Ajax email address validator - php

Question

Currently the below script works brillantly checking the database using ajax to see if the email address has been used before. My question is how do I stop it checking the database until it is a fully defined EMAIL address?
Form field
<label for="email">Email: * </label><br/>
<input id="username" name="username" type="email" value="" onblur="return check_username();" />
AJAX script
<script type="text/javascript">
$(document).ready(function() {
$('#Loading').hide();
});
function check_username(){
var username = $("#username").val();
if(username.length > 2){
$.post("../script/check_username_availablity.php", {
username: $('#username').val(),
}, function(response){
$('#Info').fadeOut();
setTimeout("finishAjax('Info', '"+escape(response)+"')", 450);
});
return false;
}
}
function finishAjax(id, response){
$('#'+id).html(unescape(response));
$('#'+id).fadeIn(1000);
}
</script>

Answer 1

Instead of if(username.length > 2){... use something like:
if (/^(([^<>()[\]\\.,;:\s#\"]+(\.[^<>()[\]\\.,;:\s#\"]+)*)|(\
".+\"))#((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA
-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/.test(username.val())) {...

Answer 2

see http://jsfiddle.net/cmontgomery/gEL7n/
very similar to what ialencar posted:
isValidateEmail = function(email) {
var re = /^(([^<>()[\]\\.,;:\s#\"]+(\.[^<>()[\]\\.,;:\s#\"]+)*)|(\".+\"))#((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/;
return re.test(email);
}
check_username = function() {
var username = $("#username").val();
if(isValidateEmail(username)) {
$("#username-error").html("");
console.log("ajax request...");
} else {
$("#username-error").html("invalid email address");
}
}