How to extract the host name from an URL (C#)

This snippet demonstrates how to extract the host/domain name from a valid URL using regular expressions.
  using System.Text.RegularExpressions;

  ...

  public static string ExtractDomainFromURL(string in_URL)
  {
    string regexPattern = @"^(?(?[^:/\?#]+):)?(?"
                        + @"//(?[^/\?#]*))?(?[^\?#]*)"
                        + @"(?\?(?[^#]*))?"
                        + @"(?#(?.*))?";

    Regex re = new Regex(regexPattern, RegexOptions.ExplicitCapture);
    Match m = re.Match(in_URL);

    return m.Groups["s1"].Value + m.Groups["a1"].Value;
  }