Python-based regular expression parser that allows patterns to call functions?

[Andrew Warkentin]

I am writing a filtering HTTP proxy (the site is 
http://xuproxy.sourceforge.net/). I want it to be compatible with 
Proxomitron (http://proxomitron.info/) filters. I need a regular 
expression parser that allows patterns to call functions (or more 
likely, class methods), to implement "matching commands" (look at the 
Proxmitron documentation to see what I mean). Does anyone know if such a 
library exists for Python, or do I have to write my own parser?