docs: def _parse_canonical_regex(doc):
      """Decode a JSON regex to bson.regex.Regex."""
      regex = doc['$regularExpression']
      if len(doc) != 1:
          raise TypeError('Bad $regularExpression, extra field(s): %s' % (doc,))
      if len(regex) != 2:
          raise TypeError('Bad $regularExpression must include only "pattern"'
                          'and "options" components: %s' % (doc,))
      return Regex(regex['pattern'], regex['options'])