{
        var pat = new Regex(@"^A(B(C(.(Z)?)?)?)?");
        var testStrings = new string[]
        {
            "ALPHA",
            "ABGOOF",
            "ABCblah",
            "ABCbZ",
            "FOOBAR"
        };
        foreach (var s in testStrings)
        {
            var m = pat.Match(s);
            if (m.Success)
            {
                Console.WriteLine("{0} matches {1}", s, m.Value);
            }
            else
            {
                Console.WriteLine("No match found for {0}", s);
            }
        }
    }

ALPHA matches A
ABGOOF matches AB
ABCblah matches ABCb
ABCbZ matches ABCbZ
No match found for FOOBAR