open source artificial ignorance-like systems

Likely everyone here has read MJR's artificial ignorance guide, but just 
in case you haven't:
http://www.ranum.com/security/computer_security/papers/ai/index.html

What I've spent a lot of time looking for, with no success, is an open 
source system that will implement something similar but smarter than a 
simple grep from cron. What I ideally want is something that reads logs 
in real time, and has a list of "rules", if you will, which would be a 
list of regex's with an action for each. Like always ignore xyz.*, 
ignore abcd.* unless it happens more than X times in Y time frame 
(minutes, hours), etc. and a default alert rule for anything that 
doesn't match any of the previous rules, since it's something I'm not 
expecting.

Seems pretty simple, but a lot of searching and browsing loganalysis.org 
and similar sites has left me with nothing. I could hack together 
something with standard Unix tools that would meet most of these 
requirements. But I figure as simple and useful as this is, somebody has 
probably already put together an open source package that does what I 
describe, I just can't find it.

Any suggestions would be appreciated. OS, if it matters, could be either 
FreeBSD or Linux.

Cheers,
Chris