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PCRE(3) PCRE(3)
NAME
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. They are all selected, or deselected, by providing options to
the configure script that is run before the make command. 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 describe certain options whose names begin with --enable or
--disable. These settings specify changes to the defaults for the configure com-
mand. 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.
UTF-8 SUPPORT
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.
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
--enable-unicode-properties
to the configure command. This implies UTF-8 support, even if you have not explic-
itly requested it.
Including Unicode property support adds around 90K of tables to the PCRE library,
approximately doubling its size. 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 treats character 10 (linefeed) as the newline character. This is
the normal newline character on Unix-like systems. You can compile PCRE to use
character 13 (carriage return) instead by adding
--enable-newline-is-cr
to the configure command. For completeness there is also a --enable-newline-is-lf
option, which explicitly specifies linefeed as the newline character.
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
--disable-shared
--disable-static
to the configure command, as required.
POSIX MALLOC USAGE
When PCRE is called through the POSIX interface (see the pcreposix documentation),
additional working storage is required for holding the pointers to capturing sub-
strings, 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 mal-
loc() 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.
LIMITING PCRE RESOURCE USAGE
Internally, PCRE has a function called match(), which it calls repeatedly (possibly
recursively) when matching a pattern. 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
million, but this can be changed by adding a setting such as
--with-match-limit=500000
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. 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 han-
dling them.
If you build PCRE with an increased link size, test 2 (and test 5 if you are using
UTF-8) will fail. Part of the output of these tests is a representation of the com-
piled pattern, and this changes with the link size.
AVOIDING EXCESSIVE STACK USAGE
PCRE implements backtracking while matching by making recursive calls to an inter-
nal function called match(). In environments where the size of the stack is lim-
ited, this can severely limit PCRE’s operation. (The Unix environment does not usu-
ally suffer from this problem.) An alternative approach that uses memory from the
heap to remember data, instead of using recursive function calls, has been imple-
mented to work round this problem. 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. Separate functions are provided 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 the standard malloc() and free() functions. PCRE runs
noticeably more slowly when built in this way.
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). PCRE can, however, be compiled
to run in an EBCDIC environment by adding
--enable-ebcdic
to the configure command.
Last updated: 09 September 2004
Copyright (c) 1997-2004 University of Cambridge.
PCRE(3)
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