function format(str){
    temp1 = '';
    temp2 = str.match(/...$/);
    while(temp2 != null){
      temp1 = temp2 + ','+temp1;
      str = str.replace(/...$/,'');
      temp2 = str.match(/...$/);
    }
    temp1 = (str!='') ? str + ',' + temp1 : temp1;
    temp1 = temp1.replace(/.$/,"");
    return temp1;
  }
  
  //正規表現を駆使した方法
  function format2(str){
    var temp1 = str.match(/./g).reverse().join("");
    temp1 = temp1.replace(/(\d{3})/g,"$1,");
    temp1 = temp1.match(/./g).reverse().join("").replace(/^,/,"");
    return temp1;
  }