clear understanding on possesive quantifiers

 

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  • Greedy vs. Reluctant vs. Possessive Quantifiers 7 answers

    • There are three types of quantifiers:

  • the "regular" quantifiers (*, +, ?) also called "greedy" quantifiers;
  • the "lazy", quantifiers (*?, +?, ??);
  • the "possessive" quantifiers (*+, ++, ?+).

    • For instance, take this input: 

    The answer is 42

    Now, take this regex: 

    .*(d+)

    The question is, what will be captured by (d+) according to which version of * you use in .* :

  • if * , what will be captured is 2 ;
  • if *? , what will be captured is 42 ;
  • if *+ , the regex does not match.

    • Why:

  • the greedy quantifier swallows everything it can; however, it retains, on its way, the positions where it has matched; when having swallowed the full text, there is still d+ to match; it will reluctantly backtrack until d+ is satisfied, and d+ is satisfied with 2 ;
  • the lazy quantifier tries and queries the following regex token: "if d+ does not match then I swallow the next character"; when encountering 4 it lets d+ do its job, therefore 42 is captured;
  • the possessive quantifier is possessive; it works like the greedy quantifier except that it does not retain any positions; therefore, when reaching the end, the regex engine asks "OK, can you give back?", .*+ says "No...", therefore no match.

    • The possessive quantifier implies that no backtracking is done.

    What happens when you try to match mdfoo against .*+foo :

  • The first part of the pattern (ie .*+ ) matches the whole string mdfoo
  • But the second part of the pattern (ie foo ) is not found after the first match
  • Since there is no backtracking, the attempt immediately fails

    • These possessive quantifiers are quite clearly explained here.

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