I wrote a full fledged interpreter by hand for a vintage BASIC language. I called it Basice (I'm Icemanind, so Basice is a play on BASIC and Iceman). It's a little outdated now though. I wrote it using C# and it doesn't use any external libraries, like yacc or flex. It's all by hand. You can see it at https://github.com/icemanind/Basice