This is a GRE practice question.

Which of the following regular expressions generate(s) no string with two consecutive 1’s? (Note that ε denotes the empty string.)

I. (1 + ε)(01 + 0)*

II. (01+10)*

III. (0+1)*(0+ε)

(A) I only

(B) II only

(C) III only

(D) I and II only

(E) II and III only

My understanding is that neither I nor III generates strings with 11. In I, a string containing 1 is either 1 or 1 surrounded by 0's. In III, all 1's are preceded by 0's. But the correct answer is A, so III must generate a string with 11 somehow. Please explain. Thanks!

  • 4
    $\begingroup$ "In III, all 1's are preceded by 0's" are you sure? Try some examples of $(0+1)^*$ (what is the meaning of $(0+1)$?) $\endgroup$
    – Vor
    Oct 15, 2012 at 7:36
  • 2
    $\begingroup$ What is the meaning of plus? Usually it is one or more times, but then it is written in superscript. Is it just regular concatenation? No, must be or... $\endgroup$ Oct 15, 2012 at 11:37
  • 2
    $\begingroup$ Isn't this a bit localized anyway? $\endgroup$ Oct 15, 2012 at 11:38
  • 1
    $\begingroup$ It would be nice if the question could be put in more general terms. $\endgroup$
    – Raphael
    Oct 16, 2012 at 7:21
  • $\begingroup$ Thank you! I did confuse the meaning of +. I thought it was concatenation; it was union. I agree it's a pretty specific question. $\endgroup$ Oct 17, 2012 at 4:03

2 Answers 2


III is $(0+1)^* (0+ \epsilon)$ which means pick a word from $\Sigma^*$ where $\Sigma = \{0, 1 \}$ and then concatenate it with either $0$ or $\epsilon$.

so III Does generate a string with two consecutive $1$'s. In fact it generates every string which contains $1$'s, $0$'s or is empty.


A is correct only. In II:0110 string present which has two consecutive 11 In III also 11 contain from (0+1)* expression...

  • $\begingroup$ I don't see what this adds over the other answer, which makes the same point more clearly. $\endgroup$
    – Raphael
    Aug 3, 2016 at 13:19

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