BASE <http://ex.example/>
PREFIX ex: <http://a.example/>
PREFIX sx:  <http://www.w3.org/ns/shex#>

[] a sx:Schema ;
  sx:start _:S1 ;
  sx:shapes (_:S1) .

_:S1 a sx:ShapeDecl ; sx:shapeExpr [ a sx:Shape ;
  sx:expression
    [ a sx:TripleConstraint ; sx:predicate ex:p1 ; sx:min 0 ; sx:max -1 ;
      sx:valueExpr _:S1 ] ] .