I have a feeling that my original code for UPC was right, as seen also
in the "Check digit calculation" section here,
http://en.wikipedia.org/wiki/Universal_Product_Code.

Inspired by Abel, I have now made an improved version of my original
code for testing if UPC code is legal. The two versions, both tested,
are shown side by side.

<?xml version="1.0"?>
<!-- legal UPC e.g.: 639382000393-->
<test>
  <upc>639382000393</upc>
</test>

<?xml version="1.0"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="2.0"
  xmlns:xs="http://www.w3.org/2001/XMLSchema" exclude-result-prefixes="xs">
  <xsl:output indent="yes"/>
  <xsl:strip-space elements="*"/>
  <xsl:template match="test/upc">
      <topelement>
          <title>Test of UPC code. Legal code returns empty elements.</title>

              <xsl:variable name="x"
select="((xs:integer(substring(., 1, 1)) + xs:integer(substring(.,
3,1)) + xs:integer(substring(., 5, 1)) + xs:integer(substring(., 7,
1)) + xs:integer(substring(., 9, 1)) + xs:integer(substring(., 11,
1))) * 3 + (xs:integer(substring(., 2, 1)) + xs:integer(substring(.,
4, 1)) + xs:integer(substring(., 6, 1)) + xs:integer(substring(., 8,
1)) + xs:integer(substring(., 10, 1)))) mod 10"/>

              <xsl:if test="(if ($x ne 0) then (10 - $x) else $x) ne
xs:integer(substring(., 12, 1))">UPC not legal</xsl:if>

              <xsl:variable name="y" select="for $i in
string-to-codepoints(substring(., 1, string-length(.) - 1)) return
codepoints-to-string($i)"/>

<xsl:variable name="z" select="sum((for $i in $y[(position()) mod 2 =
1] return xs:integer($i)*3, for $i in $y[(position()) mod 2 = 0]
return xs:integer($i))) mod 10"/>

              <xsl:if test="(if ($z ne 0) then (10 - $z) else $z) ne
xs:integer(substring(., string-length(.), 1))">UPC not legal</xsl:if>

          </improved>
      </topelement>
  </xsl:template>
</xsl:stylesheet>

Cheers,
Jesper Tverskov