build_conditions

# File actionpack/lib/action_dispatch/routing/route_set.rb, line 410
      def build_conditions(current_conditions, req_predicates, path_values)
        conditions = current_conditions.dup

        verbs = conditions[:request_method] || []

        # Rack-Mount requires that :request_method be a regular expression.
        # :request_method represents the HTTP verb that matches this route.
        #
        # Here we munge values before they get sent on to rack-mount.
        unless verbs.empty?
          conditions[:request_method] = %[^#{verbs.join('|')}$]
        end
        conditions.delete_if { |k,v| !(req_predicates.include?(k) || path_values.include?(k)) }

        conditions
      end