using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        String val = "client: aaa-bbb-ccc";

        String replaced = Regex.Replace(val, "[^a-zA-Z0-9_]", "_");

        Console.WriteLine(replaced);

​

    }

}