def rdf_find_subject(id)
      return nil unless rdf_valid?
      rdf.each_subject do |s|
        return s if s.to_s =~ Regexp.new(id, Regexp::IGNORECASE)
      end
      nil
    end
def rdf_find_subject(id)
      return nil unless rdf_valid?
      rdf.each_subject do |s|
        return s if s.to_s =~ Regexp.new(id, Regexp::IGNORECASE)
      end
      nil
    end
Regexp search to find a subject matching a string
 @param id [String] A string literal used to construct a Regexp
 @return [RDF::URI] The first subject matching the Regexp