<td headers="matches"><i>X</i> followed by <i>Y</i></td></tr>
 <tr><td valign="top" headers="construct logical"><i>X</i><tt>|</tt><i>Y</i></td>

     <td headers="matches">Either <i>X</i> or <i>Y</i></td></tr>
 <tr><td valign="top" headers="construct logical"><tt>(</tt><i>X</i><tt>)</tt></td>
     <td headers="matches">X, as a <a href="#cg">capturing group</a></td></tr>

 <tr><th>&nbsp;</th></tr>
 <tr align="left"><th colspan="2" id="backref">Back references</th></tr>

 <tr><td valign="bottom" headers="cons