Regex that matches everything but itself
PCRE flavour, 45 bytes

^(?!\^()(?R){2}\z)|\1\Q(?!\^()(?R){2}\z)|\1\Q


Try it at regex101.

This is a simple modification of jimmy23013's brilliant solution to the opposite challenge. We just need to make two changes:


In this challenges, the delimiters shouldn't be matched so we drop them from the regex as well. However, that could lead to infinite recursion, because then no characters would be matched before the (?R) call. To fix this, we match the ^ explicitly with \^ to have one character before we enter the recursion.
We need to negate the result which is as simple as wrapping everything after the ^ in a negative lookahead to ensure that the input isn't equal to the string.


For the details of this quining approach, see jimmy's amazing answer.