From: Kendall Gifford <zettabyte@...>
Date: 2010-11-05T08:06:44+09:00
Subject: Re: Regular Expression

On Thu, Nov 4, 2010 at 4:53 PM, Mike Cargal <mike@cargal.net> wrote:
>
> On Nov 4, 2010, at 1:34 PM, Kendall Gifford wrote:
>
>> 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))+$/
>>
>> However, it still has problems with "DADD", "DADBD" and such...
>>
>> --
>> Kendall Gifford
>> zettabyte@gmail.com
>>
>
>
> Oh... missed that detail... I *think* this covers the bases...
>
>
> regex = /^(A(?!.*A)|B(?!.*B)|C(?!.*C)|D{1,2}(?!.*D))+$/
>
> ["ABCDD","CA","CAD",
>  "CADD","CADDD","DAD",
>  "BAC","BBAD","AABCCD",
>  "DADD","DADBD","ABCDDA",
>  "MIKE"].each do |string|
>   puts "'#{string}' => #{string.match(regex)}"
> end
> ====================
> 'ABCDD' => ABCDD
> 'CA' => CA
> 'CAD' => CAD
> 'CADD' => CADD
> 'CADDD' =>
> 'DAD' =>
> 'BAC' => BAC
> 'BBAD' =>
> 'AABCCD' =>
> 'DADD' =>
> 'DADBD' =>
> 'ABCDDA' =>
> 'MIKE' =>
> ====================
>
> Mike Cargal
>
> mike@cargal.net
> http://blog.mikecargal.com
>

Except that "DAD" *is* legal according to OP's description. :-)

-- 
Kendall Gifford
zettabyte@gmail.com