How to bypass HTML pattern regex restriction

One of my websites in an application page. One of the fields is a 5 digit # field. Let's call it Employee ID. I have restricted it to only 5 chars with 0-9 available through the following in HTML.

<label for="empId">*Employee ID #</label> <input id="empId" tabindex="0" aria-required="true" maxlength="5" pattern="^[0-9]*$" class = "number" >

In all of my testing, I have not been able to type a non-[0-9] char in the field. However, an app that came through had a 5 alpha values. I am wondering how this is possible. Is there a way to strip the pattern off a site you do not own? Or bypass it? I understand this is close to hacking. It caused unfortunate problems down the line. Understanding how it occurred will help me to stop future instances. Thanks.