function check(str){
	exp=/[^\w]/gi;
	if(str.search(exp)!=-1)
	{return false;}
	return true;
}
function esists_user(obj){
    if (obj.value==''){
		$('chkuser_yes').style.display='none';
        $('chkuser_no').style.display='block';
        $('exists_msg').innerHTML='用户名不能为空';
		return;
	}
	if (!check(obj.value)){
		$('chkuser_yes').style.display='none';
        $('chkuser_no').style.display='block';
        $('exists_msg').innerHTML='用户名必须为字母或数字的组合';
		return;
	}
    var URL = "ajax.php" + '?username=' + obj.value + '&action=exists_user';
	var ax = new Ajax.Request(URL,{
	  method: 'get',
	  onSuccess: function(transport){
		if (transport.responseText=='OK'){
		  $('chkuser_yes').style.display='block';
          $('chkuser_no').style.display='none';
          $('exists_msg').innerHTML='该用户名可以使用!';
		}else{
		  $('chkuser_yes').style.display='none';
          $('chkuser_no').style.display='block';
          $('exists_msg').innerHTML='该用户名已被注册,请重新输入!';
		}
	  }
	});
    ax=null;
}