Question

I’m working on some homework for my compiler class and I have the following problem:

Write a regular expression for all strings of a‘s and b‘s that contain an odd number of a‘s or an odd number of b‘s (or both).

After a lot of whiteboard work I came up with the following solution:

(aa|bb)* (ab|ba|a|b) ((aa|bb)* (ab|ba) (aa|bb)* (ab|ba) (aa|bb)*)*

However, Is this is the most simplified I can get it? I’ve considered constructing the DFA trying to minimize the number of states there to see if it would help me simplify but I figured I would ask the regex gurus on SO first.

Answer 1

Take Greg D’s recommendation of starting with a(aa)*, and going from there. Sepp2k almost has it right, but the real consideration is that you don’t care about the other letter. What I mean is, when you are looking at the “odd number of a’s” constraint, you don’t care at all about what b’s are in your string. Thus, stick b*’s anywhere you can 🙂

Sepp2k’s answer is almost right, but this one is correct:

b* a b* (a b* a b* )* | a* b a* (b a* b a* )*

To elaborate, this regex figures out all strings with an odd number of a’s (first section), and OR’s those strings with any strings containing an odd number of b’s.