path-diag = [ [FWS] "!POSTED" ] [ "!" / ; match => "!!" ( [FWS] "!" ( IPv4address ; see [RFC 3986] / IPv6address ; see [RFC 3986] / path-identity ) ".MISMATCH" ) ]
If you prefer your "lazy IP-diagnostics" = Charles' .SEEN please replace the ".MISMATCH" by ( ".SEEN" / ".MISMATCH" ) in this proposal.