hilight -regexp: using zero-width negative look-ahead assertion?

I would like to make a hilight that reacts on anything containing
"foo", except when it is followed by "bar".
For instance, "I pity the fool" should produce a hilight, but "I pity
the foobar" not.
The normal way to do this in a perl regex is /foo(?!bar)/ (called a
"zero-width negative look-ahead assertion" in the perlre manpage), but
if I use "/hilight -regexp foo(?!bar)", irssi says that the regex is
"[INVALID!]"
So how do I use a ZWNLAA in irssi?