CODEWITHSUNDEEP
regexregular-languageautomataformal-languages
Elijah Williams
Elijah Williams

Reputation: 17

Regex with even instances of ab and ba

I am trying to write a regex to match strings with even instances of ab and ba with only the alphabet of {a,b}.

I am trying to do so with only | and * operators. I think it has something to do with the number of times I switch letters needing to be even.

Ex: aba is accepted (1 ab and 1 ba). It switches between characters twice.

aaaa is accepted as it has 0 instances of ab and ba and switches characters 0 times.

I am really struggling to generate the regex.

I tried regexes like (a*b*a*)|(b*a*b*) and a few others but I cannot seem to solve it.

Upvotes: 0

Views: 208

Answers (1)

ruakh
ruakh

Reputation: 183514

If a string contains only a's and b's, then the number of occurrences of ab will be equal to the number of occurrences of ba if and only if it starts and ends with the same letter.

So you can write: a(a|b)*a|b(a|b)*b

Upvotes: 2

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