I've taken a look at the kinds of expressions you can use with the <xsl: template> element's match attribute. You can use an even more involved syntax with the select attribute of the <xsl:apply-templates>, <xsl:value-of>, <xsl:for-each>, <xsl:copy-of>, and <xsl:sort> elements.
The select attribute uses XPath expressions, and XPath is a W3C recommendation as of November 16, 1999. You can find the XPath recommendation (the current version is 1.0) at www.w3.org/TR/xpath.
XPath 2.0 Working Draft
As with XSLT, version 2.0 of XPath is in the works, but it's only a working draft at this point, with frequent changes. No software supports it yet, except for some support in the Saxon XSLT processor (http://saxon.sourceforge.net/ ...