[BeckyEbook]

Try this:

def replace(match, number, file_name, metadata, data):
	if match:
		title_open = match.group(1)
		title_close = match.group(2)
		between = match.group(3)
		chapter_title = match.group(4).strip()

		return title_open + chapter_title + title_close + between

Perhaps the problem is also that you put \s* before the closing </a></h1>, which forces a space to appear before the anchor closes.

To debug functions, it's a good idea to enable the SIGIL_FUNCTION_REPLACE_LOGFILE environment variable; that way, you'll see the errors in the log file.