Only way I can think of is to set up an OnKeystroke trigger to run a script that processes every keystroke in this field. You can detect the delete key(s) and use Get(ActiveSelectionStart) to determine the current cursor position.
But i am using on object modify script .
Is there any way to find it except keystroke.
Not that I can see. onObjectModify and OnKeystroke will perform in very similar ways. I don't see why you can't modify your script to work from the other trigger.