Triples not initialized after FILTER on SPARQL

[emri mbiemri]

Dear all,

I am trying to query something from an RDF/XML file with SPARQL but when I try 
queries like this:

SELECT DISTINCT ?class
WHERE {
 
                ?s ?p ?attr .
                FILTER regex(?attr , "Emri") .
                ?s <http://www.w3.org/TR/html4/Class1> ?class . 

it should after the FILTER command initialize ?s = _:b0 , but it doesn't?

Hope for any support.