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NAME | SAVING AND RE-USING PRECOMPILED PCRE PATTERNS | SAVING A COMPILED PATTERN | RE-USING A PRECOMPILED PATTERN | COMPATIBILITY WITH DIFFERENT PCRE RELEASES | AUTHOR | REVISION | COLOPHON |
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PCREPRECOMPILE(3) Library Functions Manual PCREPRECOMPILE(3)
PCRE - Perl-compatible regular expressions
If you are running an application that uses a large number of
regular expression patterns, it may be useful to store them in a
precompiled form instead of having to compile them every time the
application is run. If you are not using any private character
tables (see the pcre_maketables() documentation), this is
relatively straightforward. If you are using private tables, it
is a little bit more complicated. However, if you are using the
just-in-time optimization feature, it is not possible to save and
reload the JIT data.
If you save compiled patterns to a file, you can copy them to a
different host and run them there. If the two hosts have
different endianness (byte order), you should run the
pcre[16|32]_pattern_to_host_byte_order() function on the new host
before trying to match the pattern. The matching functions return
PCRE_ERROR_BADENDIANNESS if they detect a pattern with the wrong
endianness.
Compiling regular expressions with one version of PCRE for use
with a different version is not guaranteed to work and may cause
crashes, and saving and restoring a compiled pattern loses any
JIT optimization data.
The value returned by pcre[16|32]_compile() points to a single
block of memory that holds the compiled pattern and associated
data. You can find the length of this block in bytes by calling
pcre[16|32]_fullinfo() with an argument of PCRE_INFO_SIZE. You
can then save the data in any appropriate manner. Here is sample
code for the 8-bit library that compiles a pattern and writes it
to a file. It assumes that the variable fd refers to a file that
is open for output:
int erroroffset, rc, size;
char *error;
pcre *re;
re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL);
if (re == NULL) { ... handle errors ... }
rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size);
if (rc < 0) { ... handle errors ... }
rc = fwrite(re, 1, size, fd);
if (rc != size) { ... handle errors ... }
In this example, the bytes that comprise the compiled pattern are
copied exactly. Note that this is binary data that may contain
any of the 256 possible byte values. On systems that make a
distinction between binary and non-binary data, be sure that the
file is opened for binary output.
If you want to write more than one pattern to a file, you will
have to devise a way of separating them. For binary data,
preceding each pattern with its length is probably the most
straightforward approach. Another possibility is to write out the
data in hexadecimal instead of binary, one pattern to a line.
Saving compiled patterns in a file is only one possible way of
storing them for later use. They could equally well be saved in a
database, or in the memory of some daemon process that passes
them via sockets to the processes that want them.
If the pattern has been studied, it is also possible to save the
normal study data in a similar way to the compiled pattern
itself. However, if the PCRE_STUDY_JIT_COMPILE was used, the
just-in-time data that is created cannot be saved because it is
too dependent on the current environment. When studying generates
additional information, pcre[16|32]_study() returns a pointer to
a pcre[16|32]_extra data block. Its format is defined in the
section on matching a pattern in the pcreapi documentation. The
study_data field points to the binary study data, and this is
what you must save (not the pcre[16|32]_extra block itself). The
length of the study data can be obtained by calling
pcre[16|32]_fullinfo() with an argument of PCRE_INFO_STUDYSIZE.
Remember to check that pcre[16|32]_study() did return a non-NULL
value before trying to save the study data.
Re-using a precompiled pattern is straightforward. Having
reloaded it into main memory, called
pcre[16|32]_pattern_to_host_byte_order() if necessary, you pass
its pointer to pcre[16|32]_exec() or pcre[16|32]_dfa_exec() in
the usual way.
However, if you passed a pointer to custom character tables when
the pattern was compiled (the tableptr argument of
pcre[16|32]_compile()), you must now pass a similar pointer to
pcre[16|32]_exec() or pcre[16|32]_dfa_exec(), because the value
saved with the compiled pattern will obviously be nonsense. A
field in a pcre[16|32]_extra() block is used to pass this data,
as described in the section on matching a pattern in the pcreapi
documentation.
Warning: The tables that pcre_exec() and pcre_dfa_exec() use must
be the same as those that were used when the pattern was
compiled. If this is not the case, the behaviour is undefined.
If you did not provide custom character tables when the pattern
was compiled, the pointer in the compiled pattern is NULL, which
causes the matching functions to use PCRE's internal tables.
Thus, you do not need to take any special action at run time in
this case.
If you saved study data with the compiled pattern, you need to
create your own pcre[16|32]_extra data block and set the
study_data field to point to the reloaded study data. You must
also set the PCRE_EXTRA_STUDY_DATA bit in the flags field to
indicate that study data is present. Then pass the
pcre[16|32]_extra block to the matching function in the usual
way. If the pattern was studied for just-in-time optimization,
that data cannot be saved, and so is lost by a save/restore
cycle.
In general, it is safest to recompile all saved patterns when you
update to a new PCRE release, though not all updates actually
require this.
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
Last updated: 12 November 2013
Copyright (c) 1997-2013 University of Cambridge.
This page is part of the PCRE (Perl Compatible Regular
Expressions) project. Information about the project can be found
at ⟨http://www.pcre.org/⟩. If you have a bug report for this
manual page, see
⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩. This page was
obtained from the tarball pcre-8.45.tar.gz fetched from
⟨ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/⟩ on
2021-08-27. If you discover any rendering problems in this HTML
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improvements to the information in this COLOPHON (which is not
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man-pages@man7.org
PCRE 8.34 12 November 2013 PCREPRECOMPILE(3)
Pages that refer to this page: pcretest(1), pcreapi(3)
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