Replacing only characters using Regex in C#
REGEX stands for Regular expression.
Using regex, we can replace the characters,numbers with a string like this. (from a to z, A to Z)

Syntax:
Regex regex = new Regex("[Regular expression]");
output = regex.Replace("input_string", "substitution_string");



using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {          

            string filename = "My name is c-sharp 007; (got it !)";
            //find all characters that are letters
            Regex regex = new Regex("[a-zA-Z]");
            filename = regex.Replace(filename, "$");

            Console.WriteLine(filename);
                        
        }

    }
}


Output:
$$ $$$$ $$ $-$$$$$ 007; ($$$ $$ !)