How to check on initials with html pattern?

I'm making a database where you can insert customers.

By the initials of every forename I want to check if it's in the right order. I want it as: "T.L.R." with the dots between every initial.

To check this, I want to use a pattern.

What I got now is this:

<input required="true" type="text" required pattern="[A-Za-z]{1}+\.[A-Za-z]{1}+\.[A-Za-z]{1}+\.">

This code doesn't do anything. Does anyone know the answer?

Thanx in advance!