Example 6 (Hard)

What regular expression describes the set of strings made up of 'a' and 'b' such that each 'a' is immediately preceded by at least one 'b' and immediately followed by at least one 'b' .

Accepted:

Ø, b, bb, bbb, bab, babab, bbbb, 
bbab, babb, bbbbb, babbb, bbabb, bbbab, babab,
babbababbbabb, bbbbabababbbabbb, 

Not Accepted:

a, ab, ba, abb, bba, aaa, aab, baa,
aaaa, abbb, baab, baba, bbaa, aabb, 
aaab, bababbabaabb

Try out some regular expressions in the applet. You will need to nest groups.