From: MonkeeSage <MonkeeSage@...>
Date: 2007-11-30T09:29:59+09:00
Subject: Re: Negate a character sequence in a regular expression?

On Nov 29, 3:47 pm, crm_...@mac.com wrote:
> For the following string:
>
> 'cat sheep horse cat tac dog'
>
> I would like to write a regular expression that matches any substring
> that is prefixed by the word 'cat', is then followed by any characters
> as long as those characters do not comprise the word 'cat', and then
> finally suffixed by the string 'dog'.  Therefore, this expression
> should match the substring 'cat tac dog' in the above string.
>
> Obviously, if I write an expression like:
>
> irb(main):002:0> /cat.*dog/.match('cat sheep horse cat tac dog').to_s
> => "cat sheep horse cat tac dog"
>
> it will match the entire string.
>
> And the non-greedy Kleene doesn't buy me anything either since the
> expression matches the first cat found anyway:
>
> irb(main):003:0> /cat.*?dog/.match('cat sheep horse cat tac dog').to_s
> => "cat sheep horse cat tac dog"
>
> What I think I want to do is to negate a sequence of characters,
> rather than just a character class, but I have looked around and not
> found anything quite right.
>
> Of course, there are ways of hacking this out, e.g. I could reverse
> the string first and match 'god' followed by the first instance of
> 'tac', but I am hoping there is a more elegant way to do this with a
> single regular expression.
>
> Thanks--

If you just want the right-most match of the substring prefixed by
'cat' and suffixed by 'dog', this should work: /.*(cat.*dog)/.match(s)
[1].

Regards,
Jordan