Ho un problema con una query SPARQL, che non ritorna il risultato atteso.

L'ontologia su cui lavoro è questa (riporto le triple rdf):

codice:
s: urn:myApp/domain/classes#tool - p: http://www.w3.org/1999/02/22-rdf-syntax-ns#type - o: http://www.w3.org/2002/07/owl#Class 
s: urn:myApp/domain/classes#tecnica - p: http://www.w3.org/1999/02/22-rdf-syntax-ns#type - o: http://www.w3.org/2002/07/owl#Class 
s: urn:myApp/domain/individuals#stateclasses - p: http://www.w3.org/1999/02/22-rdf-syntax-ns#type - o: urn:myApp/domain/classes#tecnica
s: urn:myApp/domain/individuals#mytool - p: http://www.w3.org/1999/02/22-rdf-syntax-ns#type - o: urn:myApp/domain/classes#tool 
s: urn:myApp/domain/properties#implementa - p: http://www.w3.org/1999/02/22-rdf-syntax-ns#type - o: http://www.w3.org/2002/07/owl#ObjectProperty 
s: urn:myApp/domain/properties#implementa - p: http://www.w3.org/2000/01/rdf-schema#domain - o: urn:myApp/domain/classes#tool 
s: urn:myApp/domain/properties#implementa - p: http://www.w3.org/2000/01/rdf-schema#range - o: urn:myApp/domain/classes#tecnica 
s: urn:myApp/domain/properties#nome - p: http://www.w3.org/1999/02/22-rdf-syntax-ns#type - o: http://www.w3.org/2002/07/owl#DatatypeProperty 
s: urn:myApp/domain/properties#nome - p: http://www.w3.org/2000/01/rdf-schema#domain - o: urn:myApp/domain/classes#tecnica 
s: urn:myApp/domain/properties#nome - p: http://www.w3.org/2000/01/rdf-schema#range - o: http://www.w3.org/2001/XMLSchema#string 
s: urn:myApp/domain/individuals#mytool - p: urn:myApp/domain/properties#implementa - o: urn:myApp/domain/individuals#stateclasses 
s: urn:myApp/domain/individuals#stateclasses - p: urn:myApp/domain/properties#nome - o: "stateclasses"^^http://www.w3.org/2001
e questa la query
codice:
SELECT DISTINCT ?subject WHERE 
{ ?subject <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <urn:myApp/domain/classes#tool>.  ?subject <urn:myApp/domain/properties#implementa> ?x1.  
?x1 <urn:myApp/domain/properties#nome> ?value. 
FILTER regex(str(?value), "x","i").}  
ORDER BY ?subject
Io mi aspettavo che la query non trovasse nessun risultato, invece mi ritorna 1 elemento, perchè la variabile ?value contiene il carattere x (in ...XMLSchema...). Ma la funzione str() non dovrebbe estrarre solo il valore "testuale", cioè la prima parte del literal?

Potete aiutarmi? grazie