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regexmatchrakuquantifiers

* quantifier in Perl 6

This seems to be something very basic that I don't understand here.

Why doesn't "babc" match / a * / ?

> "abc" ~~ / a /
｢a｣
> "abc" ~~ / a * /
｢a｣
> "babc" ~~ / a * /
｢｣                    # WHY?
> "babc" ~~ / a + /
｢a｣
Solution

  • The answers here are correct, I'll just try to present them in a more coherent form:

    Matching always starts from the left

    The regex engine always starts at the left of the strings, and prefers left-most matches over longer matches

    * matches empty strings

    The regex a* matches can match the strings '', 'a', 'aa' etc. It will always prefer the longest match it finds, but it can't find a match longer than the empty string, it'll just match the empty string.

    Putting it together

    In 'abc' ~~ /a*/, the regex engine starts at position 0, the a* matches as many a's as it can, and thus matches the first character.

    In 'babc' ~~ /a*/, the regex engine starts at position 0, and the a* can match only zero characters. It does so successfully. Since the overall match succeeds, there is no reason to try again at position 1.