imho, negative forms "do not match...won't exclude any links...will not be extracted" may also be confusing.

Why not use something like:

> If given, extracted links' (absolute) URLs must **match at least one of the regular expressions**. If not given (or empty), it will match all links

for `allow`.
And for `deny`,

> If given, (absolute) URLs that match any of the regular expressions will be discarded. If not given (or empty), it will match all links.