Provided by: libpcre3-dev_8.39-12_amd64 bug

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.