something like this as a 2nd pass should work I think

<xsl:for-each-group select="var" group-by="replace(@name,'.*\.([^\.]*)\.[^\.]*$','$1')">
 <group num="{current-grouping-key()}">
   <xsl:copy-of select="current-group()"/>
 </group>
</xsl:for-each-group>

David