Using regular expressions in a routing rule

I would like to examine the contents of my OBX-5 field and not route the message if it contains alphabetic characters. I've tried various combinations of the Match and Contains functions, with no luck. Should I be using the COS ? operator or plaini regular expressions?

e.g.

OBX-5 Contains "\D"

OBX-5 Contains "?.A"

OBX-5 Contains "[A-Z]"