an extremely strict check that is supposed to only let particular URLs pass and reject the rest
I think you could make your life much easier if you'd reverse the logic of your script: similiar to: 
if ($referrer =~ m!^https?://(forum\.)?robidu\.de/!)
  { print "pass\n"; }
else
  { print "reject\n"; }

[download]
That way you can concentrate on what you want instead of what to avoid.

In reply to Re: regexp: Mind-boggling negative assertions... by soonix
in thread regexp: Mind-boggling negative assertions... by Robidu

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