[xsl] substring-before doesn't copy node

[Marco <spinmar@xxxxxxxxx>]

Hi,
I'm writing a recursive template and I have some problems with 
subtring-before.
I'm trying to write a words highlighter.
In my template if my text contains the word I'm looking for, I write:

<xsl:variable name="before" select="substring-before($test-text, .)" />
<xsl:variable name="after" select="substring-after($test-text, .)"/>
<xsl:variable name="real-before" select="substring($text, 1, 
string-length($before))"/>
<xsl:variable name="real-after" select="substring($text, 
string-length($before) + string-length(.) + 1)"/>

The problem is that my test-text contains for example 
http://dominio.com/&lt;b&gt;pippo&lt;/b&gt;/pluto.html and I'm looking 
for pluto, the before string is http://dominio.com/pippo/. It seems that 
substring-before deletes my tag <b></b>. I suppose that the problem is 
in the substring-after too.
I tried to write:
<xsl:variable name="before"><xsl:copy-of 
select="substring-before($test-text, .)" /></xsl:variable>
but the problem remains.
How could I solve my problem?
Best regards
Marco