That is nearly thats what i want. But i want only PB123.
With the function substring-after i get "PB123 Rest of the Comment".

What i now what to do is the following: I will islote the terms "PB123" and
"PB456" and so on in the hole document. Therefore i need a spezial pattern i
think. The pattern is always the same: "PB" and then 3 or 4 numbers. I will
write this terms in an extra column in a HTML table.

Check out the XPath string functions http://www.w3.org/TR/xpath.  The
function substring-after seems to be what you're looking for.

<xsl:template match="comment">
<td>PB<xsl:value-of select="substring-after(.,"PB")" /></td>
</xsl:template>