Help setting value with formula

Hello!

I am building a workflow, that should tick a check box in real time if certain condition is true.

I can do this after submit.

Like so:
Empty entry (State 1)
    Transition On: After Record Submit
    Condition: REGEXP_INSTR({custrecord_my_field}, '[A-Za-z]' ) > 0
        State 2:
        Action: Set Field Value (Set field value MyCheckBox=T)

    Transition On: After Record Submit
    Condition: REGEXP_INSTR({custrecord_my_field}, '[A-Za-z]' ) < 1
        State 3:
        Action: Set Field Value (Set field value MyCheckBox=F)   

Now, In real time I have tried this:
Entry (State 1)
    Action:
        Trigger On: After Field Edit
            Triggering Client Fields: custrecord_my_field