Match First Sequence in Regular Expression?

Roger L. Cauvin roger at
Thu Jan 26 19:58:43 CET 2006

"Michael Spencer" <mahs at> wrote in message 
news:mailman.1096.1138299292.27775.python-list at
> 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?

Roger L. Cauvin
nospam_roger at (omit the "nospam_" part)
Cauvin, Inc.
Product Management / Market Research

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