var username = document.getElementById('username');
    var password = document.getElementById('password');
    var repssw = document.getElementById('password-2');
    var reg=/^[0-9a-zA-Z]{3,20}$/;
    if(!reg.test(username.value)){
        alert("用户名必须为英文或数字，且长度为3-20位！");
        return false;
    }
        if(password.value.length<6||password.value.length>20){
             alert("口令长度应为6-20位！");
             return false;
        }
        if(password.value!=repssw.value){
            alert("两次口令输入必须一致！");
            return false;
        }
    return true;