What does ?! mean?

Question

What does the ?! mean in the following regex expression?

new RegExp(\'http:\\/\\/(?!\' + location.hostname + \')\')

Answer 1

It's a negative lookahead, which means that for the expression to match, the part within (?!...) must not match. In this case the regex matches http:// only when it is not followed by the current host name (roughly, see Thilo's comment).