title: How does jQuery.expr[':'] work and where is it documented for jQuery1.8.x and later?
body: <p>I've looked all over for some clear documentation on this syntax. It is used to create custom filters to select very specific elements in the DOM.  There is a good example by Benjamin Arthur Lupton in the <a href="https://gist.github.com/balupton/854622">ajaxify-html5.js </a> gist example (see line 45 where the function is defined and line 78 where it is used), which he provides to show how to ajaxify a web site using his <a href="https://github.com/balupton/history.js">history plug-in</a>. </p>
  
  <p>There is also a fairly extensive blog by Shane Riley called <a href="http://blog.hashrocket.com/posts/jquery-tips-adding-filter-expressions">Adding Filter Expressions</a>, but I've scoured through the jQuery documentation and I can't find any documentation.  </p>
  
  <p>I would be grateful if you could either </p>
  
  <ul>
  <li>point me to the official documentation for the <code>expr</code> function</li>
  <li>show me where to find it in the source files (I've looked - can't find it)</li>
  <li>or suggest alternative syntax based on a technique that is properly documented </li>
  </ul>
  
  <p>I've looked at <code>jQuery.filter</code> but can't figure out an alternative to Lupton's technique in his ajaxify script. </p>
  
  <p>I'm keen to work with a recent jQuery.  I'm working with v 1.8.2 and will be aiming to upgrade to 1.9.x soon.</p>