For example command

zabbix_agentd -t 'vfs.file.regexp[/tmp/a,Ubuntu,,000000000000000000001ajhjahgkga]'

does not fail and produce the same result as

zabbix_agentd -t 'vfs.file.regexp[/tmp/a,Ubuntu,,1]'

File /tmp/a contains line "Ubuntu 14.02 LTS" followed by new line.

This is probably a more general issue not restricted to agent or this particular function.