using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        ValidateText("hello world").Dump(); // returns false

        ValidateText("123456789A").Dump(); // returns false

        ValidateText("1234567A").Dump(); // returns false

        ValidateText("1234567A").Dump(); // returns false

        ValidateText("12345678A").Dump(); // returns true

    }

    private static bool ValidateText(string input)

    {

       string pattern = @"^((([A-Za-z])\d{8})|(\d{8}([A-Za-z])))$";

​

       Regex r = new Regex(pattern);

​

       return r.IsMatch(input);

    }

}