Question

Which of the following is not a regular expression?

(a) [(a+b)*-(aa+bb)]*

(b) [(0+1)-(0b+a1)*(a+b)]*

(c) (01+11+10)*

(d) (1+2+0)*(1+2)*

This question was posed to me during an internship interview.

This intriguing question comes from Regular Language & Expression topic in section Regular Expressions and Languages of Automata Theory

Answer 1

Right option is (b) [(0+1)-(0b+a1)*(a+b)]*

Explanation: Except b all are regular expression*.