PCREPRECOMPILE(3)         OpenBSD Programmer's Manual        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]_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]_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]_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]_study() returns a pointer to a pcre[16]_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]_extra block itself).
       The length of the study data can be obtained by calling
       pcre[16]_fullinfo() with an argument of PCRE_INFO_STUDYSIZE. Remember
       to check that pcre[16]_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]_pattern_to_host_byte_order() if
       necessary, you pass its pointer to pcre[16]_exec() or
       pcre[16]_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]_compile()), you
       must now pass a similar pointer to pcre[16]_exec() or
       pcre[16]_dfa_exec(), because the value saved with the compiled pattern
       will obviously be nonsense. A field in a pcre[16]_extra() block is used
       to pass this data, as described in the section on matching a pattern in
       the pcreapi 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]_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]_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: 10 January 2012
       Copyright (c) 1997-2012 University of Cambridge.



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