How to ask for and limit entry of 4 digits

I am asking for 4 digits of an ID code. I've seen information on using "matches regex" but without enough detail regarding how to set this up in Custom & Content Validation. I found a code for: matches regex

^\\d{4}$. 

I can't find how & where to enter this. The menu requires multiple entries.

https://www.qualtrics.com/community/discussion/14926/allow-4-numbers-in-the-text-box was the answer I found but without sufficient specifics