var data;
data = '<div class="content"><h1>This is a heading</h1><p>This is a paragraph of text.</p><p><strong>Note:</strong> If you don\'t escape "quotes" properly, it will not work.</p></div><div class="isim">deneme</div>';

const data_text = data.match(/<div class="isim">(.*?)<\/div>/);

console.log(data_text[1]);