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.