code: def custom_object_extension_prefix_strict(instance):
      
      for key, obj in instance['objects'].items():
          if not ('extensions' in obj and 'type' in obj and
                  obj['type'] in enums.OBSERVABLE_EXTENSIONS):
              continue
          for ext_key in obj['extensions']:
              if (ext_key not in enums.OBSERVABLE_EXTENSIONS[obj['type']] and
                      not CUSTOM_TYPE_PREFIX_RE.match(ext_key)):
                  yield JSONError("Custom Cyber Observable Object extension type"
                                  " '%s' should start with 'x-' followed by a source "
                                  "unique identifier (like a domain name with dots "
                                  "replaced by hyphens), a hyphen and then the name."
                                  % ext_key, instance['id'],
                                  'custom-prefix')
docs: Ensure custom observable object extensions follow strict naming style
      conventions.