P.O. Box Validation

[Simple Simon]

Hi,
I need some help getting this P.O. Box regular expression to work, or
a new expression entirely. I got this one from a Google Groups
search. It seems to work at: http://regexlib.com/ but not at runtime.

ValidationExpre ssion="(?i)^((? <!P\.?\s?O\.?\s Box).)+(?<!P\.? \s?O\.?\sBox)$"

Please help?

TIA,
~Gordon