def METHOD_NAME(blob_key):
  """Return whether if the key is a GCS key."""
  gcs_key_pattern = re.compile(
      r'^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$')
  return bool(gcs_key_pattern.match(blob_key))