[xsl] Testing the Last Character of a String

I'd like to test the last character in a node text-string. If it's a
period, then insert the value and a space. If it's not a period, then
insert the value, a period., and a space. I think I've gotten most of
the code right, but don't have it quite right, as it's not working. Any tips?

Thanks,
Nathan

<xsl:choose>
    <xsl:when test="substring(archdesc/did/unittitle/text(),
string-length(archdesc/did/unittitle), 1)=.">
        <xsl:value-of select="normalize-space(archdesc/did/unittitle)"/>
        <xsl:text>&#160;</xsl:text>
    </xsl:when>
    <xsl:otherwise>
        <xsl:value-of select="normalize-space(archdesc/did/unittitle)"/>
        <xsl:text>.&#160;</xsl:text>
    </xsl:otherwise>
</xsl:choose>

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