title: Replace preg_replace() e modifier with preg_replace_callback
body: <p>I'm terrible with regular expressions. I'm trying to replace this:</p>
  
  <pre><code>public static function camelize($word) {
     return preg_replace('/(^|_)([a-z])/e', 'strtoupper("\\2")', $word);
  }
  </code></pre>
  
  <p>with preg_replace_callback with an anonymous function. I don't understand what the \\2 is doing. Or for that matter exactly how preg_replace_callback works. </p>
  
  <p>What would be the correct code for achieving this?</p>