The leftContext property represents the substring preceding the most recent pattern match.
<html>
<body>
<script language="JavaScript">
<!--
pat = /is*/g;
var str = "This is a test";
myArray = pat.exec(str);
document.write("In the string: " + "<b>" + "I know where the fish is tonight" + "</b><br><br>");
document.write("The RegExp.leftContext is: " + RegExp.leftContext);