If you have followed my work logs, you already know that I currently have targeted expression support for rsyslog. I have just finished the basic tokenizer/parser to parse expressions from the configuration file. They base on flexible ABNF definition and will operate on a typeless data type. Initially, expressions will only be available for filters, but further on they will move to more and more places in rsyslog.
The surprising fact is that expressions will also bring a virtual machine implementation to rsyslog. I thought a while, but a vm is actually the easiest (and cleanest) way to implement arbitrary expressions in rsyslog. As a side-effect, the vm, once there, will probably lay the foundation for very interesting future developments inside rsyslog. It also affects the future rsyslog.conf file format. Maybe we extend it into a programming-language like construct ("syslogscript" comes up my mind) ;)
In the mean time, here is a preliminary object model for the expression part of rsyslog as I plan to implement it (click on the image for a larger version):