According to the XSLT Recommendation, Section 5.3 "Defining Template Rules",

[quote]It is an error for the value of the "match" attribute to contain a
"VariableReference".[/quote]

Here is the sample XLST which incorrectly uses a variable:

[code]
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<!-- -->

<xsl:variable name="a" select="2"/>

<xsl:template match="/*[$a=2]">
  <out/>
</xsl:template>

</xsl:stylesheet>
[/code]

Executed with xsltproc, the stylesheet produces the following result:

[screen]
<?xml version="1.0"?>
<out/>
[/screen]

But instead of running XSLT, xsltproc should report the error, something like it:

Error at xsl:template on line 6 of file:<filename>:
  The match pattern in xsl:template may not contain references to variables