pcre/doc/pcre2perform.3

.TH PCRE2PERFORM 3 "02 January 2015" "PCRE2 10.00"
.SH NAME
PCRE2 - Perl-compatible regular expressions (revised API)
.SH "PCRE2 PERFORMANCE"
.rs
.sp
Two aspects of performance are discussed below: memory usage and processing
time. The way you express your pattern as a regular expression can affect both
of them.
.
.SH "COMPILED PATTERN MEMORY USAGE"
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.sp
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
.sp
  (abc|def){2,4}
.sp
is compiled as if it were
.sp
  (abc|def)(abc|def)((abc|def)(abc|def)?)?
.sp
(Technical aside: It is done this way so that backtrack points within each of
the repetitions can be independently maintained.)
.P
For regular expressions whose quantifiers use only small numbers, this is not
usually a problem. However, if the numbers are large, and particularly if such
repetitions are nested, the memory usage can become an embarrassment. For
example, the very simple pattern
.sp
  ((ab){1,1000}c){1,3}
.sp
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 repetition 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.
.P
One way of reducing the memory usage for such patterns is to make use of
PCRE2's
.\" HTML <a href="pcre2pattern.html#subpatternsassubroutines">
.\" </a>
"subroutine"
.\"
facility. Re-writing the above pattern as
.sp
  ((ab)(?2){0,999}c)(?1){0,2}
.sp
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
.\" HTML <a href="pcre2pattern.html#atomicgroup">
.\" </a>
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.
.
.
.SH "STACK USAGE AT RUN TIME"
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When \fBpcre2_match()\fP is used for matching, certain kinds of pattern can
cause it to use large amounts of the process stack. In some environments the
default process stack is quite small, and if it runs out the result is often
SIGSEGV. Rewriting your pattern can often help. The
.\" HREF
\fBpcre2stack\fP
.\"
documentation discusses this issue in detail.
.
.
.SH "PROCESSING TIME"
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Certain items in regular expression patterns are processed more efficiently
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.
.P
Using Unicode character properties (the \ep, \eP, and \eX 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.
.P
By default, the escape sequences \eb, \ed, \es, and \ew, 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 \ed, when matched with \fBpcre2_match()\fP; the performance loss is
less with a DFA matching function, and in both cases there is not much
difference for \eb.
.P
When a pattern begins with .* not in atomic parentheses, nor in parentheses
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 optimization can be disabled by the
PCRE2_NO_DOTSTAR_ANCHOR option, and is automatically disabled if the pattern
contains (*PRUNE) or (*SKIP).
.P
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 character immediately
following one of them instead of from the very start. For example, the pattern
.sp
  .*second
.sp
matches the subject "first\enand second" (where \en 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.
.P
If you are using such a pattern with subject strings that do not contain
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 subject looking for a newline to restart at.
.P
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
.sp
  ^(a+)*
.sp
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.
.P
An optimization catches some of the more simple cases such as
.sp
  (a+)*b
.sp
where a literal character follows. Before embarking on the standard matching
procedure, PCRE2 checks that there is a "b" later in the subject string, and if
there is not, it fails the match immediately. However, when there is no
following literal this optimization cannot be used. You can see the difference
by comparing the behaviour of
.sp
  (a+)*\ed
.sp
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.
.P
In many cases, the solution to this kind of performance issue is to use an
atomic group or a possessive quantifier.
.
.
.SH AUTHOR
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.nf
Philip Hazel
University Computing Service
Cambridge, England.
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.SH REVISION
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.nf
Last updated: 02 January 2015
Copyright (c) 1997-2015 University of Cambridge.
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