Regular Expression (Regex) to exclude Special Characters in JavaScript

Name:
<input type="text" id="txtName" />
<br />
<br />
<input type="button" value="Submit" onclick="Validate()" />
<script type="text/javascript">
    function Validate() {
        var regex = /^[a-zA-Z0-9 ._]+$/;
        var isValid = regex.test(document.getElementById("txtName").value);
        if (!isValid) {
            alert("Invalid");
        } else {
            alert("Valid");
        }

        return isValid;
    }
</script>