这下对了

var uname = document.getElementById('username');

    var pwd = document.getElementById('password');

    var pwd2 = document.getElementById('password-2');

    var re1 = /^[0-9a-zA-Z]{3,10}$/;

    var re2 = /^\w{6,20}$/;

 if(re1.test(uname.value)&&re2.test(pwd.value)&&(pwd.value===pwd2.value)){

   return true;

}

else{

   return false;

}

}