From: Kendall Gifford <zettabyte@...>
Date: 2010-11-05T02:34:57+09:00
Subject: Re: Regular Expression

On Thu, Nov 4, 2010 at 11:27 AM, Kendall Gifford <zettabyte@gmail.com> wrote:
> 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...
>

Okay, now this is even closer:

regex = /^(A(?=[^A]+$)|B(?=[^B]+$)|C(?=[^C]+$)|D{1,2}(?!D))+$/

However, it still has problems with "DADD", "DADBD" and such...

-- 
Kendall Gifford
zettabyte@gmail.com