Character entity references cannot be used in the ruleset unless they have been declared there. They can be declared in the internal subset of the XMLProbe ruleset, just as for any other XML document:

<?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE silcn:silcn [
        <!ENTITY reg '&#x00AE;'>
    ]>
    <silcn:silcn xmlns:silcn="http://silcn.org/200309" xmlns:probe="http://xmlprobe.com/200312">
    <!--etc.-->

It is then possible to use the entity reference in an XPath expression:

<silcn:expression>//foo[ contains( ., '&reg;' ) ]</silcn:expression>

Otherwise, character entities must be addressed by their Unicode code point, rather than their name, e.g.

<silcn:expression>//foo[ contains( ., '&#x00AE;' ) ]</silcn:expression>