CONSTRUCT {
 ?topic <http://www.w3.org/2004/02/skos/core#inScheme> <http://www.openvocabulary.info/thesauri/ot/pl/> .
} WHERE {
  ?topic a ?t .
  OPTIONAL { ?topic <http://www.w3.org/2004/02/skos/core#inScheme> ?scheme }
  FILTER ( ! bound( ?scheme ) && 
           regex ( str ( ?topic ), "http://www.openvocabulary.info/thesauri/ot/pl/(.+)", "" )
         )
}