title: regular expression to add a target="blank" to all external links
body: <p>Is it possible to add a <code>target="_blank"</code> to all <code>&lt;a&gt;</code> tags, with just one regular expression?  I've been experimenting with negative and positive look aheads/behinds, to no avail.  This should:</p>
  
  <ul>
  <li>Match anchor tags whose <code>href</code> starts with <code>http://</code></li>
  <li>If the anchor tag does not have a <code>target</code> tag, adds a <code>target="_blank"</code> tag</li>
  <li>If it does have a <code>target</code> tag, it checks if the <code>target</code> tag is not already set to <code>"_blank"</code>, and if not it is replaced to <code>target="_blank"</code></li>
  </ul>
  
  <p>Is this possible? If not, what would be the least computationally intensive way to do this?</p>