I like the graphic examples in this question (besides the obvious attempts at answers): https://stackoverflow.com/questions/1050222/what-is-the-diff...
At first look I tougth it was brainfuck but it's valid JavaScript:
https://stackoverflow.com/questions/7202157/why-does-return-...
You can't parse [X]HTML with regex. Because HTML can't be parsed by regex.
https://stackoverflow.com/questions/1732348/regex-match-open...
This answer [1] is my favorite as it resonates with many work/life experiences.
[1] - https://security.stackexchange.com/questions/33470/what-tech...