pcre2perform(3) — Linux manual page

NAME | PCRE2 PERFORMANCE | COMPILED PATTERN MEMORY USAGE | STACK AND HEAP USAGE AT RUN TIME | PROCESSING TIME | AUTHOR | REVISION | COLOPHON

PCRE2PERFORM(3)         Library Functions Manual         PCRE2PERFORM(3)

NAME         top

       PCRE2 - Perl-compatible regular expressions (revised API)

PCRE2 PERFORMANCE         top


       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         top


       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         top


       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         top


       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         top


       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.

REVISION         top


       Last updated: 27 July 2022
       Copyright (c) 1997-2022 University of Cambridge.

COLOPHON         top

       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 2024-06-14.  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 [email protected]

PCRE2 10.41                   27 July 2022               PCRE2PERFORM(3)