Converting URLs Into Links With Regex

Following on from the recent spate of extensions methods I've posted, here's another I use to convert URLs and email addresses into links within HTML. You may want to restrict users from submitting HTML tags via forms in your application, which means that URLs and email addresses that they submit appear as plain text unless they are subjected to some kind of processing.

/// <summary>
/// Finds web and email addresses in a string and surrounds then with the appropriate HTML anchor tags 
/// </summary>
/// <param name="s"></param>
/// <returns>String</returns>
public static string WithActiveLinks(this string s)
  //Finds URLs with no protocol
  var urlregex = new Regex(@"\b\({0,1}(?<url>(www|ftp)\.[^ ,""\s<)]*)\b", 
    RegexOptions.IgnoreCase | RegexOptions.Compiled);
  //Finds URLs with a protocol
  var httpurlregex = new Regex(@"\b\({0,1}(?<url>[^>](http://www\.|http://|https://|ftp://)[^,""\s<)]*)\b", 
    RegexOptions.IgnoreCase | RegexOptions.Compiled);
  //Finds email addresses
  var emailregex = new Regex(@"\b(?<mail>[a-zA-Z_0-9.-]+\@[a-zA-Z_0-9.-]+\.\w+)\b", 
    RegexOptions.IgnoreCase | RegexOptions.Compiled);
  s = urlregex.Replace(s, " <a href=\"http://${url}\" target=\"_blank\">${url}</a>");
  s = httpurlregex.Replace(s, " <a href=\"${url}\" target=\"_blank\">${url}</a>");
  s = emailregex.Replace(s, "<a href=\"mailto:${mail}\">${mail}</a>");
  return s;

This will convert most URLs, but not all. Parsing URLs is not the easiest thing to do so you need to make a judgement on what type of URLs your users/visitors are most likely to provide and alter the regex patterns accordingly. One thing to point out is that the second pattern (the one that matches URLs with a protocol - http, https etc) also checks to make sure that it isn't already a hyperlink. By the time the second Replace() operations takes place, URLs without protocols will already be fitted with them, and have HTML surrounding them.