The Customer Account Fields app allows you to set validation rules for any customer field. Validation allows you to specify the allowed values you'll accept for a given field.
In the app, go to the Fields tab, then edit a field. Validation is at the bottom of the form.
The available options are flexible and multiple validation rules can be applied to the same field.
Regular Expression Support
The app allows you to define your own highly customized validation rules using Regular Expressions. We recommend the site, RegExr to test your custom expressions.