On BSD systems where syslogd is configured with the "-v" or "-vv" options, the log file format is changed slightly to include the message facility and priority. This causes the __prefix_line regex not to recognise valid syslog entries.
Standard syslog format example:
May 25 13:38:00 hostname sshd: Invalid user testuser from 192.168.6.3
Verbose syslog format example:
May 25 13:38:00 <auth.info> hostname sshd: Invalid user testuser from 192.168.6.3
I've attached a fix to support both modes.