I finished the first function in pcre-grok's predicate feature.

% ifconfig | ./grokre '%{IP}' IP
Entry: IP => 192.168.0.5
Entry: IP => 127.0.0.1

# Now, with a predicate:
% ifconfig | ./grokre '%{IP =~ /^192/}' IP
Entry: IP => 192.168.0.5

The eventual plan is to allow users to register their own predicates. The first target of this will be a python module wrapping grok allowing you to use grok and additionally write predicate functions in python, executed inside the regular expression.

So far, PCRE has not let me down.