Validate Passport number in JavaScript

Passport:
<input name="txtPassport" type="text" id="txtPassport" maxlength="8" />
<input type="submit" value="Submit" onclick="return Check();" />
<script type="text/javascript">
    function Check() {
        var patt = new RegExp("^([A-Z a-z]){1}([0-9]){7}$")
        var advalue = document.getElementById("txtPassport").value;
        if (patt.test(advalue)) {
            return true;
        }
        else {
            alert("Passport is not valid");
        }
        return false;
    }
</script>