[kovidgoyal]

That assumes that there is already a Chapter x. prefix. If you want to handle both the case where there is a prefix and one where there is not, use this:

import re

def replace(match, number, file_name, metadata, dictionaries, data, functions, *args, **kwargs):
    section_number = '%d. ' % number
    return match.group(1) + 'Chapter ' +  section_number + re.sub(r'^Chapter \d+\. ', '', match.group(2))