to find a SIP packet:
if (packet_string contains the word 'sip' but NOT the word 'sdp') {
	print "packet appears to be sip";
}

##</code><code>##

to find a SDP packet:
if (packet_string contains the word 'sip' and ALSO CONTAINS the word 'sdp) {
	print "packet appears to be sdp";
}

##</code><code>##

to find an RTP packet:
if (packet_string cotains NEITHER 'sdp and sdp') {
	print "packet appears to be rtp";
}


##</code><code>##

if ($packet !~ m/sip && sdp/) {
    print "packet is RTP\n";
 }
 elsif ($packet =~ m/sip/g) {
    print "packet is sip\n";
 }
 elsif ($packet =~ m/sdp/ && m/sip/) {
    print "packet is sdp!\n";
 }