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NAME | PCRE2 PERFORMANCE | COMPILED PATTERN MEMORY USAGE | STACK AND HEAP USAGE AT RUN TIME | PROCESSING TIME | AUTHOR | REVISION | COLOPHON |
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PCRE2PERFORM(3) Library Functions Manual PCRE2PERFORM(3)
PCRE2 - Perl-compatible regular expressions (revised API)
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.
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.)
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().
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.
Philip Hazel
Retired from University Computing Service
Cambridge, England.
Last updated: 27 July 2022
Copyright (c) 1997-2022 University of Cambridge.
This page is part of the PCRE (Perl Compatible Regular
Expressions) project. Information about the project can be found
at ⟨http://www.pcre.org/⟩. If you have a bug report for this
manual page, see
⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩. This page was
obtained from the tarball fetched from
⟨https://github.com/PhilipHazel/pcre2.git⟩ on 2023-12-22. If you
discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for
the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original
manual page), send a mail to man-pages@man7.org
PCRE2 10.41 27 July 2022 PCRE2PERFORM(3)
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HTML rendering created 2023-12-22 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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