title: Regex replace part of url with string only when url doesn't start with http://
body: <p>Tried to find the answer on stack<b>overflow</b> but I'm a complete RegEx noob!</p>
  
  <p>All I need (if it's possible) is to match a PDF URL in some HTML and if it doesn't start with <code>http://</code> add <code>/content/</code> to the start, if it does start with <code>http://</code> do nothing.</p>