javascript input只输入数字和字母

<input type="text" placeholder="请输入您的用户名...">
<script type="text/javascript">
        $("input[name='username']").bind('input',function(){
            var pattern = new RegExp("[^a-zA-z1-9]");
            var rs = '';
            var str = $(this).val();

            for (var i=0;i< str.length;++i){
                rs = rs + str.substr(i, 1).replace(pattern,'');
            }

            $(this).val(rs);
        });
</script>