using System;

using System.Collections.Generic;

using System.Linq;

using System.Globalization;

using System;

using System.Web;

using System.IO;using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        var text = "1POB1677";

Console.WriteLine( Regex.IsMatch(text, "^[0-9]+[A-Z]{0,3}[0-9]+$") && text.Length > 4);

        Console.WriteLine( Regex.IsMatch(text, "^[0-9]+$"));

​

    }

}