Question

The set of all strings over ∑ ={a,b} in which a single a is followed by any number of b’s a single b                   followed by any number of a’s is?

(a) ab* + ba*

(b) ab*ba*

(c) a*b + b*a

(d) None of the mentioned

The question was posed to me by my school teacher while I was bunking the class.

Origin of the question is Obtaining the regular Expression from the Finite automata topic in division Finite Automata and Regular Expression of Compiler

Answer 1

Right option is (a) ab* + ba*

Easiest explanation: ab*+ba* is the expression in which a single a is followed by any number of b’s a single b followed by any number of a’s.