<p>using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.IO;
using System.Xml;
using System.Text.RegularExpressions;

public interface IStringUtilities
{
  string ConvertPlainTextUrlsToHtmlHyperlinks(string input, HyperlinkOptions options = HyperlinkOptions.None);
}

public class Solution : IStringUtilities
{
  public string ConvertPlainTextUrlsToHtmlHyperlinks(string input, HyperlinkOptions options = HyperlinkOptions.None)
  {
    Regex r = new Regex("(https?://[^ ]+)");
    var target = (options &amp; HyperlinkOptions.OpenInNewWindow) == HyperlinkOptions.OpenInNewWindow ?
      " target='_blank'" : String.Empty;
    var noFollow = (options &amp; HyperlinkOptions.NoFollow) == HyperlinkOptions.NoFollow ?
      " rel='nofollow'" : String.Empty;
    return r.Replace(input, String.Format("<a href="%5C%22%241%5C%22%7B0%7D%7B1%7D">$1</a>", target, noFollow));
  }
}

public enum HyperlinkOptions
{
	None = 0,
	OpenInNewWindow = 1,
	NoFollow = 2
}</p>