The lastIndex RegExp property allows you to call those methods repeatedly, to loop through all matches in a string, and works only if the "g" modifier is set.
This property is read/write, so you can set it at any time to specify where in the target string, the next search should begin. exec() and test() automatically reset the lastIndex to 0 when they fail to find a match (or another match).
Example
You can try to run the following code to work with lastIndex RegExp property in JavaScript −
<html>
<head>
<title>JavaScript RegExp lastIndex Property</title>
</head>
<body>
<script>
var str = "JavaScript is an interesting scripting language";
var re = new RegExp( "script", "g" );
re.test(str);
document.write("Test 1 - Current Index: " + re.lastIndex);
re.test(str);
document.write("<br />Test 2 - Current Index: " + re.lastIndex);
</script>
</body>
</html>