Prepare reserved tokens and a regex for splitting them out of strings.

def _prepare_reserved_tokens(reserved_tokens):
  """Prepare reserved tokens and a regex for splitting them out of strings."""
  reserved_tokens = [tf.compat.as_text(tok) for tok in reserved_tokens or []]
  dups = _find_duplicates(reserved_tokens)
  if dups:
    raise ValueError("Duplicates found in tokens: %s" % dups)
  reserved_tokens_re = _make_reserved_tokens_re(reserved_tokens)
  return reserved_tokens, reserved_tokens_re