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 providing options to configure before running the make com-
mand. However, the same options can be selected in both Unix-like and non-Unix-like envi-
ronments 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
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
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() function.
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 sup-
port 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 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
to the configure command. This implies UTF-8 support, even if you have not explicitly
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
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 adding
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 you specify
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.
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 of
to the configure command, as required.
POSIX MALLOC USAGE
When PCRE is called through the POSIX interface (see the pcreposix documentation), addi-
tional 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 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. Neverthe-
less, 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
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 backtracking by making recur-
sive 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 maxi-
mum stack size. There is a discussion in the pcrestack 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
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 rele-
vant for the the pcre_dfa_exec() function.
LIMITING PCRE RESOURCE USAGE
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 documentation. The default is 10 mil-
lion, 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 func-
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 con-
trols 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 time.
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_charta-
bles.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, cre-
ated 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
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 oper-
ating systems. PCRE can, however, be compiled to run in an EBCDIC environment by adding
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 main-
frame 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 relevant 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-licenced, so if you distrib-
ute 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. How-
ever, 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.
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Last updated: 17 March 2009
Copyright (c) 1997-2009 University of Cambridge.