sets by adding a setting such as
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.
AVOIDING EXCESSIVE STACK USAGE
When matching with the pcre_exec() function, PCRE implements backtrack‐
ing by making recursive calls to an internal function called match().
In environments where the size of the stack is limited, this can se‐
verely 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 docu‐
mentation.) 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
to the configure command. With this configuration, PCRE will use the
pcre_stack_malloc and pcre_stack_free variables to call memory manage‐
ment functions. By default these point to malloc() and free(), but you
can replace the pointers so that your own functions are used instead.
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 pcre_dfa_exec().
LIMITING PCRE RESOURCE USAGE
Internally, PCRE has a function called match(), which it calls repeat‐
edly (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 documen‐
tation. The default is 10 million, but this can be changed by adding a
setting such as
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,
to the configure command. This value can also be overridden at run
CREATING CHARACTER TABLES AT BUILD 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
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".)
USING EBCDIC CODE
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, how‐
ever, be compiled to run in an EBCDIC environment by adding
to the configure command. This setting implies --enable-rebuild-charta‐
bles. You should only use it if you know that you are in an EBCDIC
environment (for example, an IBM mainframe operating system). The
--enable-ebcdic option is incompatible with --enable-utf8.
PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT
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
to the configure command. These options naturally require that the rel‐
evant libraries are installed on your system. Configuration will fail
if they are not.
PCRETEST OPTION FOR LIBREADLINE SUPPORT
If you add
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-licensed, so if you distribute a binary of
pcretest linked in this way, there may be licensing issues.
Setting this option causes the -lreadline option to be added to the
pcretest build. In many operating environments with a sytem-installed
libreadline this is sufficient. However, in some environments (e.g. if
an unmodified distribution version of readline is in use), some extra
configuration may be necessary. The INSTALL file for libreadline says
"Readline uses the termcap functions, but does not link with the
termcap or curses library itself, allowing applications which link
with readline the to choose an appropriate library."
If your environment has not been set up so that an appropriate library
is automatically included, you may need to add something like
immediately before the configure command.
SEE ALSOpcreapi(3), pcre_config(3).
University Computing Service
Cambridge CB2 3QH, England.
Last updated: 29 September 2009
Copyright (c) 1997-2009 University of Cambridge.