XPath with regex match on an attribute value

I've searched and tinkered around for hours in an effort to crack this one, but I'm still having problems. I have the XML data below:

<game id="2009/05/02/arimlb-milmlb-1" pk="244539">
  <team id="109" name="Arizona" home_team="false">
    <event number="9" inning="1" description="Felipe Lopez doubles to left fielder Chris Duffy.  "/>
    <event number="15" inning="1" description="Augie Ojeda flies out to center fielder Mike Cameron.  "/>
    <event number="23" inning="1" description="Chad Tracy doubles to right fielder Joe Sanchez.  "/>
    <event number="52" inning="2" description="Mark Reynolds lines out to left fielder Chris Duffy.  "/>
    <!-- more data here -->
  </team>
</game>

I'm trying to get the total number of event nodes that contain the text ' doubles ' in the value of the description attribute. This is what I've been trying so far, to no avail (irb throws an error):

"/game/team/event/@description[matches(.,' doubles ')]"

Since I'm just trying to match a fragment of the value of the description attribute, it's possible to use the XPath 2.0 function 'matches', right? If so, what am I doing wrong?