  <td headers="matches"><i>X</i>, via zero-width positive lookahead</td></tr>
 <tr><td valign="top" headers="construct special"><tt>(?!</tt><i>X</i><tt>)</tt></td>
     <td headers="matches"><i>X</i>, via zero-width negative lookahead</td></tr>

 <tr><td valign="top" headers="construct special"><tt>(?&lt;=</tt><i>X</i><tt>)</tt></td>
     <td headers="matches"><i>X</i>, via zero-width positive lookbehind</td></tr>
 <tr><td valign="top" headers="construct special"><tt>(?&lt;!</tt><i>X</i><tt>)</tt></td>
    