@jimmy7, I will try that...hope it works.
@masonos, I will try that also; for one filter I will try what Jimmy7 said, and for another show I will try your method.
I have no prior experience with autodl-irssi, but I understand that my filter must be setup ahead of the IRC announcement and autodl-irssi must be listening in the IRC, i.e. the filter doesn't know about previous announcements, it only takes into consideration the announcements made after the filter is created.