From: Ammar Ali <ammarabuali@...>
Date: 2010-11-24T00:36:02+09:00
Subject: Re: Regex negative look-behind bug?

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On Tue, Nov 23, 2010 at 4:36 PM, Ruby Nuby <b1st@hotmail.com> wrote:

> irb, Ruby 1.9.1
>
> What am I missing here?
>
> "b T T W b".match(/(?<!t t|a b) w/i)
> => nil
>
> #The second look-behind is now just a
> "b T T W b".match(/(?<!t t|a) w/i)
> => #<MatchData " W">
>
> #Regex stays the same, the T T are now in lower case
> "b t t W b".match(/(?<!t t|a) w/i)
> => nil
>
> #Look-behind only contains the t t condition now and, T T are back to
> upper case
> "b T T W b".match(/(?<!t t) w/i)
> => nil
>
>

No bug here. It is doing exactly what you asked: only match a w if it is not
preceded by 't t'. In all cases the w is preceded by 't t', and in the case
that did match (?<!t t|a), the w was preceded by a 't t' but not an 'a', as
you asked, so it did match.

>> "b Y T W b".match( /(?<!t t) w/i )
=> #<MatchData " W">

Regards,
Ammar

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