[otrs] Rejecting requestes based on missing information.

Hello, Im trying to setup OTRS to reject mail that doesnt contain [OS] or [OSS] or [DBA] (list goes on)..... I want to send a mail back to those users telling them that they need to resend their mail. Ive setup a autoreject email ready for this. But I cant seem to figue out how to filter these messages out. Im guessing regex is the way to go but im hopeless when it comes to regex. Any help would be much appreciated. Thanks in advance. Gordon