Something like this:
<xsl:variable name="quot">"</xsl:variable>
<xsl:variable name="hrefmarker"
select="concat('href=', $quot)"/>
<xsl:variable name="s1" select="substring-after(@attribute, $hrefmarker)"/>
<xsl:variable name="s2" select="substring-before($s, $quot)"/>
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference