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PCRESTACK(3)									     PCRESTACK(3)

NAME
       PCRE - Perl-compatible regular expressions

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 proceeds 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.  Further-
       more,  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 recur-
       sion at all. The limit on its complexity is the amount of workspace it is given. The  com-
       ments 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 recur-
       sively. 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 recur-
       sion, and therefore 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 character. 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. Recursion happens only when a "<"  charac-
       ter 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-"<"  charac-
       ters, but that is not related to stack usage.

       This  example  shows  that  one	way of avoiding stack problems when matching 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

       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. 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_stack_malloc and pcre_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 efficient than the standard functions.

   Limiting PCRE's stack usage

       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 pcre-
       build 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  recur-
       sions. 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 the size  of  its
       stack.

   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 com-
       mand:

	 ulimit -s

       Unfortunately, the effect of running out of stack is often  SIGSEGV,  though  sometimes	a
       more explicit 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().

   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: http://developer.apple.com/qa/qa2005/qa1419.html.

AUTHOR

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

REVISION

       Last updated: 09 July 2008
       Copyright (c) 1997-2008 University of Cambridge.

										     PCRESTACK(3)
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