derive_named_filter

# File railties/lib/rails/test_unit/runner.rb, line 137
        def derive_named_filter(filter)
          if filter.respond_to?(:named_filter)
            filter.named_filter
          elsif filter =~ %/(.*)/% # Regexp filtering copied from minitest.
            Regexp.new $1
          elsif filter.is_a?(String)
            filter
          end
        end