var usernamePatrn = /^([a-zA-Z0-9]|[_]){6,20}$/;
if(!usernamePatrn.test(form.username.value)){
		alert("用户名输入按照规则"); 
         return false; 
	}

//去掉空格
function  ltrim(s){
  	 for(var i=0;i<s.length;i++)
        if(s.charAt(i)!=' ')
         return s.substring(i,s.length);
     return "";
}

<input type=button name="checkall" value="全选" onClick="this.value=check(this.form.checkbox)">

var checkflag = "false";
function check(field) {
	if (checkflag == "false") {
		for (i = 0; i < field.length; i++) {
			field[i].checked = true;
		}
		checkflag = "true";
		return "取消"; 
	}else {
		for (i = 0; i < field.length; i++) {
			field[i].checked = false; 
		}
		checkflag = "false";
		return "全选"; 
	}
}

<input type=submit name="del" value="删除" onClick="return checkDel();">
//选择删除
function checkDel(){
	var  check_id=document.getElementsByName("checkbox");
	var  id="";  
	for(i=0;i<check_id.length;i++){  
	   if(check_id[i].checked){
	   		id+=check_id[i].value+"##";  
	   }  
	}
	if (id=="") {
	   alert ("请至少选择1个");
	   return false;
	}
	document.getElementById("userid").value=id;
}