Can a parser type be made to optionally match? For example, given the message of 'DROP IN=vlan2 OUT= MAC=48:5b:39:e8:44:c5:00:1d:5a:1c:37:b9:08:00:45:00:00:28 SRC=1.2.3.4 DST=172.16.0.1 LEN=40 TOS=0x00 PREC=0x00 TTL=45 ID=61318 PROTO=TCP SPT=443 DPT=45872 SEQ=1548679084 ACK=0 WINDOW=0 RES=0x00 RST URGP' OUT= may or may not be defined. I would like to match it if it exists with a parser and maybe just set it to nul if it doesn't exist. Similarly, in this message: 'DROP IN=vlan2 OUT= MAC=48:5b:39:e8:44:c5:00:1d:5a:1c:37:b9:08:00:45:00:00:28 SRC=1.2.3.4 DST=172.16.0.2 LEN=40 TOS=0x00 PREC=0x00 TTL=54 ID=3265 DF PROTO=TCP SPT=443 DPT=44616 SEQ=880418731 ACK=0 WINDOW=0 RES=0x00 RST UR' the DF field sometimes exists and sometimes doesn't. Whenever I define something like 'DROP IN=@ESTRING:s1: @OUT=@ESTRING:s0: @' the OUT= string doesn't match if there is no value.