How do I modify front-end input validation?

[safius]

I want to use a different phone validation regex, since the built-in one allows even single-digit numbers. 

I marked my input with `data-validate="isPhoneNumber"` and modified the `isPhoneNumber` function in validate.js, but the [X] or [tick] sign on the input field is still assigned according to the original regex.

Is this functionality overridden somewhere else? Is it validated on the back-end via AJAX?