How to use regex for register name ?

Votirono

Hi,
I'd like to use a regex in register form on the username : [A-Z][a-zA-z]{2,}_[A-Z][a-zA-z]{2,} but I can't find out where I have to place it and if there is a way of using it ?
Thank you in advance !

River

https://vanillaforums.org/discussion/31507/how-to-limit-usernames-to-just-letters-numbers-underscores-like-twitter