[Javascript] RegEx for validating a file name

[rod.coate at falkirkcollege.ac.uk (Rod Coate)]

Hi,
I am tring to use a regular expression to validate user input (a file name) on my html using javascript.

What I have tried so far is:
=====================================================
<script type="text/javascript">
function validateFileName(fileName){
	var validReg="\W+\.\W{3}$";
	var regex= new RegExp(validReg);
	return regex.test(fileName);
}

function checkFileName(f){
	var fileName=f.value;
	if(!validateFileName(fileName)){
		alert("file name is not valid");
		f.value="";
		document.myForm.fileName.focus();
	}
}

</script>

<form action="" name="myForm" method="post">
	<input type="text" name="fileName" value="" onChange="checkFileName(this);">
</form>
=====================================================
but the regex does not seem to work correctly. What I want is to make sure that fileName is alphanumeric, contains only one . and has three alpha characters after the .

It also should not contain things like \/*?| ect or any spaces.

Is anyone able to help with the regex to get it to work? (I have tried google, but can't find what I am looking for).

Cheers,
Rod.