The way I see it, the following should hold
empty_flag = RegexFlag(0)
any_case = RegexFlag.IGNORECASE
any_case_on_any_line = RegexFlag.IGNORECASE | RegexFlag.MULTILINE
any_case in empty_flag is False
any_case_on_any_line in empty_flag is False
empty_flag in any_case is False
empty_flag in any_case_on_any_line is False
The latter two defy all logic. Please don't. Your 'in' operator clearly means "is a subset of", and the empty set emphatically is a subset of all sets (this is the most basic mainstream set theory you can think of).