The article explains that (?~foo) is not the same as (?!foo), since the latter fails the match if it matches, but the former succeeds if it can match anything t

The article explains that (?~foo) is not the same as (?!foo), since the latter fails the match if it matches, but the former succeeds if it can match anything that's not 'foo' (including 'oo'). The first examples anchor the patterns, so that this behaviour: "foo" ~= /(?~foo)/ # => 1 isn't revealed.