Animate letters in a label or text view

I need to perform an effect like this in my app:

How can I do this? I need to animate single letters in the text and not the entire label. I'm using Swift.

EDIT (more details):

I need to animate the letters from right to left using a sort of spring damping (like UIView animation), but I don't know how to achieve this. The most important part of this animation is the first one, where the letters enter from left to right with a variable space between the first ones and the others