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.
Last updated: 05 June 2007 Copyright (c) 1997-2007 University of Cambridge.
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This manual page is taken from the PCRE library, which is distributed under the BSD license. |