using System;

using System.Text.RegularExpressions;

// see: https://stackoverflow.com/review/low-quality-posts/20001794

public class Program

{

    public static void Main()

    {

        var p1 = "aaaa999999"; 

        CheckMatch(p1);

        p1 = "aaaa9999999";

        CheckMatch(p1);

        p1 = "aaaa99999999";

        CheckMatch(p1);

        p1 = "aaa999999";

        CheckMatch(p1);

    }

    public static void CheckMatch(string p1)

    {

        var reg = new Regex(@"^\w{4}\d{6,7}$");

        if (!reg .IsMatch(p1))

        {

            Console.WriteLine($"{p1} doesn't match");

        }

        else

        {

            Console.WriteLine($"{p1} matches");

        }

    }

}

​