regex.cs

using System;
using System.Text.RegularExpressions;

namespace ConsoleApplication6
{
    class Program
    {
        static void Main(string[] args)
        {
            string test = "ABC123D";

            var regex = new Regex("[A-Z]*(.*)");

            var result = regex.Matches(test);

            Console.WriteLine(result[0].Groups[1]);
        }
    }
}

Skriver ut:
123D