Currently, the Zabbix Agent (I tested against 2.0 and 2.2) does not support any kind of lookahead syntax.

For example, this item should return the first line in the zabbix agent's log file that does not contain "permission denied". When queried, though, the agent responds with "ZBX_NOTSUPPORTED.

vfs.file.regexp[/var/log/zabbix/zabbix_agentd.log,".(?!permission denied).","UTF-8"

Negative lookahead would be useful (in my circumstance) to be able exclude certain spammy lines in logrt items when I otherwise want all the content in the file.