How to parse text nodes in xml using XSLT 2.0 functions

Let's say I have this temporary document:

<xsl:variable name="var">
    <root>
        <sentence>Hello world</sentence>
        <sentence>Foo foo</sentence>
    </root>
</xsl:variable>

I am using analyze-string to find "world" string and wrap it with element called <match>

<xsl:function name="my:parse">
    <xsl:param name="input"/>
        <xsl:analyze-string select="$input" regex="world">
            <xsl:matching-substring>
                <match><xsl:copy-of select="."/></match>
            </xsl:matching-substring>
            <xsl:non-matching-substring>
                <xsl:copy-of select="."/>
            </xsl:non-matching-substring>
        </xsl:analyze-string>
</xsl:function>

This function will return:

Hello <match>world</match>Foo foo

Naturally I want this output:

    <root>
        <sentence>Hello <match>world</match></sentence>
        <sentence>Foo foo</sentence>
    </root>

Nevertheless, I know why is my function doing this, but I just can't figure it out how to copy elements and inject them with new content. I know there is issue with context item but I tried so many other ways and nothing works for me.

On the other hand, using template that match all text works fine. But requirement is to use function (because I am working on multiphase transform and I want to use functions that represent each step). I wonder is there solution to this problem? Am I missing something fundamental?