Question

Which one of the following regular expressions represents the language: the set of all binary strings having two consecutive 0s and two consecutive 1s?

(A) (0+1)*0011(0+1)*+ (0+1)*1100(0+1)*

(B) (0+1)*(00(0+1)*11 + 11(0+1)*00) (0+1)*

(C) (0+1)*00(0+1)*+ (0+1)*11(0+1)*

(D) 00(0+1)*11 + 11(0+1)*00

Answer 1

(B) (0+1)*(00(0+1)*11 + 11(0+1)*00) (0+1)*

(a) contains 00 & 11 consecutively which is not the required condition. 

(c) Doesn’t guaranty that both 00 & 11 will be present in the string. 

(d) Says string should start with 11 & ends with 00 or vice versa.