Jeff Moser writes How .NET Regular Expressions Really Work.
I've got a soft spot for regular expressions (programming Perl can do that to you), and while I understand backtracking and greedy / lazy matching, I've never actually read the source code for a regex library before.
If you haven't either, and want to benefit from someone else's description of the 14,000 lines of .NET regular expression library, you'll enjoy this post.
Kirk