I'm trying to convert the following NFA to a regular expression.

I've attached my work below and end up with the expression $aa^*bb^*$. As far as I can tell, this doesn't seem correct but I've been working at it for quite a while. Can anyone tell me where I went wrong? And if it happens to be correct, can you tell me why?

Thanks a lot in advance.

  • $\begingroup$ How does the NFA in the 1st image relate to the top-left NFA in the 2nd image? $\endgroup$
    – frabala
    Mar 6, 2020 at 15:35
  • $\begingroup$ @frabala, the top left NFA is the GNFA corresponding to the 1st image (with the exclusion of the phi transitions. $\endgroup$
    – Pwned1760
    Mar 7, 2020 at 2:46
  • $\begingroup$ In the elimination of (1,2), you've lost the transition from (2,3) on a. $\endgroup$
    – rici
    Mar 7, 2020 at 3:56

1 Answer 1


Think of it this way. You can reach 2 from 1 in two ways

  • $1\rightarrow 2$
  • $1\rightarrow 3 \rightarrow 2$

Then from 2, you can move to 1 by $2\rightarrow 1$. From there again, move to 2 from 1 in the above two ways. So this NFA can be expressed as an RE as follows:

$$RE = (1\rightarrow 2 + 1\rightarrow 3 \rightarrow 2) \cdot [(2\rightarrow 1)\cdot (1\rightarrow 2 + 1\rightarrow 3 \rightarrow 2)] ^*$$

Now its just the matter of finding the REs for $1\rightarrow 2$, $1\rightarrow 3 \rightarrow 2$ and $2 \rightarrow 1$.


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