PCRE DISCUSSION OF STACK USAGE

       When  you call pcre_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  pro-
       ceeds  deeper  and deeper into the tree of possibilities, the recursion
       depth increases.

       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 pcre_dfa_exec() function operates in an entirely different way, and
       hardly uses recursion at all. The limit on its complexity is the amount
       of  workspace  it  is  given.  The comments that follow do NOT apply to
       pcre_dfa_exec(); they are relevant only for pcre_exec().

       You can set limits on the number of times that match() is called,  both
       in  total  and  recursively. If the limit is exceeded, an error occurs.
       For details, see the section on  extra  data  for  pcre_exec()  in  the
       pcreapi documentation.

       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:

         ([^<]|<(?!inet))+

       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
       either  one  character that is not "<" or a "<" that is not followed by
       "inet". However, each time a  parenthesis  is  processed,  a  recursion
       occurs, so this formulation uses a stack frame for each matched charac-
       ter. For a long string, a lot of stack is required. Consider  now  this
       rewritten pattern, which matches exactly the same strings:

         ([^<]++|<(?!inet))

       This  uses very much less stack, because runs of characters that do not
       contain "<" are "swallowed" in one item inside the parentheses.  Recur-
       sion  happens  only when a "<" character that is not followed by "inet"
       is encountered (and we assume this is relatively  rare).  A  possessive
       quantifier  is  used  to stop any backtracking into the runs of non-"<"
       characters, but that is not related to stack usage.
       an error message is given. You can normally 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_exec().

       PCRE has an internal counter that can be used to  limit  the  depth  of
       recursion,  and  thus cause pcre_exec() to give an error code before it
       runs out of stack. By default, the limit is very  large,  and  unlikely
       ever  to operate. It can be changed when PCRE is built, and it can also
       be set when pcre_exec() is called. For details of these interfaces, see
       the pcrebuild and 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. The pcretest  test  program
       has  a command line option (-S) that can be used to increase its stack.

Last updated: 29 June 2006
Copyright (c) 1997-2006 University of Cambridge.



                                                                  PCRESTACK(3)