Forms >>> Validation Num Or Char.
Validates an input field to make sure that only a number or character is entered. If you enter a number or a letter everything you can continue on. But, try entering another value like an exclamation point (!), an ampersand (&), or a dollar sign ($) and see what happens. It even highlights the incorrect entry field for you.
Add the below code to the <body> section of your page: