When  you  call  pcre[16]_exec(),  it makes use of an internal function
       called match(). This calls itself recursively at branch points  in  the
       pattern,  in  order  to  remember the state of the match so that it can
       back up and try a different alternative if  the  first  one  fails.  As
       matching proceeds deeper and deeper into the tree of possibilities, the
       recursion depth increases. The match() function is also called in other
       circumstances,  for  example,  whenever  a parenthesized sub-pattern is
       entered, and in certain cases of repetition.

       Not all calls of match() increase the recursion depth; for an item such
       as  a* it may be called several times at the same level, after matching
       different numbers of a's. Furthermore, in a number of cases  where  the
       result  of  the  recursive call would immediately be passed back as the
       result of the current call (a "tail recursion"), the function  is  just
       restarted instead.

       The  above  comments  apply  when  pcre[16]_exec() is run in its normal
       interpretive  manner.   If   the   pattern   was   studied   with   the
       PCRE_STUDY_JIT_COMPILE  option, and just-in-time compiling was success-
       ful, and the options passed to pcre[16]_exec() were  not  incompatible,
       the  matching process uses the JIT-compiled code instead of the match()
       function. In this case, the memory requirements  are  handled  entirely
       differently. See the pcrejit documentation for details.

       The pcre[16]_dfa_exec() function operates in an entirely different way,
       and uses recursion only when there is a regular expression recursion or
       subroutine  call in the pattern. This includes the processing of asser-
       tion and "once-only" subpatterns, which  are  handled  like  subroutine
       calls.  Normally,  these are never very deep, and the limit on the com-
       plexity of pcre[16]_dfa_exec() is controlled by the amount of workspace
       it  is  given.   However, it is possible to write patterns with runaway
       infinite recursions; such patterns will  cause  pcre[16]_dfa_exec()  to
       run out of stack. At present, there is no protection against this.

       The  comments that follow do NOT apply to pcre[16]_dfa_exec(); they are
       relevant only for pcre[16]_exec() without the JIT optimization.

   Reducing pcre[16]_exec()'s stack usage

       Each time that match() is actually called recursively, it  uses  memory
       from  the  process  stack.  For certain kinds of pattern and data, very
       large amounts of stack may be needed, despite the recognition of  "tail
       recursion".   You  can often reduce the amount of recursion, and there-
       fore the amount of stack used, by modifying the pattern that  is  being
       matched. Consider, for example, this pattern:


       It  matches  from wherever it starts until it encounters "<inet" or the
       end of the data, and is the kind of pattern that  might  be  used  when
       processing an XML file. Each iteration of the outer parentheses matches
       quantifier is used to stop any backtracking into the  runs  of  non-"<"
       characters, but that is not related to stack usage.

       This  example shows that one way of avoiding stack problems when match-
       ing long subject strings is to write repeated parenthesized subpatterns
       to match more than one character whenever possible.

   Compiling PCRE to use heap instead of stack for pcre[16]_exec()

       In  environments  where  stack memory is constrained, you might want to
       compile PCRE to use heap memory instead of stack for remembering  back-
       up points when pcre[16]_exec() is running. This makes it run a lot more
       slowly, however.  Details of how to do this are given in the  pcrebuild
       documentation. When built in this way, instead of using the stack, PCRE
       obtains and frees memory by calling the functions that are  pointed  to
       by  the  pcre[16]_stack_malloc  and  pcre[16]_stack_free  variables. By
       default, these point to malloc() and free(), but you  can  replace  the
       pointers to cause PCRE to use your own functions. Since the block sizes
       are always the same, and are always freed in reverse order, it  may  be
       possible  to  implement  customized memory handlers that are more effi-
       cient than the standard functions.

   Limiting pcre[16]_exec()'s stack usage

       You can set limits on the number of times that match() is called,  both
       in  total  and  recursively.  If  a  limit is exceeded, pcre[16]_exec()
       returns an error code. Setting suitable limits should prevent  it  from
       running  out of stack. The default values of the limits are very large,
       and unlikely ever to operate. They can be changed when PCRE  is  built,
       and they can also be set when pcre[16]_exec() is called. For details of
       these interfaces, see the pcrebuild documentation and  the  section  on
       extra data for pcre[16]_exec() in the pcreapi documentation.

       As a very rough rule of thumb, you should reckon on about 500 bytes per
       recursion. Thus, if you want to limit your  stack  usage  to  8Mb,  you
       should  set  the  limit at 16000 recursions. A 64Mb stack, on the other
       hand, can support around 128000 recursions.

       In Unix-like environments, the pcretest test program has a command line
       option (-S) that can be used to increase the size of its stack. As long
       as the stack is large enough, another option (-M) can be used  to  find
       the  smallest  limits  that allow a particular pattern to match a given
       subject string. This is done by calling pcre[16]_exec() repeatedly with
       different limits.

   Obtaining an estimate of stack usage

       The  actual  amount  of  stack used per recursion can vary quite a lot,
       depending on the compiler that was used to build PCRE and the optimiza-
       tion or debugging options that were set for it. The rule of thumb value
       of 500 bytes mentioned above may be larger  or  smaller  than  what  is
       actually needed. A better approximation can be obtained by running this

       instead of the stack for recursion, the value that  is  output  is  the
       size of each block that is obtained from the heap.

   Changing stack size in Unix-like systems

       In  Unix-like environments, there is not often a problem with the stack
       unless very long strings are involved,  though  the  default  limit  on
       stack  size  varies  from system to system. Values from 8Mb to 64Mb are
       common. You can find your default limit by running the command:

         ulimit -s

       Unfortunately, the effect of running out of  stack  is  often  SIGSEGV,
       though  sometimes  a more explicit error message is given. You can nor-
       mally increase the limit on stack size by code such as this:

         struct rlimit rlim;
         getrlimit(RLIMIT_STACK, &rlim);
         rlim.rlim_cur = 100*1024*1024;
         setrlimit(RLIMIT_STACK, &rlim);

       This reads the current limits (soft and hard) using  getrlimit(),  then
       attempts  to  increase  the  soft limit to 100Mb using setrlimit(). You
       must do this before calling pcre[16]_exec().

   Changing stack size in Mac OS X

       Using setrlimit(), as described above, should also work on Mac OS X. It
       is also possible to set a stack size when linking a program. There is a
       discussion  about  stack  sizes  in  Mac  OS  X  at  this   web   site:


       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


       Last updated: 21 January 2012
       Copyright (c) 1997-2012 University of Cambridge.

PCRE 8.30                       21 January 2012                   PCRESTACK(3)
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