[Deceptor]

<script type="text/javascript">
<!--

var teststring = '<img src="dir/cas.jpg"><img src="dir/ast.jpg">';

function fetch_image_names(code)
{
	matches = new Array();

	while (code.match(/(<img src="dir\/(.+?)\.jpg">)/i))
	{
		matches.push(RegExp.$2);

		code = code.replace(RegExp.$1, '', code);
	}

	return matches;
}

alert('The string yielded the following results: ' + fetch_image_names(teststring).join(','));

-->
</script>

Should do the job for you, following the exact code you posted that is.