how to counts the character in string using c#

  string inputstr;
            Console.WriteLine("Enter Tex");
            inputstr=Console.ReadLine();
             string a;
            Console.WriteLine("Enter character to find");
            a =Console.ReadLine();
            
            Regex exp = new Regex(a, RegexOptions.IgnoreCase);
            int count = exp.Matches(inputstr).Count;
            Console.WriteLine(count);
            Console.ReadKey();