pcre2perform(3) Library Functions Manual pcre2perform(3)
NAME
PCRE2 - Perl-compatible regular expressions (revised API)
PCRE2 PERFORMANCE
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.
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 for storing the
compiled version. However, there is one case where the memory usage of a
compiled pattern can be unexpectedly large. If a parenthesized group has
a quantifier with a minimum greater than 1 and/or a limited maximum, the
whole group 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
particularly 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 over 50KiB 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 65535 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.
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 around 16KiB, and indeed it remains
under 20KiB even with the outer repetition increased to 100. However,
this kind of pattern is not always exactly equivalent, because any
captures within subroutine calls are lost when the subroutine completes.
If this is not a problem, this kind of rewriting will allow you to
process patterns that PCRE2 cannot otherwise handle. The matching
performance of the two different versions of the pattern are roughly the
same. (This applies from release 10.30 - things were different in earlier
releases.)
STACK AND HEAP USAGE AT RUN TIME
From release 10.30, the interpretive (non-JIT) version of pcre2_match()
uses very little system stack at run time. In earlier releases recursive
function calls could use a great deal of stack, and this could cause
problems, but this usage has been eliminated. Backtracking positions are
now explicitly remembered in memory frames controlled by the code.
The size of each frame depends on the size of pointer variables and the
number of capturing parenthesized groups in the pattern being matched. On
a 64-bit system the frame size for a pattern with no captures is 128
bytes. For each capturing group the size increases by 16 bytes.
Until release 10.41, an initial 20KiB frames vector was allocated on the
system stack, but this still caused some issues for multi-thread
applications where each thread has a very small stack. From release 10.41
backtracking memory frames are always held in heap memory. An initial
heap allocation is obtained the first time any match data block is passed
to pcre2_match(). This is remembered with the match data block and re-
used if that block is used for another match. It is freed when the match
data block itself is freed.
The size of the initial block is the larger of 20KiB or ten times the
pattern's frame size, unless the heap limit is less than this, in which
case the heap limit is used. If the initial block proves to be too small
during matching, it is replaced by a larger block, subject to the heap
limit. The heap limit is checked only when a new block is to be
allocated. Reducing the heap limit between calls to pcre2_match() with
the same match data block does not affect the saved block.
In contrast to pcre2_match(), pcre2_dfa_match() does use recursive
function calls, but only for processing atomic groups, lookaround
assertions, and recursion within the pattern. The original version of the
code used to allocate quite large internal workspace vectors on the
stack, which caused some problems for some patterns in environments with
small stacks. From release 10.32 the code for pcre2_dfa_match() has been
re-factored to use heap memory when necessary for internal workspace when
recursing, though recursive function calls are still used.
The "match depth" parameter can be used to limit the depth of function
recursion, and the "match heap" parameter to limit heap memory in
pcre2_dfa_match().
PROCESSING TIME
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.
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 function, and in both cases
there is not much difference for \b.
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).
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
.*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 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.
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 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
(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. This can often reduce memory
requirements as well. As another example, consider 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 backtracking
position is passed, so this formulation uses a memory frame for each
matched character. For a long string, a lot of memory is required.
Consider now this rewritten pattern, which matches exactly the same
strings:
([^<]++|<(?!inet))+
This runs much faster, because sequences of characters that do not
contain "<" are "swallowed" in one item inside the parentheses, and a
possessive quantifier is used to stop any backtracking into the runs of
non-"<" characters. This version also uses a lot less memory because
entry to a new set of parentheses happens only when a "<" character that
is not followed by "inet" is encountered (and we assume this is
relatively rare).
This example shows that one way of optimizing performance when matching
long subject strings is to write repeated parenthesized subpatterns to
match more than one character whenever possible.
SETTING RESOURCE LIMITS
You can set limits on the amount of processing that takes place when
matching, and on the amount of heap memory that is used. 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() or pcre2_dfa_match() is called. For details of these
interfaces, see the pcre2build documentation and the section entitled
"The match context" in the pcre2api documentation.
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 pattern to match. This is done by repeatedly matching with
different limits.
AUTHOR
Philip Hazel
Retired from University Computing Service
Cambridge, England.
REVISION
Last updated: 27 July 2022
Copyright (c) 1997-2022 University of Cambridge.
PCRE2 10.41 27 July 2022 pcre2perform(3)
pcre2 10.42 - Generated Mon Jan 9 15:11:27 CST 2023