PCREPRECOMPILE(3)                                            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.

       If you save compiled patterns to a file, you can copy them to a
       different host and run them there. This works even if the new host has
       the opposite endianness to the one on which the patterns were compiled.
       There may be a small performance penalty, but it should be
       insignificant. However, compiling regular expressions with one version
       of PCRE for use with a different version is not guaranteed to work and
       may cause crashes.

SAVING A COMPILED PATTERN

       The value returned by pcre_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_fullinfo() with an
       argument of PCRE_INFO_SIZE. You can then save the data in any
       appropriate manner. Here is sample code 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 study
       data in a similar way to the compiled pattern itself. When studying
       generates additional information, pcre_study() returns a pointer to a
       pcre_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_extra block itself). The length of the study data can be obtained
       by calling pcre_fullinfo() with an argument of PCRE_INFO_STUDYSIZE.
       Remember to check that pcre_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, you pass its pointer to pcre_exec() or
       pcre_dfa_exec() in the usual way. This should work even on another
       host, and even if that host has the opposite endianness to the one
       where the pattern was compiled.

       However, if you passed a pointer to custom character tables when the
       pattern was compiled (the tableptr argument of pcre_compile()), you
       must now pass a similar pointer to pcre_exec() or pcre_dfa_exec(),
       because the value saved with the compiled pattern will obviously be
       nonsense. A field in a pcre_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
       pcre_exec() 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_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_extra block to pcre_exec() or pcre_dfa_exec() in the
       usual way.

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: 17 November 2010
       Copyright (c) 1997-2010 University of Cambridge.



                                                             PCREPRECOMPILE(3)
