How to capture the cursor position of a text field?

Hi,

I need to permanently capture the position of the cursor of a text field and the write the position to another text field. E.g. if the text field contains "Hello world" and the cursor is placed after the word "Hello", then the position is "5". How can I monitor the position? I think I need JavaScript to do this, but I do not know how...

Can anyone advice?

Thanks in advance,

Michael