Something

using System;
using System.Text.RegularExpressions;

namespace ConsoleApplication1
{
	public class Program
	{
		public static void Main( string[ ] args )
		{
			Console.WriteLine( "Enter a hyperlink: " );
			string hyper = Console.ReadLine( );

			Regex r = new Regex( @"^(\bhttp://|\bhttps://|\bftp://)\w+\w{2,6}", RegexOptions.IgnoreCase );
			string hyperReplace = r.Replace( hyper, hyper + ".com" );

			Console.WriteLine( hyperReplace );
		}
	}
}