PCRE2PERFORM(3) Introduction to Library Functions 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: 06 December 2022
Copyright (c) 1997-2022 University of Cambridge.
PCRE2 10.45 06 December 2022 PCRE2PERFORM(3)