If I recall, an entity can be only alphanums, so this should do it: 
s/&(?![A-Za-z0-9]+;)/REPLACED/g;

[download]
Literally: an ampersand not followed by (1 or more alphanums followed by a semicolon).

If that's not the correct definition of an entity, season to taste.

Not sure why you were looking to the left to determine ampersandy-ness. The interesting part is to the right.

-- Randal L. Schwartz, Perl hacker
Be sure to read my standard disclaimer if this is a reply.

update: Ahh yes, the pound form. And the #x form. OK, try this: 
s/&(?!(?:[A-Za-z0-9]+|#\d+|#x[0-9A-Fa-f]+);)/REPLACED/g;

[download]

In reply to Re: Can't figure out how to invert this regex by merlyn
in thread Can't figure out how to invert this regex by Anonymous Monk

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