The RegExp.$' property represents the substring following the most pattern match.
This is the same as using the rightContext property.
<html>
<body>
<script language="JavaScript">
<!--
pat = /is*/gi;
var str = "This is a test.";
myArray = pat.exec(str);
document.write("The RegExp.rightContext is: " + RegExp.rightContext);
-->
</script>
</body>
</html>