Remove non letters characters from a string but leave alone the spaces(C#)

Am working on a decipher algorithm that is supposed to take a string input of en english sentence flipped backwards and contains numbers, spaces and delimiters.
I got code that removes numbers and special characters but also removes spaces, Can you help me tell this code to not bully the spaces and just leave them in my string, Thank You.

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

public class Program
{
    static List<string> mywords=new List<string>();
    public static void Main()
    {
        string inp=Console.ReadLine();
        string answer=Regex.Replace(inp,"[^a-zA-Z]", "");
        string replacement=reverse(answer);
        Console.WriteLine(replacement);
    }
    private static string reverse(string b){
     string reverse="";int Length=0;
          Length = b.Length - 1;  
            while(Length>=0)  
            {  
                reverse = reverse + b[Length];  
                Length--;  
            }  
        return reverse;
    }
}