Check Email Address with JavaScript and Regular Expressions

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A while ago I put up a Password Strength Checker using JavaScript and Regular Expressions. On that same note, you can also check the structure of an email address utilizing the same methodology:

If your form element has the id=emailaddress and you add a form onSubmit=”return checkEmail();”, this is a Javascript function that you can utilize to return an alert if the email address has a valid structure or not:
<script language="javascript">
function checkEmail() {
var email = document.getElementById('emailaddress');
var filter = /^([a-zA-Z0-9_\.\-])+\@(([a-zA-Z0-9\-])+\.)+([a-zA-Z0-9]{2,4})+$/;
if (!filter.test(email.value)) {
alert('Please provide a valid email address');
return false;

The function also returns the focus back to the email address field!