<script type="text/javascript">
function isEmail(str){
       var reg = /^([a-zA-Z0-9_-])+@([a-zA-Z0-9_-])+((\.[a-zA-Z0-9_-]{2,3}){1,2})$/;
       alert( reg.test(str));
}

</script>

<input id="zz" type="text"/><input type="button" onclick="isEmail(zz.value)"/>