function banli(){
    var select_11 = document.getElementById("select_11");
    var test = document.getElementById("test");
    if(select_11.value=="bbb"){
        test.style.display = "block";
        test.value = "";
    }else if(select_11.value=="aaa"){
        test.style.display = "block";
        test.value = select_11.value;
        test.readonly = "readonly";
    }else{
        test.style.display = "none";
    }
}

------解决方案--------------------

验证小于12的js，用正则了，你稍微改一下就能用了
JScript code

function isURL(strURL){
    var reg2 = /^([0-9]{1,12})?$/g
       if (reg2.test(strURL)){
        return true;  
       }else{
           return false;  
       }
}
function check(){
   var myurl = document.getElementById("myurl").value;
   if(isURL(myurl)){
    alert(myurl+"\nright");
   }else{
    alert(myurl+"\nwrong");
   }
}
<input type="text" id="myurl" value="55555"><a href="javascript:check()">test</a>