> Sorry for the confusion. The correct pattern should reject
> all strings except those in which the first sequence of the
> letter 'a' that is followed by the letter 'b' has a length of
> exactly three.
Ah...a little more clear.
r = re.compile("[^a]*a{3}b+(a+b*)*")
matches = [s for s in listOfStringsToTest if r.match(s)]
or (as you've only got 3 of 'em)
r = re.compile("[^a]*aaab+(a+b*)*")
matches = [s for s in listOfStringsToTest if r.match(s)]
should do the trick. To exposit:
[^a]* a bunch of stuff that's not "a"
a{3} or aaa three letter "a"s
b+ one or more "b"s
(a+b*) any number of "a"s followed optionally by "b"s
Hope this helps,
-tkc
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