Try testing the following form with valid and invalid email addresses. The
code uses javascript to match the users input with a regular expression.
函數代碼：

復制代碼 代碼如下:

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;
}
}

In the forms ‘onsubmit' code call javascript:return validate(‘form_id','email_field_id')
使用方法：

復制代碼 代碼如下:

<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>

You should not rely purely on client side validation on your website / web application, if the user has javascript disabled this will not work. Always validate on the server.
from:　http://www.white-hat-web-design.co.uk/blog/javascript-validation/