SPARQL: How to restrict a variable to a namespace?

Question

Consider the following SPARQL query:

PREFIX basics: <http://example.org/basic-knowledge/>
PREFIX special: <http://example.org/special-predicates/>

SELECT ?S ?P ?O
WHERE
{

 ?S ?P ?O.

}

This yields all triples.

How can I restrict the result to those triples where the predicate ?P is from the namespace with the prefix special:?

Answer 1

This can be done with a string test. str() turns URIs into strings.

FILTER(strStarts(str(?P), str(special:)))

note the special: is replaced by the parser with the full URI: http://example.org/special-predicates so the executiuon is

FILTER(strStarts(str(?P), str(<ttp://example.org/special-predicates>)))

Runnable example:

PREFIX ns1: <http://example/>
PREFIX ns2: <http://ex/>

SELECT * {
 VALUES ?x { ns1:abc ns2:xyz }
 FILTER(strstarts(str(?x), str(ns1:)))
}