PREFIX   :      <http://example/>
PREFIX  rdf:    <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX  rdfs:   <http://www.w3.org/2000/01/rdf-schema#>
PREFIX  xsd:    <http://www.w3.org/2001/XMLSchema#>

SELECT ?v ?y
WHERE
 { :x ?p1 ?y .
   OPTIONAL
   {
     ?y ?p ?v 
     FILTER regex(?v, "^a")
   }
 }