Match First Sequence in Regular Expression?

Say I have some string that begins with an arbitrary sequence of characters
and then alternates repeating the letters 'a' and 'b' any number of times,
e.g.

"xyz123aaabbaabbbbababbbbaaabb"

I'm looking for a regular expression that matches the first, and only the
first, sequence of the letter 'a', and only if the length of the sequence is
exactly 3.

Does such a regular expression exist? If so, any ideas as to what it could
be?

--
Roger L. Cauvin
nospam_roger@xxxxxxxxxx (omit the "nospam_" part)
Cauvin, Inc.
Product Management / Market Research
http://www.cauvin-inc.com


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