Rule to catch PO#

[Ray Jette]

Good morning,
I am trying to write a negative scoring rule that files on the following:
PO
PO#
PO #
Following is the rule I am using:

header PO_AND_ORDERS        Subject =~ /\bPO*?#?/i
score PO_AND_ORDERS        -0.50
describe PO_AND_ORDERS    A negative scoring rule that searches the 
subject for PO #'s.   

Thanks for any help you can provide.