help with a regular expression please

[akanevsky]

Say I have a string:
x!!x bbbbbdddd x!!x bbbbaaaaddddd x!!x bbbbcccccddddd
How do I match the bbbb(.*)dddd that do NOT have an "x!!x" substring between them? Don't ask what for, just tell me how do to such regex. explode() is not an option for my purposes. Thanks.

Ideally, it would be something like ([^x!!x]*), but that won't work for some reason.