ReGeX is a Android game that's all about "Regular Expressions". In different modes the user can try to solve tasks, which range from having to match only certain strings but not others, or extracting parts of a string with a regular expression, making it enjoyable (under a certain definition of pleasure) for beginners and people with more experience alike.
More information can be found on the project homepage, the app can be downloaded from F-Droid, and most of the development (including issues and suggestions) is to be found on GitHub. ReGeX is published under GPLv3.