pcrestack

PCRE - Perl-compatible regular expressions

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. 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 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.

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.

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_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 the size of its stack. . .

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

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Last updated: 05 June 2007
Copyright (c) 1997-2007 University of Cambridge.