That will also keep <supfoo> and <foosup>. A partial improvement may be 
$tag = qr{</?(?:(?![< ]sup[ >])[^>])*>}i;

[download]
It works, however I'm not terribly sure about why it works...

In reply to Re^4: Remove all html tag Except 'sup' by waldner
in thread Remove all html tag Except 'sup' by jai_dgl

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