[AndrewStephens]

I hate the way companies try to do custom formatting and validation as you type. It breaks form handling in many different and exciting ways and pretty much ensures that autofill (or even just copy/paste) will fail.

If you must do client-side massaging of form data, at least wait until the user has pressed Submit before messing with the form's contents. And don't assume that the keypresses are the only way your inputs will change.

[martin-adams]

When I was a UI tester, one thing that would almost always break these validators was to select some text on the the surrounding page, then drag it into the text box. Right-click paste is another way.