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

Try this regular expression:

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

I think you can see the pattern and expand it for ABCD and ABCDE like:

^(A(B(C(D)?)?)?)?$
^(A(B(C(D(E)?)?)?)?)?$

Now each part depends on the preceeding parts (B depends on A, C depends on B, etc.).