ASP.NET PO.BOX REG EXPR in regular expression validator control

Hello Everybody,

The following Validation Expression string for my regular expression validator control, gives me an error:
(?i)^((?<!P\.?\s?O\.?\sBox) .)+(?<!P\.?\s?O\.?\sBox)$

The error is: Syntax error in regular expression, but several websites gives this reg-expr as valid (??)

I read that  (?i) could be an issue for some browsers, I am working on IE7.0.

Any ideas?
Thanks!
PD.

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