Link: Russ Cox's articles on regular expressions
If you have any interest in regular expression matching, especially efficient regexps and understanding why Perl, Python, and so on have sometimes oddly slow implementations, you really want to read Russ Cox's series of articles on regular expressions.
The core things to read are his three part series, Regular Expression Matching Can Be Simple And Fast, Regular Expression Matching: the Virtual Machine Approach, and Regular Expression Matching in the Wild.
(I know, this is late, since Hacker News discussed this a couple of years ago (plus the comment here). The gears of my link-pointing machinery evidently grind very slowly, but better late than never.)