Digit validation

Could you help me figuring out something? Is there a way to set a specified number or characters needed to validate the number field when specifying to validate digits?
I want to use the “Number” field to collect ZIP codes validating for digits. But the “Number” field doesn’t have a limit for number of characters to check against. It will validate anything as as long as it is a digit.
Would that be a rule I write in the my-js.js file? If so how?
http://ww.wp.xz.cn/extend/plugins/visual-form-builder/