Constructs compiled regex to parse out reserved tokens.

def _make_reserved_tokens_re(reserved_tokens):
  """Constructs compiled regex to parse out reserved tokens."""
  if not reserved_tokens:
    return None
  escaped_tokens = [_re_escape(rt) for rt in reserved_tokens]
  pattern = "(%s)" % "|".join(escaped_tokens)
  reserved_tokens_re = _re_compile(pattern)
  return reserved_tokens_re