using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
        var html = @"
            <div>
              <a href=""http://g...content-available-to-author-only...1.com:1139"" target=""_blank"">http://g...content-available-to-author-only...2.com:1139</a> 
              http://g...content-available-to-author-only...3.com:1139
            </div>
            <div>
              <a href=""http://g...content-available-to-author-only...4.com:1139"" target=""_blank"">http://g...content-available-to-author-only...5.com:1139</a>
              http://g...content-available-to-author-only...6.com:1139
            </div>
        ";
        html = Regex.Replace(html,
            @"<a.*?</a>|(?:https?|ftp)://[\w_.-]+:\d+",
            m => m.Value.StartsWith("<") 
                ? m.Value : string.Format("<a target='_blank' href='{0}'>{0}</a>", m.Value));
        Console.WriteLine(html);
	}
}