xsl:analyze-string trouble or Saxon 7.3 bug?

Hi, where does the XSLT 2.0 spec say that neither matching-substring
nor non-matching-substring is executed if there is no occurrence of
any text matching the regex? Is this a bug in Saxon 7.3?

<xsl:transform version="2.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

  <xsl:variable name="data">
Palaver palaver pullover andover hover mover bother brother mother.
  </xsl:variable>

  <xsl:template match="/">
    <xsl:analyze-string select="$data" regex="diagnosis">
      <xsl:matching-substring>
      	<FOUND><xsl:value-of select="."/></FOUND>
      </xsl:matching-substring>
      <xsl:non-matching-substring>
        <NOT><xsl:value-of select="."/></NOT>
      </xsl:non-matching-substring>
    </xsl:analyze-string>
  </xsl:template>

this finds nothing at all, just discards the string, when I ask
for some regex that is in the string then it works as expected.

I can't find the specification saying that the string not matching
at all is a special case. And if it does, then isn't it a bit awkward?
Wouldn't the intuitive behavior be that the whole non-matching text
gets fed to the non-matching-substring side of it?

thanks,
-Gunther

--
Gunther Schadow, M.D., Ph.D.                    gschadow@xxxxxxxxxxxxxxx
Medical Information Scientist      Regenstrief Institute for Health Care
Adjunct Assistant Professor        Indiana University School of Medicine
tel:1(317)630-7960                         http://aurora.regenstrief.org