code: def custom_property_prefix_strict(instance):
      
      for prop_name in instance.keys():
          if (instance['type'] in enums.PROPERTIES and
                  prop_name not in enums.PROPERTIES[instance['type']] and
                  prop_name not in enums.RESERVED_PROPERTIES and
                  not CUSTOM_PROPERTY_PREFIX_RE.match(prop_name)):
  
              yield JSONError("Custom property '%s' should have a type that "
                              "starts with 'x_' followed by a source unique "
                              "identifier (like a domain name with dots "
                              "replaced by hyphen), a hyphen and then the name."
                              % prop_name, instance['id'],
                              'custom-prefix')
docs: Ensure custom properties follow strict naming style conventions.
  
      Does not check property names in custom objects.