PCRE2PERFORM(3)        (03 February 2019)         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

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          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. An initial 20KiB vector of frames is allocated on the
          system stack (enough for about 100 frames for small
          patterns), but if this is insufficient, heap memory is used.
          The amount of heap memory can be limited; if the limit is
          set to zero, only the initial stack vector is used.
          Rewriting patterns to be time-efficient, as described below,
          may also reduce the memory requirements.

          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

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          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

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          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,

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          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
          University Computing Service
          Cambridge, England.

     REVISION

          Last updated: 03 February 2019
          Copyright (c) 1997-2019 University of Cambridge.

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