Regex for password with one letter and one number

HTML

<input id="pass" type="text"></input>
<input type="button" value="go!" onclick="checkPwd()"></input>

JavaScript

checkPwd = function () {
    var str = document.getElementById('pass').value;
    if (str.length < 6) {
        alert("too_short");
        return ("too_short");
    } else if (str.length > 50) {
        alert("too_long");
        return ("too_long");
    } else if (str.search(/\d/) == -1) {
        alert("no_num");
        return ("no_num");
    } else if (str.search(/[a-zA-Z]/) == -1) {
        alert("no_letter");
        return ("no_letter");
    } else if (str.search(/[^a-zA-Z0-9\!\@\#\$\%\^\&\*\(\)\_\+\.\,\;\:]/) != -1) {
        alert("bad_char");
        return ("bad_char");
    }
    alert("oukey!!");
    return ("ok");
}