程式客户端验证：如身份证栏位只能输入数字或X/x

<script language = "JavaScript">
function checkField(){
     var card=frm.id_card.value;//身份证号码
     var letters   =   "1234567890xX";
     var i=0; 
  
     for   (i=0;i<card.length;i++) {  
         var   CheckChar   =   card.charAt(i);  
         if   (letters.indexOf(CheckChar)==-1){
             alert("the id card must be integer or x/X");
             frm.id_card.focus();
             return false;
        }  
     }
}
</script>