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>