250+ TOP MCQs on Regular Language & Expression

Automata Theory Multiple Choice Questions on “Regular Language & Expression”.

1. How many strings of length less than 4 contains the language described by the regular expression (x+y)*y(a+ab)*?
a) 7
b) 10
c) 12
d) 11

Answer: c
Clarification: string of length 0 = Not possible (because y is always present).
string of length 1 = 1 (y)
string of length 2 = 3 (xy,yy,ya)
string of length 3 = 8 (xxy,xyy,yxy,yyy,yaa,yab,xya,yya)

2. Which of the following is true?
a) (01)*0 = 0(10)*
b) (0+1)*0(0+1)*1(0+1) = (0+1)*01(0+1)*
c) (0+1)*01(0+1)*+1*0* = (0+1)*
d) All of the mentioned

Answer: d
Clarification: None.

3. A language is regular if and only if
a) accepted by DFA
b) accepted by PDA
c) accepted by LBA
d) accepted by Turing machine

Answer: a
Clarification: All of above machine can accept regular language but all string accepted by machine is regular only for DFA.

4. Regular grammar is
a) context free grammar
b) non context free grammar
c) english grammar
d) none of the mentioned

Answer: a
Clarification: Regular grammar is subset of context free grammar.

5. Let the class of language accepted by finite state machine be L1 and the class of languages represented by regular expressions be L2 then
a) L1<L2
b) L1>=L2
c) L1 U L2 = .*
d) L1=L2

Answer: d
Clarification: Finite state machine and regular expression have same power to express a language.

6. 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)*

Answer: b
Clarification: Except b all are regular expression*.

7. Regular expression are
a) Type 0 language
b) Type 1 language
c) Type 2 language
d) Type 3 language

Answer: a
Clarification: According to Chomsky hierarchy .

8. Which of the following is true?
a) Every subset of a regular set is regular
b) Every finite subset of non-regular set is regular
c) The union of two non regular set is not regular
d) Infinite union of finite set is regular

Answer: b
Clarification: None.

9. L and ~L are recursive enumerable then L is
a) Regular
b) Context free
c) Context sensitive
d) Recursive

Answer: d
Clarification:If L is recursive enumerable and its complement too if and only if L is recursive.

10. Regular expressions are closed under
a) Union
b) Intersection
c) Kleen star
d) All of the mentioned

Answer: d
Clarification: According to definition of regular expression.