提示缺少对象,可是我觉得没错阿
<HTML>  
 	 <HEAD>  
 		 <title> Default </title>  
 	 </HEAD>  
 	 <body>  
 		 <form   name= "Form1 "   id= "Form1 ">  
 			 <input   name= "txtUser "   type= "text "   id= "txtUser "   onBlur= "checkusername() "   />  
 			 <div   id= "checkusername ">  </div>  
 			 <input   type= "submit "   name= "btnSubmit "   value= "提   交 "   id= "btnSubmit "   />  
 		 </form>  
 		 <script>  
 var   profile_username_toolong   =    '太长了 '; 
 var   profile_username_tooshort   =    '太短了 '; 
 var   lastusername   =    ' '; 
 function   checkusername()   { 
 var   username   =   trim(document.Form1.txtUser.value); 
 if(username   ==   lastusername   )   { 
 return; 
 }   else   { 
 lastusername   =   username; 
 } 
 var   cu   =   $( 'checkusername '); 
 var   unlen   =   username.replace(/[^\x00-\xff]/g,    "** ").length;   
 if(unlen    <   1   ||   unlen   >    10)   { 
 warning(cu,   unlen    <   1   ?   profile_username_tooshort   :   profile_username_toolong); 
 return; 
 } 
 }   
 function   warning(obj,   msg)   { 
 obj.style.display   =    ' '; 
 obj.innerHTML   =    ' <img   src= "images/check_error.gif ">         '   +   msg; 
 obj.className   =    "warning "; 
 } 
 		 </script>  
 	 </body>  
  </HTML>    
 用鼠标将光标放在textbox上   然后让它失去焦点   这个时候提示缺少对象   这是怎么回事? 
 还有我想问问   username.replace(/[^\x00-\xff]/g,    "** ").length; 
 这个是什么意思?谢谢
------解决方案--------------------$是正则表表达式的引用很有用代表正则表达式的分组匹配结果。$( 'checkusername ');你应该把它改为RegExp.$(1)或者直接$(1);他代表的是username.replace(/[^\x00-\xff]/g,  "** ").中与正则表达式匹配的项。