I am creating a glossary and I want to group entries under each letter of the alphabet. I am using this and it is working fine:

<xsl:for-each-group select="glossentry" group-by="upper-case(substring(glossterm/text(),1,1))">

<topicref navtitle="{current-grouping-key()}" toc="no">

...

</topicref>

</xsl:for-each-group>

However, I want to group entries that start with a symbol or number all together under one group. I am not sure what to use for the group-by attribute so that letters are separate from each other and all of the non-letter characters are in the same group.