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
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
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
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
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).
Last updated: 21 September 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. |