docs: def regex_query(expression):
      """Apply regular expression to result of expression."""
      def _regex(index, expression=expression):
          """Return store key for documents that satisfy expression."""
          pattern = re.compile(expression)
          return [
              store_key
              for value, store_keys
              in index.get_index().items()
              if (isinstance(value, six.string_types)
                  and re.match(pattern, value))
              for store_key in store_keys
          ]
  
      return _regex