How about:
pattern = re.compile('^([^a]|(a+[^ab]))*aaab')
Which basically says, "precede with arbitrarily many non-a's
or a sequences ending in non-b, then must have 3 as followed by a b."
cases = ["xyz123aaabbab", "xayz123aaabab", "xaaayz123aaabab",
"xyz123aaaababaaabab", "xyz123aabbaaab", "xaaayz123abab"]
[re.search(pattern, case) is not None for case in cases]
[True, True, True, False, False, False]
--Scott David Daniels
[EMAIL PROTECTED]
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