title: Check if given string is an URL
body: <p>In my rails application I need to verify if the URL given by a user is really an URL. I'm only concerned with the HTTP protocol (and perhaps HTTPS, I have not looked into that at all), which leads me to believe that there might something already in rails that can do this work for me.</p>
  
  <p>If not: can you recommend a regex string that does this? I've found some after googling but they all seem to have a problem or two according to user comments.</p>
  
  <p>Thanks</p>