Match First Sequence in Regular Expression?

[Peter Hansen]

Roger L. Cauvin wrote:
> "Michael Spencer" <mahs at telcopartners.com> wrote in message 
> news:mailman.1096.1138299292.27775.python-list at python.org...
> 
>>Roger L. Cauvin wrote:
>>>>>"xyz123aaabbab" accept
>>>>>"xyz123aabbaaab" reject
>>>>>"xayz123aaabab" accept
>>>>>"xaaayz123abab" reject
>>>>>"xaaayz123aaabab" accept
>>>>>
>>
>>This passes your tests.  I haven't closely followed the thread for other 
>>requirements:
>>
>> >>> pattern = ".*?(?<![a+b])aaab" #look for aaab not preceded by any a+b
> 
> Very interesting.  I think you may have solved the problem.  The key seems 
> to be the "not preceded by" part.  I'm unfamiliar with some of the notation. 
> Can you explain what "[a+b]" and the "(?<!" do?

I think you might need to add a test case involving a pattern of aaaab 
prior to another aaab.  From what I gather (not reading too closely), 
you would want this to be rejected.  Is that true?

xyz123aaaababaaabab

-Peter