PCREPRECOMPILE(3) Introduction to 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() 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.

SAVING A COMPILED PATTERN

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
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.

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 12 November 2013 PCREPRECOMPILE(3)