From: Lars Christensen <larsch@...>
Date: 2002-05-08T02:10:58+09:00
Subject: Re: RegExp zero-width negative lookahead

On Wed, 8 May 2002 nobu.nokada@softhome.net wrote:

> Hi,
>
> At Tue, 7 May 2002 23:33:50 +0900,
> Lars Christensen wrote:
> > I'm puzzled why this doesn't work the way I expect it to.
> >
> > "xxoxxxoxx" =~ /(?!x)ox+o(?!x)/      #=> nil (ok)
> > "xx oxxxoxx" =~ /(?!x)ox+o(?!x)/     #=> nil (ok)
> > "xx oxxxo xx" =~ /(?!x)ox+o(?!x)/    #=> 3 (ok)
> > "xxoxxxo xx" =~ /(?!x)ox+o(?!x)/     #=> 2 (huh!?)
> >
> > Why is the string matched in the last case?
>
> Already pointed out the reason, however you really want look
> behind?
>
> "xxoxxxo xx" =~ /(?<!x)ox+o(?!x)/      #=> nil (with Oniguruma)

Yes, this is was what I am missing.

My current work-around is /([^x]|^)ox+o(?!x)/, but then I need to prepend
$1 to the replacement, e.g.

str.gsub(/([^x]|^)ox+o(?!x)/) { $1 + "replacement" }

.. and it gets even worse/impossible if one wants to non-match multiple
characters.

btw; Thanks to all for the explanations!

-- 
Lars Christensen