I keep forgetting that the sort expression needn't be a single xpath in
xslt2, so in that case you don't need to use the tricky replace regexp
to add the zero, you can do what I had in mind really and use
format-number to express all numbers with teh same number of digits.

This version works up to 6 digits (and without the
restrictions on digit runs being close)

  <xsl:for-each select="server">
      <xsl:sort>
      <xsl:value-of>
      <xsl:analyze-string select="@dn" regex="[0-9]+">
       <xsl:matching-substring>
          <xsl:value-of select="format-number(number(.),'000000')"/>
       </xsl:matching-substring>
       <xsl:non-matching-substring>
	 <xsl:value-of select="."/>
       </xsl:non-matching-substring>
      </xsl:analyze-string>
      </xsl:value-of>
      </xsl:sort>
      <xsl:copy-of select="."/>
    </xsl:for-each>
  

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