flowgrep is a basic IDS/IPS tool written in python as a way to help you
investigate and manage your network. it works by sniffing traffic, reassembling
TCP streams, and IP and UDP fragments into single packets, and allowing you to
"grep" through their payloads using regular expressions. the quality of the
regular expression engine is similar to Perl's. think of it as a marriage of
tcpflow, tcpkill, and ngrep.

WWW: http://www.monkey.org/~jose/software/flowgrep/