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.)