{} quantifiers in regex

I have this XML
<?xml version="1.0" encoding="UTF-8"?>
<test>43:12</test>

If I change it to this
(removing \d{2} in favour of \d\d)

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="2.0">
<xsl:template match="test">
    <xsl:variable name="this-duration" select="."/>
    <xsl:analyze-string select="$this-duration" regex="((\d\d):)?(\d\d):\d\d">
        <xsl:matching-substring>
            <xsl:value-of select="regex-group(3)"></xsl:value-of>
        </xsl:matching-substring>
      </xsl:analyze-string>
</xsl:template>
</xsl:stylesheet>

Am I overlooking something?
I am using Saxon 9B in Oxygen,
I had the same issue with older versions, but failed to report it