Example 4

What regular expression specifies the set of strings consisting only of a's and b's which start with at least one 'a' and end with at least one 'b':

Accepted:

ab, aab, abb, aaab, abbb, abab, aabb,
aaaab, aaabb, aabbb, abbbb, abaab, ababb,  
aabbababbabababb

But Not:

a, b, aa, bb, ba, aaa, bbb, bab, aba, bba,
aaaa, bbbb, baaa, bbaa, bbba, abaa, ...