Suppose you have a timestamp string, and want to check whether it fits to one of the following formats with leading and trailing spaces:

• YYYY-MM-DD-HH.MM.SS.NNNNNN
• YYYY-MM-DD-HH.MM.SS
• YYYY-MM-DD

We decided to use regex and its capture groups to extract timestamp parts. This left us with only solution: xsl:analyze-string instruction. It took a couple more minutes to reach a final solution:

<xsl:variable name="parts" as="xs:string*">
  <xsl:analyze-string select="$value"
    regex="
      ^\s*(\d\d\d\d)-(\d\d)-(\d\d)
      (-(\d\d)\.(\d\d)\.(\d\d)(\.(\d\d\d\d\d\d))?)?\s*$"
    flags="x">
    <xsl:matching-substring>
      <xsl:sequence select="regex-group(1)"/>
      <xsl:sequence select="regex-group(2)"/>
      <xsl:sequence select="regex-group(3)"/>

      <xsl:sequence select="regex-group(5)"/>
      <xsl:sequence select="regex-group(6)"/>
      <xsl:sequence select="regex-group(7)"/>

      <xsl:sequence select="regex-group(9)"/>
    </xsl:matching-substring>
  </xsl:analyze-string>
</xsl:variable>

<xsl:choose>
  <xsl:when test="exists($parts)">
    ...
  </xsl:when>
  <xsl:otherwise>
    ...
  </xsl:otherwise>
</xsl:choose>

How would you solve the problem? Is it the best solution?