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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()(3) 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(3) 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(3)
documentation.
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.
Last updated: 24 June 2012 Copyright (c) 1997-2012 University of Cambridge.
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This manual page is taken from the PCRE library, which is distributed under the BSD license. |