xpath regular expressions example?

[Mary Gardiner]

When Sam first described the xpath-sifter filter, one of the advantages
people talked about was the ability to use regular expressions in the
filters. Is this in fact possible? I know XPath 2.0 introduces regular
expressions, but I can't figure out if or how to use the matches()
function.

Something like this does not work:

require:
  //atom:title[matches(.,'Release')]

I get the following Python error:

ERROR:planet.runner:xmlXPathCompOpEval: function matches not found
Unregistered function
xmlXPathEval: 3 object left on the stack

-Mary