PREFIX : <http://example.org/>
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX xsd: <http://www.w3.org/2001/XMLSchema#>
SELECT ?s WHERE {
    ?s :p ?v .
    FILTER(datatype(xsd:string(?v)) = xsd:string) .
}