9. 独自の抽出ロジックを実装
var html = UrlFetchApp.fetch(url).getContentText();
var username = find(html, '', ''),
function find(text, from, to) {
var fromIndex = text.indexOf(from);
if (fromIndex === -1) return '';
text = text.substring(fromIndex + from.length);
var toIndex = text.indexOf(to);
if (toIndex === -1) return '';
return text.substring(0, toIndex);
}