nice discussion. I missed it somehow.
I wonder if the regex engine could give it. A negated compiled form seems possible. Then there would be the problem of giving a regex that corresponds to a given compiled form. Still a special flag could be used to say "already compiled" and somehow could make it useful as part of a bigger regex (not fully compiled).
This seems to ring some bells of a discussion on p5p about the actual scheme of adding qr// parts.
--stephan the rewrite feeling is strong today;)