Question

A string can be viewed as alternating sequences of a’s and b’s. For example, a string that starts with an ‘a’ will have some number of a’s, followed by some number of b’s, followed by some number of a’s, etc. For the language that consists of all strings where there is an odd number of occurrences of sequences of a’s….

• Valid strings: aa, aababaa, bbaaa, abbaababbbaaaba, aaab

• Invalid strings: b, aba, baaba, abaaaabbaabbbba

To iterate: I want alternate a's and b's and it is important that a is odd number. I'm not sure how to process the odd part.

Answer 1

1. b1*(ab*ab*)*ab*

This enumerate all possibilities of even number of A's .then at last ,sn extra a has to exist to make it odd.

This regular expression is equivalent to

b*a(b*ab*a)*b*

2. even number of b's and odd number of a's.

(a*ba*ba*)*