I'm trying to write a regex that only matches a string that includes lowercase a & b such that each a has the letter b immediately before it and after it.

e.g., the Regex should match the string 'bbbabbababbbbab' but not 'abbbbaabbab'

I have written an expression that does satisfy the example however, the regex should also match a string such as 'bab' but it does not.

My current expression is ^[b] (ab) [ab]*[b]$

Anyone have any advice on what I'm doing wrong?

CodePudding user response：

You are repeating single chars ab in the group (ab)

You can repeat the a's and the b's as well:

^b (?:a b )*$

Regex demo

If you only want to match a single a and the a should be there at least once:

^b (?:ab ) $

Regex demo