Email Validation - JavaScript

<input type='text' id='txtEmail'/>
<input type='submit' name='submit' onclick='Javascript:checkEmail();'/>
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<script language="javascript">

function checkEmail() {

    var email = document.getElementById('txtEmail');
    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');
    email.focus;
    return false;
 }
}</script>