regexp-php-reference-php-repetition-8

  • PCRE regex
    syntax
  • Repetition

  • Repetition
  • Repetition

    Repetition

    Repetition is specified by quantifiers, which can
    follow any of the following items:

    • a single character,
      possibly escaped
    • the .
      metacharacter
    • a character
      class
    • a back reference (see
      next section)
    • a parenthesized
      subpattern (unless it is an assertion – see below)

    The general repetition quantifier specifies a
    minimum and maximum number of permitted matches, by giving the two
    numbers in curly brackets (braces), separated by a comma. The
    numbers must be less than 65536, and the first must be less than or
    equal to the second. For example: z{2,4} matches “zz”,
    “zzz”, or “zzzz”. A closing brace on its own is not a special
    character. If the second number is omitted, but the comma is
    present, there is no upper limit; if the second number and the
    comma are both omitted, the quantifier specifies an exact number of
    required matches. Thus [aeiou]{3,} matches at least 3
    successive vowels, but may match many more, while \d{8}
    matches exactly 8 digits. An opening curly bracket that appears in
    a position where a quantifier is not allowed, or one that does not
    match the syntax of a quantifier, is taken as a literal character.
    For example, {,6} is not a quantifier, but a literal string of four
    characters.

    The quantifier {0} is permitted, causing the
    expression to behave as if the previous item and the quantifier
    were not present.

    For convenience (and historical compatibility) the
    three most common quantifiers have single-character
    abbreviations:

    Single-character quantifiers
    * equivalent to {0,}
    + equivalent to {1,}
    ? equivalent to {0,1}

    It is possible to construct infinite loops by
    following a subpattern that can match no characters with a
    quantifier that has no upper limit, for example: (a?)*

    Earlier versions of Perl and PCRE used to give an
    error at compile time for such patterns. However, because there are
    cases where this can be useful, such patterns are now accepted, but
    if any repetition of the subpattern does in fact match no
    characters, the loop is forcibly broken.

    By default, the quantifiers are “greedy”, that is,
    they match as much as possible (up to the maximum number of
    permitted times), without causing the rest of the pattern to fail.
    The classic example of where this gives problems is in trying to
    match comments in C programs. These appear between the sequences /*
    and */ and within the sequence, individual * and / characters may
    appear. An attempt to match C comments by applying the pattern
    /\*.*\*/ to the string /* first comment */ not comment
    /* second comment */
    fails, because it matches the entire
    string due to the greediness of the .* item.

    However, if a quantifier is followed by a question
    mark, then it becomes lazy, and instead matches the minimum number
    of times possible, so the pattern /\*.*?\*/ does the right
    thing with the C comments. The meaning of the various quantifiers
    is not otherwise changed, just the preferred number of matches. Do
    not confuse this use of question mark with its use as a quantifier
    in its own right. Because it has two uses, it can sometimes appear
    doubled, as in \d??\d which matches one digit by
    preference, but can match two if that is the only way the rest of
    the pattern matches.

    If the PCRE_UNGREEDY option is set (an option which is not
    available in Perl) then the quantifiers are not greedy by default,
    but individual ones can be made greedy by following them with a
    question mark. In other words, it inverts the default
    behaviour.

    Quantifiers followed by + are
    “possessive”. They eat as many characters as possible and don’t
    return to match the rest of the pattern. Thus .*abc
    matches “aabc” but .*+abc doesn’t because .*+
    eats the whole string. Possessive quantifiers can be used to speed
    up processing.

    When a parenthesized subpattern is quantified with
    a minimum repeat count that is greater than 1 or with a limited
    maximum, more store is required for the compiled pattern, in
    proportion to the size of the minimum or maximum.

    If a pattern starts with .* or .{0,} and the
    PCRE_DOTALL option (equivalent to Perl’s /s) is set,
    thus allowing the . to match newlines, then the pattern is
    implicitly anchored, because whatever follows will be tried against
    every character position in the subject string, so there is no
    point in retrying the overall match at any position after the
    first. PCRE treats such a pattern as though it were preceded by \A.
    In cases where it is known that the subject string contains no
    newlines, it is worth setting PCRE_DOTALL when the pattern begins with .* in order to
    obtain this optimization, or alternatively using ^ to indicate
    anchoring explicitly.

    When a capturing subpattern is repeated, the value
    captured is the substring that matched the final iteration. For
    example, after (tweedle[dume]{3}\s*)+ has matched
    “tweedledum tweedledee” the value of the captured substring is
    “tweedledee”. However, if there are nested capturing subpatterns,
    the corresponding captured values may have been set in previous
    iterations. For example, after /(a|(b))+/ matches “aba”
    the value of the second captured substring is “b”.