using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		string pattern1 = @"^([0-9]{2,}?)[A-Z]?[0-9]?$";
		string pattern2 = @"^([0-9]{2})([0-9]+?)[A-Z]?[0-9]?$";
        string[] strings = { "171A1", "171A1", "171A", "171A0", "15211", "15211", "15211", "15210", "15190" };
        
        foreach (String s in strings)
        {
            Console.WriteLine(Regex.Replace(s, pattern1, "$1"));
            Console.WriteLine(Regex.Replace(s, pattern2, "$1.$2"));
        }
	}
}