А>

class Program {
    static void Main() {
        var regex = new Regex(@"AB_(?<num>(0?\d{1,5})|100000)_CD");
        Test(regex, "AB_012_CD");
        Test(regex, "AB_000000_CD");
        Test(regex, "AB_0000000_CD");
        Test(regex, "AB_100000_CD");
        Test(regex, "AB_200000_CD");
        Test(regex, "AB_012345_CD");
    }

    static void Test(Regex regex, string s) {
        var match = regex.Match(s);
        Console.WriteLine("{0}: {1}", s, match.Success);
        if (match.Success) {
            string num = match.Groups["num"].Value;
            Console.WriteLine("  {0} - {1}", num, int.Parse(num));
        }
    }
}