Question

Which of the following regular expressions represents the set of strings which do not contain a substring ‘rt’ if ∑= {r, t}

(a) (rt)*

(b) (tr)*

(c) (r*t*)

(d) (t*r*)

I got this question in examination.

Enquiry is from Building Regular Expressions in section Regular Expressions and Languages of Automata Theory

Answer 1

Right choice is (d) (t*r*)

To explain: As Kleene operation is not on the whole of the substring, it will not repeat and maintain the order of t, r.