in reply to Re^2: Integrating match counts into regex matching
in thread Integrating match counts into regex matching

  1. What does (?<!a) do?

    Zero-length negative lookbehind. Ensures that the string of a's is not preceded by another a.

  2. What does (?!b) do?

    Zero-length negative lookahead. Ensures the matched string of b's is not followed by another b.

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Re^4: Integrating match counts into regex matching
by ikegami (Patriarch) on Dec 19, 2008 at 08:21 UTC
    Why? I can't think of a situation where omitting the lookahead and the lookbehind will change the answer.

    Nevermind, I didn't realize this was a new thread with a different question.

[reply]

      I'm not sure that it is a different question? Anyhow, my limited testing threw up one case where the negative lookbehind was necessary:

      With and without negative lookbehind: 

      Perl> 'aaabb' =~ m[ 
    (?{ local( $a, $b ) }) 
    ( 
        (?<!a) 
        ( a (?{++$a}) )+ 
        ( b (?{++$b}) )+
    )
    (?(?{ $a+1 == $b }) (?=) | (?!) ) 
]x and print "'$1'";;

Perl> 'aaabb' =~ m[ 
    (?{ local( $a, $b ) }) 
    ( 
        ( a (?{++$a}) )+ 
        ( b (?{++$b}) )+ 
    )
    (?(?{ $a+1 == $b }) (?=) | (?!) ) 
]x and print "'$1'";;
'abb'

      [download]

      A breif further test suggests that the negative lookahead may be unnecessary, but I threw it in on the basis that it would do no harm, and may catch a case I haven't thought to test. With a regex using closures, it's doubtful if it will slow things down noticably?

      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice.
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