I had a question on an assignment where we were supposed to write a regular expression for a language where every $a$ in $w$ is immediately preceded and followed by a $b$. My answer was $\epsilon + (b + bab)^*$. The teacher pointed out that my answer makes an $a$ surrounded by at least two $b$s, which isn't correct. I can't for the life of me figure out how to fix it to make it correct. Can anyone help?