Forbid and deactivate alphanumeric keys in a form?

1. How do you restrict alphabets in input field?
2. What is valid alphanumeric key?

How do you restrict alphabets in input field?

To set the maximum character limit in input field, we use <input> maxlength attribute. This attribute is used to specify the maximum number of characters enters the <input> field. To set the minimum character limit in input field, we use <input> minlength attribute.

What is valid alphanumeric key?

Alphanumeric characters: alphabetic characters A through Z. numeric characters 0 through 9.