From: Kendall Gifford <zettabyte@...>
Date: 2010-11-05T02:27:26+09:00
Subject: Re: Regular Expression

On Thu, Nov 4, 2010 at 5:46 AM, Mike Cargal <mike@cargal.net> wrote:
> regex = /^(A(?!A)|B(?!B)|C(?!C)|D{1,2}(?!D))+$/
>
> ["ABCDD","CA","CAD","CADD","CADDD","DAD","BAC","BBAD","AABCCD"].each do |string|
>   puts "'#{string}' => #{string.match(regex)}"
> end
>
> ==============
> 'ABCDD' => ABCDD
> 'CA' => CA
> 'CAD' => CAD
> 'CADD' => CADD
> 'CADDD' =>
> 'DAD' => DAD
> 'BAC' => BAC
> 'BBAD' =>
> 'AABCCD' =>
> ==============

However, this also appears by my test to match "DADD", "ABA", etc:

["ABA", "DADD", "CAC"].each do |string|
  puts "'#{string}' => #{string.match(regex)}"
end
'DADD' => DADD
'ABA' => ABA
'CAC' => CAC

It does get you closer though. I rarely remember to make use of
look-ahead (and look-behind and other "(?X" style patterns) since when
switching languages/regexp engines, I'm never sure what features will
be there (and will still work the same). I guess I'm too conservative,
sticking with core/basic features.

This does make me curious how short of a regex using these features
could be written for this one case...

-- 
Kendall Gifford
zettabyte@gmail.com