Regular expression to match A, AB, ABC, but not AC. (“starts with”)

Question

I\'m banging my head against a wall. I want a regex that matches: empty string, A, AB, and ABC, but not AC. I have this,

Answer 1

/^A(?:B(?:C)?)?$/

should do it.

This is using the non-capturing group construct (?: xxx ) so as not to mess up any match capturing you may be doing.