Replacing only characters using Regex in string C#

How to get only letters from a string in C#.

using System;
using System.Text.RegularExpressions;

public class Program
{
    public static void Main()
    {
        Console.WriteLine("Replacing only characters using Regex in string C#\n");
        string sampleText = "I like .NET 4.5. This is awesome.";
        //find all characters that are letters
        Regex regex = new Regex("[a-zA-Z]");
        sampleText = regex.Replace(sampleText, "$");
        Console.WriteLine(sampleText);
    }
}

DEMO