using System;
using System.Text.RegularExpressions;

class Demo
{
  static string ReverseAlpha(string s)
  {
    string t = Regex.Replace(s, "[^A-Za-z]", "");
    char[] c = s.ToCharArray();
    for (int i = 0, j = t.Length - 1; i < c.Length; i++)
    {
      if (Char.IsLetter(c[i])) // c[i] >= 'A' && c[i] <= 'Z' || c[i] >= 'a' && c[i] <= 'z'
      {
        c[i] = t[j];
        j--;
      }
    }
    return new string(c);
  }

  static void Main()
  {
    string s = "1w4rt,5t7?9u";
    Console.WriteLine(s);
    string t = ReverseAlpha(s);
    Console.WriteLine(t);
  }
}