ECMAv262 (in section 7.6) says:
A token is identified as an identifier if it consists of some combination of '$', '_', Unicode Letter ,Unicode Escape sequence and Unicode Digit..(i know this is not the complete and exact statement..I just want to talk about Unicode Letter right now)

Now, the Unicode Letter contains Uppercase, Lowercase, Titlecase, Modifier and Other letters...and as far as i know that there is no sequence that...