.net

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);
}
}

private string reverse(string src)
        {
            char[] s = src.ToCharArray();
            char c='\0';
            int i = 0;
            int j = s.Length - 1;
            for (; i < j; )
                if (s[i] >= 65 && s[i] <= 90 || s[i] >= 97 && s[i] <= 122)
                    if (s[j] >= 65 && s[j] <= 90 || s[j] >= 97 && s[j] <= 122)
                    { c = s[i]; s[i] = s[j]; s[j] = c; i++; j--; }
                    else
                        j--;
                else
                    if (s[j] >= 65 && s[j] <= 90 || s[j] >= 97 && s[j] <= 122)
                        i++;
                    else
                    { i++; j--; }
            return new string(s);
        }