Let's test you skills, what does this do?

mystery_regex = (
    r"^(?!string$)(?=.{3,16}$)[a-z]([0-9a-z]|[0-9a-z\-](?=[0-9a-z]))"
    r"{2,}([\.](?=[a-z][0-9a-z\-][0-9a-z\-])[a-z]([0-9a-z]"
    r"|[0-9a-z\-](?=[0-9a-z])){1,}){0,}$"
)