Matching string positions

[Jérôme]

Hello,

I am a new Jena/Joseki user and i would like to get matching string 
positions in order to highligth them.
Queries are LARQ queries.
Here the client side code:

        Query query = QueryFactory.create(queryString) ;
        QueryExecution qExec = QueryExecutionFactory.sparqlService(url, 
query);
        ResultSet set = qExec.execSelect();
        String xml = ResultSetFormatter.asXMLString(set) ;
        qExec.close();

I probably have to use an other way than the sparqlService method. I 
know that positions are available with Lucene, but how to recover this 
information with the Jena API ?

What is the a simple way to do that?

Thanks.

Jerome.