codemastr, like this?
$replace(string,substring,newsubstring,substring,newsubstring,N)
N being left out replaces all.
$replace(string,substring,newsubstring,substring,newsubstring,2-5) replaces 2nd through 5th.
$replace(string,substring,newsubstring,substring,newsubstring,2 3 7 -2 19-25) replaces the 2nd, 3rd, 7th, 2nd to the last, and the 19th through 25th.