You must make a regex that matches everything except itself. For example,
PPCG must not match
PPCG, but must match everything else, like
Surrounding slashes are not considered part of the regex in flavors where they are usually part of the syntax. The same is true of all delimiters.
The regex needs to match the entire string, not just a substring.
You may choose your flavor, but you can't use a flavor newer than this challenge. Please pick a flavor that is easily tested.
Shortest regex in bytes wins.