using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string source = "123gdfg";

        //string source = null;

        var r = new Regex("[0-9]", RegexOptions.Compiled | RegexOptions.IgnoreCase);

        var res = r.Replace(source, "x");

        Console.WriteLine(res.ToString());

    }

}

​

​