// botとして扱うアカウントIDを設定しないと、作成する処理によっては無限ループに入るので注意
var BOT_ACCOUNT_ID = '必要ならここにbotとして扱うアカウントIDを書く';
var API_TOKEN = '先述の項で取得したチャットワークAPIのTokenをここに書く';

function doPost(event) {
  var contents = JSON.parse(event.postData.contents);
  var accountId = contents.webhook_event.account_id;

  if (accountId == BOT_ACCOUNT_ID) {
    return;
  }

  var contentsBody = contents.webhook_event.body
  var addAccountIdArray = getAddedAccountIdArray(contentsBody);

  if (addAccountIdArray.length > 0) {
    var replyMessage = "初めまして。";
    var params = {
      headers : {"X-ChatWorkToken" : API_TOKEN},
      method : "post",
      payload : {body : createHelloMessage(addAccountIdArray) + replyMessage}
    };
    var url = "https://api.chatwork.com/v2/rooms/" + contents.webhook_event.room_id + "/messages";
    UrlFetchApp.fetch(url, params);
  }
}

function getAddedAccountIdArray(contentsBody) {
  var accountIdArray = [];
  if (contentsBody.match(/dtext:chatroom_member_is\S+piconname\S+dtext:chatroom_added/)) {
    var regex = /(piconname:(\d+))+/g
    while ((piconname = regex.exec(contentsBody)) != null) {
        accountIdArray.push(piconname[2]);
    }
  }
  return accountIdArray;
}

function createHelloMessage(accountIdArray) {
  var message = "";
  for (var i=0; i<accountIdArray.length; i++) {
    message += "[To:" + accountIdArray[i] + "][piconname:" + accountIdArray[i] + "] さん\n";
  }
  return message;
}