title: XSL - Invalid Xpath Extension on Replace
body: <p>Im getting on error when I try and use the following:</p>
  
  <pre><code>&lt;xsl:variable name="url" select="guid"/&gt;        
  &lt;xsl:variable name="vid" select="substring-after($url,'podcast/')"/&gt;
  &lt;xsl:variable name="pre" select="substring-before($vid,'.mp4')"/&gt;
  
  &lt;&lt;xsl:variable name="p" select="replace($pre,'_','-')"/&gt;
  &lt;xsl:variable name="p1" select="concat($p,'.embed_thumbnail.jpg')"/&gt;
  &lt;xsl:variable name="p2" select="concat('http://images.ted.com/images/ted/tedindex/embed-posters/',$p1)"/&gt;
  </code></pre>
  
  <p>Can anyone see a problem, it all looks good to me?</p>