def METHOD_NAME(key):
  """Check that key begins with lower-case letter and contains only supported characters."""
  valid = True
  # must be alphanumeric
  if not re.match("^[a-z][a-zA-Z0-9_]*$", key):
    valid = False
  if args.verbose:
    print("key {}: {}".format(key, valid))
  if not valid:
    print("ERROR: invalid key {}".format(key), file=sys.stderr)
  return valid