Regex: what does (?! …) mean?

Question

The following regex finds text between substrings FTW and ODP.

/FTW(((?!FTW|ODP).)+)ODP+/

What does the (?!...) do?

Answer 1

From perldoc:

A zero-width negative look-ahead assertion. For example /foo(?!bar)/ matches any occurrence of "foo" that isn't followed by "bar". Note however that look-ahead and look-behind are NOT the same thing. You cannot use this for look-behind.

If you are looking for a "bar" that isn't preceded by a "foo", /(?!foo)bar/ will not do what you want. That's because the (?!foo) is just saying that the next thing cannot be "foo"--and it's not, it's a "bar", so "foobar" will match. You would have to do something like /(?!foo)...bar/ for that. We say "like" because there's the case of your "bar" not having three characters before it. You could cover that this way: /(?:(?!foo)...|^.{0,2})bar/ . Sometimes it's still easier just to say:

if (/bar/ && $` !~ /foo$/)