Reg expressions have always been a little complex for me to understand. Great tutorial by the way. One thing that I have found that works really well and I use it with a lot of the recipes i write for calibre is this site here.
http://www.txt2re.com/index.php3
you can type the string that you want to search for. then hit the language which in our case would be python and it spits out the code to find the regex.