Regular expressions using forward slashes and square brackets

Am trying to create a filter using forward slashes and square brackets. I can use semi-colons, numbers and letters but the moment I introduce special characters, it fails to match. Any advice on correct formatting for this?

syslog-ng: 3.5

RHEL: 6.10

Text:

"[10/11/23 11:12:13 NZDT] ......"

Formats tried

message('^\[[0-9]*'); # Fails

message('//[0-9]*[ ]*'); # Fails

Regards, Ian