in reply to brackets matching problem

What do you find odd?. With alternation | Perl will match the first element if it can, if not the next ... so in the first case it finds a 1 which satisfies the first capture and then the second capture takes the optional 2. In the second case you get 12 as the first capture and then the second captuere 2? 'matches' the 3 as 3 is 0 or more 2s..... (actually it matches the null string between 2 and 3 to be technically correct.

cheers

tachyon

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Re^2: brackets matching problem
by lucky8 (Initiate) on Aug 19, 2004 at 08:45 UTC
    Does this mean that leftmost-longest rule is not true for perl?
[reply]
      Out of perldoc perlre:
      Alternatives are tried from left to right, so the first alternative found for which the entire expression matches, is the one that is chosen. This means that alternatives are not necessarily greedy. For example: when matching "foo|foot" against "barefoot", only the "foo" part will match, as that is the first alternative tried, and it successfully matches the target string. (This might not seem important, but it is important when you are capturing matched text using parentheses.)

      Let's look at a simply modified test sample:
      /(1|12)(3?)/

      [download]
      will match 1 because 3 ist optional

      and
      /(1|12)(3?)$/  # use anchor to force the engine to match the entire ex
+pression

      [download]
      will match 12 because it has to satisfy the entire expression and so it will match 3 at the end of the string

      Add:
      For clarifying, the first example will have '1' in $1 and nothing in $2
      Second example will have '12' in $1 and '3' in $2
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[d/l]
[select]
        Thank you, now it's clear
[reply]
      Greedy matching applies to the closures, not to alternatives. Besides, in this case, whether 12 matches or 1 then 2, it's the same length, is it not? From perlre, "alternatives are not necessarily greedy." You can make your matches greedy by arranging them how you like.
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