pcreprecompile(3)
PCREPRECOMPILE(3) C LIBRARY FUNCTIONS PCREPRECOMPILE(3)
NAME
PCRE - Perl-compatible regular expressions
SAVING AND RE-USING PRECOMPILED PCRE PATTERNS
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() documen-
tation), 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 func-
tions return PCRE_ERROR_BADENDIANNESS if they detect a pat-
tern with the wrong endianness. Compiling regular expres-
sions with one version of PCRE for use with a different ver-
sion is not guaranteed to work and may cause crashes, and
saving and restoring a compiled pattern loses any JIT optim-
ization data.
SAVING A COMPILED PATTERN
The value returned by pcre[16|32]_compile() points to a sin-
gle 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 appropri-
ate 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 pat-
tern are copied exactly. Note that this is binary data that
may contain any of the 256 possible byte values. On systems
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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, preced-
ing 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 simi-
lar 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 try-
ing to save the study data.
RE-USING A PRECOMPILED PATTERN
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 non-
sense. 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 com-
piled, 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
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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 func-
tion 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.
COMPATIBILITY WITH DIFFERENT PCRE RELEASES
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.
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 12 November 2013
Copyright (c) 1997-2013 University of Cambridge.
PCRE 8.34 Last change: 12 November 2013 3
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