PCRE2PERFORM(3)PCRE2PERFORM(3)NAME
PCRE2 - Perl-compatible regular expressions (revised API)
PCRE2 PERFORMANCE
Two aspects of performance are discussed below: memory usage and pro‐
cessing time. The way you express your pattern as a regular expression
can affect both of them.
COMPILED PATTERN MEMORY USAGE
Patterns are compiled by PCRE2 into a reasonably efficient interpretive
code, so that most simple patterns do not use much memory. However,
there is one case where the memory usage of a compiled pattern can be
unexpectedly large. If a parenthesized subpattern has a quantifier with
a minimum greater than 1 and/or a limited maximum, the whole subpattern
is repeated in the compiled code. For example, the pattern
(abc|def){2,4}
is compiled as if it were
(abc|def)(abc|def)((abc|def)(abc|def)?)?
(Technical aside: It is done this way so that backtrack points within
each of the repetitions can be independently maintained.)
For regular expressions whose quantifiers use only small numbers, this
is not usually a problem. However, if the numbers are large, and par‐
ticularly if such repetitions are nested, the memory usage can become
an embarrassment. For example, the very simple pattern
((ab){1,1000}c){1,3}
uses 51K bytes when compiled using the 8-bit library. When PCRE2 is
compiled with its default internal pointer size of two bytes, the size
limit on a compiled pattern is 64K code units in the 8-bit and 16-bit
libraries, and this is reached with the above pattern if the outer rep‐
etition is increased from 3 to 4. PCRE2 can be compiled to use larger
internal pointers and thus handle larger compiled patterns, but it is
better to try to rewrite your pattern to use less memory if you can.
One way of reducing the memory usage for such patterns is to make use
of PCRE2's "subroutine" facility. Re-writing the above pattern as
((ab)(?2){0,999}c)(?1){0,2}
reduces the memory requirements to 18K, and indeed it remains under 20K
even with the outer repetition increased to 100. However, this pattern
is not exactly equivalent, because the "subroutine" calls are treated
as atomic groups into which there can be no backtracking if there is a
subsequent matching failure. Therefore, PCRE2 cannot do this kind of
rewriting automatically. Furthermore, there is a noticeable loss of
speed when executing the modified pattern. Nevertheless, if the atomic
grouping is not a problem and the loss of speed is acceptable, this
kind of rewriting will allow you to process patterns that PCRE2 cannot
otherwise handle.
STACK USAGE AT RUN TIME
When pcre2_match() is used for matching, certain kinds of pattern can
cause it to use large amounts of the process stack. In some environ‐
ments the default process stack is quite small, and if it runs out the
result is often SIGSEGV. Rewriting your pattern can often help. The
pcre2stack documentation discusses this issue in detail.
PROCESSING TIME
Certain items in regular expression patterns are processed more effi‐
ciently than others. It is more efficient to use a character class like
[aeiou] than a set of single-character alternatives such as
(a|e|i|o|u). In general, the simplest construction that provides the
required behaviour is usually the most efficient. Jeffrey Friedl's book
contains a lot of useful general discussion about optimizing regular
expressions for efficient performance. This document contains a few
observations about PCRE2.
Using Unicode character properties (the \p, \P, and \X escapes) is
slow, because PCRE2 has to use a multi-stage table lookup whenever it
needs a character's property. If you can find an alternative pattern
that does not use character properties, it will probably be faster.
By default, the escape sequences \b, \d, \s, and \w, and the POSIX
character classes such as [:alpha:] do not use Unicode properties,
partly for backwards compatibility, and partly for performance reasons.
However, you can set the PCRE2_UCP option or start the pattern with
(*UCP) if you want Unicode character properties to be used. This can
double the matching time for items such as \d, when matched with
pcre2_match(); the performance loss is less with a DFA matching func‐
tion, and in both cases there is not much difference for \b.
When a pattern begins with .* not in atomic parentheses, nor in paren‐
theses that are the subject of a backreference, and the PCRE2_DOTALL
option is set, the pattern is implicitly anchored by PCRE2, since it
can match only at the start of a subject string. If the pattern has
multiple top-level branches, they must all be anchorable. The optimiza‐
tion can be disabled by the PCRE2_NO_DOTSTAR_ANCHOR option, and is
automatically disabled if the pattern contains (*PRUNE) or (*SKIP).
If PCRE2_DOTALL is not set, PCRE2 cannot make this optimization,
because the dot metacharacter does not then match a newline, and if the
subject string contains newlines, the pattern may match from the char‐
acter immediately following one of them instead of from the very start.
For example, the pattern
.*second
matches the subject "first\nand second" (where \n stands for a newline
character), with the match starting at the seventh character. In order
to do this, PCRE2 has to retry the match starting after every newline
in the subject.
If you are using such a pattern with subject strings that do not con‐
tain newlines, the best performance is obtained by setting
PCRE2_DOTALL, or starting the pattern with ^.* or ^.*? to indicate
explicit anchoring. That saves PCRE2 from having to scan along the sub‐
ject looking for a newline to restart at.
Beware of patterns that contain nested indefinite repeats. These can
take a long time to run when applied to a string that does not match.
Consider the pattern fragment
^(a+)*
This can match "aaaa" in 16 different ways, and this number increases
very rapidly as the string gets longer. (The * repeat can match 0, 1,
2, 3, or 4 times, and for each of those cases other than 0 or 4, the +
repeats can match different numbers of times.) When the remainder of
the pattern is such that the entire match is going to fail, PCRE2 has
in principle to try every possible variation, and this can take an
extremely long time, even for relatively short strings.
An optimization catches some of the more simple cases such as
(a+)*b
where a literal character follows. Before embarking on the standard
matching procedure, PCRE2 checks that there is a "b" later in the sub‐
ject string, and if there is not, it fails the match immediately. How‐
ever, when there is no following literal this optimization cannot be
used. You can see the difference by comparing the behaviour of
(a+)*\d
with the pattern above. The former gives a failure almost instantly
when applied to a whole line of "a" characters, whereas the latter
takes an appreciable time with strings longer than about 20 characters.
In many cases, the solution to this kind of performance issue is to use
an atomic group or a possessive quantifier.
AUTHOR
Philip Hazel
University Computing Service
Cambridge, England.
REVISION
Last updated: 02 January 2015
Copyright (c) 1997-2015 University of Cambridge.
PCRE2 10.00 02 January 2015 PCRE2PERFORM(3)
