Reqular Expressions Questions

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21.

If ? = {a, b, c, d, e, f} then number of strings in ? of length 4 such that no symbol is used more than once in a string is

A

35

B

360

C

49

D

720

Correct Answer: B

Here string length is 4 so we create string of length 4 by 6 values firstly we arrange any value by 6 methods. Then Remaining numbers are 5 so we can arrange them by 5 methods then remaining numbers are 4 so we arrange them by 4 methods and then 3.Thus 6*5*4*3=360.

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22.

Which one of the following statement is FALSE?

A

Context-free languages are closed under union

B

Context-free languages are closed under concatenation

C

Context-free languages are closed under intersection

D

Context-free languages are closed under Kleene closure

Correct Answer: C

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23.

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

Correct Answer: C

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)

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24.

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

Correct Answer: D

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25.

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

Correct Answer: A

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

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26.

Regular grammar is

A

context free grammar

B

non context free grammar

C

english grammar

D

none of the mentioned

Correct Answer: A

Regular grammar is subset of context free grammar.

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27.

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

B

L1>=L2

C

L1 U L2 = .*

D

L1=L2

Correct Answer: D

Finite state machine and regular expression have same power to express a language.

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28.

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)*

Correct Answer: B

Except b all are regular expression*.

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29.

Regular expression are

A

Type 0 language

B

Type 1 language

C

Type 2 language

D

Type 3 language

Correct Answer: D

According to Chomsky hierarchy, Type 0

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30.

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

Correct Answer: B

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