XPath regex for file extension

XPath regex for file extension

<root>
<url>
<str>http://test.com/test.csv</str>
<str>http://example.org/rsgis.html</str>
<str>http://something.edu/thro.zip</str>
</url>
</root>
Using Xpath can I find ONLY the file extensions (csv or html or zip) for
the url's. Any help is greatly appreciated..
Thanks in advance.