As usual, use the HTML interstage to create the chapter descriptions. It may just be that in the source, the word "chapter" occurs four times at the beginning of each chapter. Also, I believe the chapter detection is done by XPath instead of regexes, so if you do use regexes, results may vary.
Edit: Oh, and I assume you do know that
only works for single-digit chapter headings? Use
if you have more than 9 chapters.