I create text like "test".
and I want to know each letters( "t" "e" "s" "t") position
How to do this with expression or script?
I don't know of a good way to do it with an expression. With a script, you can add letters one at at time and measure the text with sourceRectAtTime(). So you would start with "t", measure that, then update the text to "te" and measure that, etc. That will give you the right edge of each character.
Dan