PCRE2STACK(3)PCRE2STACK(3)NAME
PCRE2 - Perl-compatible regular expressions (revised API)
PCRE2 DISCUSSION OF STACK USAGE
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 recur‐
sion depth increases. The match() function is also called in other cir‐
cumstances, 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 -fsani‐
tize=address option of the GCC compiler, the stack requirements are
greatly increased.
The above comments apply when pcre2_match() is run in its normal inter‐
pretive manner. If the compiled pattern was processed by pcre2_jit_com‐
pile(), 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 details.
The pcre2_dfa_match() function operates in a different way to
pcre2_match(), and uses recursion only when there is a regular expres‐
sion 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 pat‐
terns with runaway infinite recursions; such patterns will cause
pcre2_dfa_match() to run out of stack. At present, there is no protec‐
tion 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. Con‐
sider, 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 charac‐
ter. 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. Recur‐
sion 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 match‐
ing 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 documen‐
tation. 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 imple‐
ment customized a memory handler that is more efficient than the stan‐
dard 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 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 nor‐
mally 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:
http://developer.apple.com/qa/qa2005/qa1419.html.
AUTHOR
Philip Hazel
University Computing Service
Cambridge, England.
REVISION
Last updated: 21 November 2014
Copyright (c) 1997-2014 University of Cambridge.
PCRE2 10.00 21 November 2014 PCRE2STACK(3)
