GATE | GATE CS Mock 2018 | Question 42

A regular expression is ambiguous when there exists a string which can be constructed in two different ways from the regular expression. Which of the following regular expressions are unambiguous?
(A) a((ab)*cd)* ∪ a(ababcb*)*a*
(B) aab*(ab)* ∪ ab* ∪ a*bba*
(C) aaba* ∪ aaaba ∪ aabba* ∪ a
(D) None of these

Answer: (C)

Explanation: (A) Ambiguous. For instance, the string a can be constructed by using a((ab)*cd)* or a(ababcb*)*a*.
(B) Ambiguous. The string abb can be constructed either by ab* or a*bba*.
(C) Unambiguous.

Option (C) is correct.

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