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SPARQL is the standard query language for RDF graphs. In its strictinstantiation, it only offers querying according to the RDF semantics and wouldthus ignore the semantics of data expressed with respect to (RDF) schemas or(OWL) ontologies. Several extensions to SPARQL have been proposed to query RDFdata modulo RDFS, i.e., interpreting the query with RDFS semantics and/orconsidering external ontologies. We introduce a general framework which allowsfor expressing query answering modulo a particular semantics in an homogeneousway. In this paper, we discuss extensions of SPARQL that use regularexpressions to navigate RDF graphs and may be used to answer queriesconsidering RDFS semantics. We also consider their embedding as extensions ofSPARQL. These SPARQL extensions are interpreted within the proposed frameworkand their drawbacks are presented. In particular, we show that the PSPARQLquery language, a strict extension of SPARQL offering transitive closure,allows for answering SPARQL queries modulo RDFS graphs with the same complexityas SPARQL through a simple transformation of the queries. We also considerlanguages which, in addition to paths, provide constraints. In particular, wepresent and compare nSPARQL and our proposal CPSPARQL. We show that CPSPARQL isexpressive enough to answer full SPARQL queries modulo RDFS. Finally, wecompare the expressiveness and complexity of both nSPARQL and the correspondingfragment of CPSPARQL, that we call cpSPARQL. We show that both languages havethe same complexity through cpSPARQL, being a proper extension of SPARQL graphpatterns, is more expressive than nSPARQL.

Answering SPARQL queries modulo RDF Schema with paths