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Repetition

 

 

 

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

 

  a literal data character

  the dot metacharacter

  the \C escape sequence

  the \X escape sequence

  the \R escape sequence

  an escape such as \d or \pL that matches a single character

  a character class

  a back reference (see next section)

  a parenthesized subpattern (including assertions)

  a subroutine call to a subpattern (recursive or otherwise)

 

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.


In UTF modes, quantifiers apply to characters rather than to individual data units. Thus, for example, \x{100}{2} matches two characters, each of which is represented by a two-byte sequence in a UTF-8 string. Similarly, \X{3} matches three Unicode extended grapheme clusters, each of which may be several data units long (and they may be of different lengths).

 

The quantifier {0} is permitted, causing the expression to behave as if the previous item and the quantifier were not present. This may be useful for subpatterns that are referenced as subroutines from elsewhere in the pattern (but see also the section entitled "Defining subpatterns for use by reference only" below). Items other than subpatterns that have a {0} quantifier are omitted from the compiled pattern.

 

For convenience, the three most common quantifiers have single-character abbreviations:

 

  *    is equivalent to {0,}

  +    is equivalent to {1,}

  ?    is 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 /* and */ and within the comment, 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 owing to the greediness of the .* item.

However, if a quantifier is followed by a question mark, it ceases to be greedy, 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 that is not available in Perl), 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.

 

When a parenthesized subpattern is quantified with a minimum repeat count that is greater than 1 or with a limited maximum, more memory 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 dot to match newlines, 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 normally 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 in order to obtain this optimization, or alternatively using ^ to indicate anchoring explicitly.

 

However, there are some cases where the optimization cannot be used. When .* is inside capturing parentheses that are the subject of a back reference elsewhere in the pattern, a match at the start may fail where a later one succeeds. Consider, for example:

 

  (.*)abc\1

 

If the subject is "xyz123abc123" the match point is the fourth character. For this reason, such a pattern is not implicitly anchored.

Another case where implicit anchoring is not applied is when the leading .* is inside an atomic group. Once again, a match at the start may fail where a later one succeeds. Consider this pattern:


  (?>.*?a)b

 

It matches "ab" in the subject "aab". The use of the backtracking control verbs (*PRUNE) and (*SKIP) also disable this optimization.

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".

 


 


 

Philip Hazel

University Computing Service

Cambridge CB2 3QH, England.

Last updated: 12 November 2013

Copyright © 1997-2013 University of Cambridge.


 


 

 

 

 

 

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