using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        //string source = "123gd789fg";

        string source = null;

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

        string res = null;

        try{

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

        }

        catch(Exception){

            res = source;

        }

        Console.WriteLine(res?.ToString());

    }

}

​

​