Canvas App Form - Regular Expression for Custom Requirements

I am building a canvas app and one screen has a form to submit a request, which saves data to a SP list.  I have a form field that requires additional validation beyond being a required fields.  

Starts with "OPX" + "-" + 10 numbers

This needs to throw an error if the first 3 characters are NOT "OPX".  There must be a hyphen next, then 10 numbers.  Any recommendations are greatly appreciated.