Javascript Validation Part 7 ( checking all letters and numbers)

<html lang="en">
<head>
    <meta charset="utf-8">
    <title>JavaScript form validation - checking all letters and numbers</title>
    <link rel='stylesheet' href='form-style.css' type='text/css' />
</head>
<body onload='document.form1.text1.focus()'>
    <div class="mail">
        <h2>
            Enter your Registration Number and Submit</h2>
        <form name="form1" action="#">
        <ul>
            <li>
                <input type='text' name='text1' /></li>
            <li class="rq">*Enter numbers and alphabets only.</li>
            <li>&nbsp;</li>
            <li>


                <input type="submit" name="submit" value="Submit" onclick="alphanumeric(document.form1.text1)" /></li>
            <li>&nbsp;</li>
        </ul>
        </form>
    </div>
    <script type="text/javascript">
        function alphanumeric(inputtxt) {
            var letters = /^[0-9a-zA-Z]+$/;
            if (inputtxt.value.match(letters)) {
                alert('Your registration number have accepted : you can try another');
                document.form1.text1.focus();
                return true;
            }
            else {
                alert('Please input alphanumeric characters only');
                return false;
            }
        }
    </script>
</body>
</html>