The main difference between these and Project Euler is that these are far more "programming" challenges, whereas Project Euler often boils down to brute forcing a solution once you've figured out the one or two mathematical tricks to reducing the problem space.
Googling for regular expressions puzzle turns up a few more of a slightly different but related flavor.
It would be fun to develop a taxonomy and categorize the different recreational challenges that people find amusing, basing the categories on the type of "aha" and skillset required to solve it.
