Peter Linz Edition 4 Exercise 3.1 Question 11 (Page No. 76)

Question

Find a regular expression for $L =$ {$ab^nw: n ≥ 3, w ∈$ {$a, b$}$^+$}.

Answer 1

Language L = {$ab^nw : n$ ≥ 3, $w$ ∈ $(a+b)^+$ }

Some Strings in this Language = { $ab^3w , ab^4w , ab^5w , ab^6w , ......$} where $w$ =Any String except epsilon

So Regular Expression for the Language = $abbb^+ (a+b)^+$, here $bbb^+$ensures that number of b’s will always be greater than 3. You could also write $bbb^+$ as $bbbb^*$