不規則字符交換字符串中字母位置

//交換字符串首尾位置

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);
        }

 

string t = ReverseAlpha("1w4rt,5t7?9u");//1w4rt,5t7?9u、字符串換位後輸出1u4tt,5r7?9w