This Javascript function validates an email address field in a form. It uses a regular expression to test the email address against a pattern to check for a valid format. If the test fails, an alert is displayed and the form submission is cancelled by returning false. The function is called on form submission, passing the form ID and email field name to validate just that field.

1. Javascript Validation
function validate(form_id,email) {
var reg = /^([A-Za-z0-9_-.])+@([A-Za-z0-9_-.])+.([A-Za-z]{2,4})$/;
var address = document.forms[form_id].elements[email].value;
if(reg.test(address) == false) {
alert('Invalid Email Address');
return false;
}
}
<form id="form_id" method="post" action="action.php"
onsubmit="javascript:return validate('form_id','email');">
<input type="text" id="email" name="email" />
<input type="submit" value="Submit" />
</form>