The RegExp.$1,$2,..$9 property represents parenthesized substring matches.
<html>
<body>
<script language="JavaScript1.2">
<!--
function swap(){
re = /(\w+)\D(\w+)/;
str = document.form1.text1.value;
newstr=str.replace(re, "$2, $1");
document.form1.text2.value = newstr;
}
-->
</script>
<form name="form1">
Enter your 7 digit phone number in the form xxx-xxxx
<br><br><br>
Phone Number (7 digits):<input type="text" name="text1" size=10>
<br><br>
<input type="button" value="Swap" onClick='swap()'>
<br><br><br>
Output: <input type="text" name="text2" size=10
</form>
</body>
</html>