<script language="javascript">
var str = "R1C1 + R2C2 - R3C3 + R15C20";
var reg = /R(\d+)C(\d+)/gi;
alert(str.replace(reg,"R$1C$2.aa"))
</script>
var str = "R1C1 + R2C2 - R3C3 + R15C20";
var reg = /R(\d+)C(\d+)/gi;
alert(str.replace(reg,"R$1C$2.aa"))
</script>
str = str.replace(/(R\d+C\d+)/gi, "$1.aa");