$ cat file
acl add name=ACL_RULE_NAME pos=400 action=allow agent=proxy authneeded=no \
dest=netgroup:destinations_ssh destburb=external nataddr= \
service=ssh source=ipaddr:1.2.3.4 sourceburb=internal \
comments='this is comments' \
lastchangedby='changed by the guy'

eval  `nawk '/source=[^ ]*/
             /sourceburb=[^ ]*/
             /dest=[^ ]*/
             /destburb=[^ ]*/
             /name=[^ ]*/
             /service=[^ ]*/' RS=' '  file`

$ echo $source
ipaddr:1.2.3.4
$ echo $service
ssh
$ echo $name
ACL_RULE_NAME
$ echo $destburb
external
$ echo $dest
netgroup:destinations_ssh
$ echo $sourceburb
internal