A variant of this is to use xsl:where-populated:

>       <xsl:analyze-string select="." regex="^SUBTASK
(\d+)-(\d+)-(\d+)-((\d+)-(\d+) - \((\d+)\))?$">
>            <xsl:matching-substring>
                <xsl:where-populated>
>                 <xsl:attribute name="chapnbr" select="regex-group(1)"/>
>                 <xsl:attribute name="sectnbr" select="regex-group(2)"/>
>                 <xsl:attribute name="subnbr" select="regex-group(3)"/>
>                 <xsl:attribute name="func" select="regex-group(4)"/>
>                 <xsl:attribute name="seq" select="regex-group(5)"/>
>                 <xsl:attribute name="revdate" select="regex-group(6)"/>
                </xsl:where-populated>
>            </xsl:matching-substring>
>         </xsl:analyze-string>
>

The xsl:where-populated instruction filters out any attributes whose string
value would be zero-length.

Michael Kay
Saxonica