Determine Cursor Position On Textfield

I have a problem with currency formatter function I made. On onkeyup's
method of a textfield I add function below:

<snip>
function formatCurrency(elm) {
var str=elm.value;
var re = /(-?d+)(d{3})(,d*)?/
var re1 = /./g

str = str.replace(re1,'')
arr = str.split(',')
str = arr[0]

while (re.test(str))
str = str.replace(re, '$1.$2$3')

str += (arr.length 1) ? (',' + arr[1]) : ''

elm.value = str
}
</snip>

It works as I expected, except that after the function finished its
execution the cursor moves to the end of the textfield. So if I want
to insert two or more characters, the rest of it goes to the end of
textfield.

How I determine the position of the cursor in the textfield before
executing the regex?