I've been working on a toy compiler on-and-off, which is basically just a "reinvent-the-wheel simulator" since I really haven't looked at much existing literature. A very janky, bug-prone part of it is a sort-of mini parser generator, which you can feed a dictionary of rules (as strings) to. This, while slowing down the compilation speed, has allowed me to expand the grammar incrementally from dead simple to more complex, which has been a nice thing.