It's in preprocess.py You'd need to download a local version, edit it, and then update your local install.
On OS X I'd use the following command from the directory containing the source file to update my calibre installation (fixed according to Kovid's input):
Code:
calibre-debug --update module calibre.ebooks.preprocess ./preprocess.py
The directories vary a bit by platform, everything else is the same.
These are the regexes doing the chapter detection there:
Code:
# Detect Chapters to match default XPATH in GUI
(re.compile(r'(?=<(/?br|p))(<(/?br|p)[^>]*)?>\s*(?P<chap>(<i><b>|<i>|<b>)?(Chapter|Epilogue|Prologue|Book|Part)\s*(\d+|\w+)?(</i></b>|</i>|</b>)?)(</?p[^>]*>|<br[^>]*>)\n?((?=(<i>)?\s*\w+(\s+\w+)?(</i>)?(<br[^>]*>|</?p[^>]*>))((?P<title>(<i>)?\s*\w+(\s+\w+)?(</i>)?)(<br[^>]*>|</?p[^>]*>)))?', re.IGNORECASE), chap_head),
(re.compile(r'(?=<(/?br|p))(<(/?br|p)[^>]*)?>\s*(?P<chap>([A-Z \'"!]{5,})\s*(\d+|\w+)?)(</?p[^>]*>|<br[^>]*>)\n?((?=(<i>)?\s*\w+(\s+\w+)?(</i>)?(<br[^>]*>|</?p[^>]*>))((?P<title>.*)(<br[^>]*>|</?p[^>]*>)))?'), chap_head),