Provided by: libpcre2-dev_10.21-1_amd64 bug


       PCRE2 - Perl-compatible regular expressions (revised API)


       When  you  call  pcre2_match(),  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 after a failure. 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.

       Each time the internal match() function is 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". Note that if PCRE2 is  compiled  with
       the  -fsanitize=address  option  of  the  GCC compiler, the stack requirements are greatly

       The above comments apply when pcre2_match() is run in its normal interpretive  manner.  If
       the  compiled pattern was processed by pcre2_jit_compile(), and just-in-time compiling was
       successful, and the options passed to pcre2_match() 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 pcre2jit  documentation  for

       The  pcre2_dfa_match()  function  operates  in  a different way to pcre2_match(), 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  pcre2_dfa_match()  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  pcre2_dfa_match()  to  run  out  of stack. At present, there is no protection
       against this.

       The comments that follow do NOT apply to pcre2_dfa_match(); they  are  relevant  only  for
       pcre2_match() without the JIT optimization.

   Reducing pcre2_match()'s stack usage

       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:


       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


       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 PCRE2 to use heap instead of stack for pcre2_match()

       In environments where stack memory is constrained, you might want to compile PCRE2 to  use
       heap memory instead of stack for remembering back-up points when pcre2_match() is running.
       This makes it run more slowly, however. Details of  how  to  do  this  are  given  in  the
       pcre2build  documentation.  When built in this way, instead of using the stack, PCRE2 gets
       memory for remembering backup points from the heap. By default, the memory is obtained  by
       calling  the  system  malloc()  function,  but  you  can arrange to supply your own memory
       management function. For details, see the section entitled  "The  match  context"  in  the
       pcre2api  documentation.  Since the block sizes are always the same, it may be possible to
       implement customized a memory handler that is more efficient than the  standard  function.
       The  memory  blocks  obtained  for this purpose are retained and re-used if possible while
       pcre2_match() is running. They are all freed just before it exits.

   Limiting pcre2_match()'s stack usage

       You can set limits on the number of times the internal match() function is called, both in
       total  and  recursively.  If  a  limit  is  exceeded, pcre2_match() 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 PCRE2 is
       built, and they can also be set  when  pcre2_match()  is  called.  For  details  of  these
       interfaces,  see the pcre2build documentation and the section entitled "The match context"
       in the pcre2api 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 pcre2test test program has a modifier called "find_limits"  which,  if  applied  to  a
       subject  line, causes it to find the smallest limits that allow a a pattern to match. This
       is done by calling pcre2_match() repeatedly with different limits.

   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

         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 pcre2_match().

   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, England.


       Last updated: 21 November 2014
       Copyright (c) 1997-2014 University of Cambridge.