DouKaku?

//http://ja.doukaku.org/148/　投稿用
using System;
static class Program {
    static string Tr(this string target, string from, string to) {
        for(int i = 0; i <= from[2] - from[0]; i++) {
            target = target.Replace((char)(from[0] + i), (char)(to[0] + i));
        }
        return target;
    }
    static void Main(string[] args) {
        Console.WriteLine("ABCDEF".Tr("A-D", "a-d"));
        Console.ReadLine();
    }
}