1PCRESTACK(3) Library Functions Manual PCRESTACK(3)
2
6 PCRE - Perl-compatible regular expressions
7
9 When you call pcre_exec(), it makes use of an internal function called
11 match(). This calls itself recursively at branch points in the pattern,
12 in order to remember the state of the match so that it can back up and
13 try a different alternative if the first one fails. As matching pro‐
14 ceeds deeper and deeper into the tree of possibilities, the recursion
15 depth increases.
16 Not all calls of match() increase the recursion depth; for an item such
18 as a* it may be called several times at the same level, after matching
19 different numbers of a's. Furthermore, in a number of cases where the
20 result of the recursive call would immediately be passed back as the
21 result of the current call (a "tail recursion"), the function is just
22 restarted instead.
23 The pcre_dfa_exec() function operates in an entirely different way, and
25 hardly uses recursion at all. The limit on its complexity is the amount
26 of workspace it is given. The comments that follow do NOT apply to
27 pcre_dfa_exec(); they are relevant only for pcre_exec().
28 You can set limits on the number of times that match() is called, both
30 in total and recursively. If the limit is exceeded, an error occurs.
31 For details, see the section on extra data for pcre_exec() in the
32 pcreapi documentation.
33 Each time that match() is actually called recursively, it uses memory
35 from the process stack. For certain kinds of pattern and data, very
36 large amounts of stack may be needed, despite the recognition of "tail
37 recursion". You can often reduce the amount of recursion, and there‐
38 fore the amount of stack used, by modifying the pattern that is being
39 matched. Consider, for example, this pattern:
40 ([^<]|<(?!inet))+
42 It matches from wherever it starts until it encounters "<inet" or the
44 end of the data, and is the kind of pattern that might be used when
45 processing an XML file. Each iteration of the outer parentheses matches
46 either one character that is not "<" or a "<" that is not followed by
47 "inet". However, each time a parenthesis is processed, a recursion
48 occurs, so this formulation uses a stack frame for each matched charac‐
49 ter. For a long string, a lot of stack is required. Consider now this
50 rewritten pattern, which matches exactly the same strings:
51 ([^<]++|<(?!inet))+
53 This uses very much less stack, because runs of characters that do not
55 contain "<" are "swallowed" in one item inside the parentheses. Recur‐
56 sion happens only when a "<" character that is not followed by "inet"
57 is encountered (and we assume this is relatively rare). A possessive
58 quantifier is used to stop any backtracking into the runs of non-"<"
59 characters, but that is not related to stack usage.
60 This example shows that one way of avoiding stack problems when match‐
62 ing long subject strings is to write repeated parenthesized subpatterns
63 to match more than one character whenever possible.
64 In environments where stack memory is constrained, you might want to
66 compile PCRE to use heap memory instead of stack for remembering back-
67 up points. This makes it run a lot more slowly, however. Details of how
68 to do this are given in the pcrebuild documentation. When built in this
69 way, instead of using the stack, PCRE obtains and frees memory by call‐
70 ing the functions that are pointed to by the pcre_stack_malloc and
71 pcre_stack_free variables. By default, these point to malloc() and
72 free(), but you can replace the pointers to cause PCRE to use your own
73 functions. Since the block sizes are always the same, and are always
74 freed in reverse order, it may be possible to implement customized mem‐
75 ory handlers that are more efficient than the standard functions.
76 In Unix-like environments, there is not often a problem with the stack
78 unless very long strings are involved, though the default limit on
79 stack size varies from system to system. Values from 8Mb to 64Mb are
80 common. You can find your default limit by running the command:
81 ulimit -s
83 Unfortunately, the effect of running out of stack is often SIGSEGV,
85 though sometimes a more explicit error message is given. You can nor‐
86 mally increase the limit on stack size by code such as this:
87 struct rlimit rlim;
89 getrlimit(RLIMIT_STACK, &rlim);
90 rlim.rlim_cur = 100*1024*1024;
91 setrlimit(RLIMIT_STACK, &rlim);
92 This reads the current limits (soft and hard) using getrlimit(), then
94 attempts to increase the soft limit to 100Mb using setrlimit(). You
95 must do this before calling pcre_exec().
96 PCRE has an internal counter that can be used to limit the depth of
98 recursion, and thus cause pcre_exec() to give an error code before it
99 runs out of stack. By default, the limit is very large, and unlikely
100 ever to operate. It can be changed when PCRE is built, and it can also
101 be set when pcre_exec() is called. For details of these interfaces, see
102 the pcrebuild and pcreapi documentation.
103 As a very rough rule of thumb, you should reckon on about 500 bytes per
105 recursion. Thus, if you want to limit your stack usage to 8Mb, you
106 should set the limit at 16000 recursions. A 64Mb stack, on the other
107 hand, can support around 128000 recursions. The pcretest test program
108 has a command line option (-S) that can be used to increase the size of
109 its stack.
110
112 Philip Hazel
114 University Computing Service
115 Cambridge CB2 3QH, England.
116
118 Last updated: 05 June 2007
120 Copyright (c) 1997-2007 University of Cambridge.
121 PCRESTACK(3)