NAME

       PCRE - Perl-compatible regular expressions

PCRE DISCUSSION OF STACK USAGE

       When  you call pcre[16|32]_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|32]_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  successful,  and  the
       options  passed  to  pcre[16|32]_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|32]_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  assertion  and  "once-only" subpatterns, which are handled like subroutine calls. Normally, these are
       never very deep, and the limit on the complexity of pcre[16|32]_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|32]_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|32]_dfa_exec();  they  are  relevant  only   for
       pcre[16|32]_exec() without the JIT optimization.

   Reducing pcre[16|32]_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 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 "<" 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.

       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 for pcre[16|32]_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|32]_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|32]_stack_malloc and pcre[16|32]_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[16|32]_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|32]_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|32]_exec() is called. For
       details of these interfaces,  see  the  pcrebuild  documentation  and  the  section  on  extra  data  for
       pcre[16|32]_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|32]_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 optimization 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 command:

         pcretest -m -C

       The -C option causes pcretest to output information about the options with which PCRE was compiled.  When
       -m is also given (before -C), information about stack use is given in a line like this:

         Match recursion uses stack: approximate frame size = 640 bytes

       The value is approximate because some recursions need a bit more (up to perhaps 16 more bytes).

       If  the  above command is given when PCRE is compiled to use the heap 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 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[16|32]_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: 24 June 2012
       Copyright (c) 1997-2012 University of Cambridge.

PCRE 8.30                                         24 June 2012                                      PCRESTACK(3)