How can I test the last preceding text character?

How do I test if the very last character of any preceding text - be it
in a parent or a sibling - has a certain value?
Example:

<a>
    This is parent text a not ending with an x.
    <d>This ends with an x</d>
    <e>This should issue a message.</e>
    This is parent text ending with an x
</a>

Now I want to know for each child element is if it is preceded by an
"x".
I can check the sibling with

<xsl:if test="matches(preceding-sibling::*[1]/text()[last()],'x\$') ">
    <xsl:message>
        Element <xsl:value-of select="name()"/> is preceded by an x.
    </xsl:message>
</xsl:if>

But I cannot test the surrounding parent content with the same model
because the parent text may surround the child text.

What's the way to do it?