ngrep strives to provide most of GNU grep's common features, applying
them to the network layer.

ngrep a pcap-aware tool that will allow you to specify extended
regular expressions to match against data payloads of packets. It
currently recognizes TCP, UDP, and ICMP across Ethernet, PPP, SLIP,
FDDI, Token Ring and null interfaces, and understands BPF filter
logic in the same fashion as more common packet sniffing tools,
like tcpdump and snoop.

WWW: http://ngrep.sourceforge.net/