regex - Regular expression to avoid exceeding character repeatance on a string -

regex - Regular expression to avoid exceeding character repeatance on a string -

i have alphabet {a,b} , want give regular look satisfies set of strings l in symbol a appears @ twice

this have:

(b*|a)b*(b*|a|ε) ε empty string

am right or missing?

i'd simplify following:

b* (a|ε) b* (a|ε) b*

in pcre notation be:

^b*a?b*a?b*$

on side note, think kind of questions belongs more cs.se.

regex