RegEx: text not inside

Hello,

I've read a difficult problem:
a RegEx, that matches a string, which doesn't include a 2nd string.
foo-xxx--bar -> no match
foo-xyx--bar -> match
It should match, if "foo.+?bar" doesn't contain "xx", something like
"foo.+(?!xx)bar", but that matches always.

Any idea?

-davidoff12001