I'm a self-taught programmer, starting in the mid-70s, and learned how to write compilers in a way that probably wouldn't work nowadays, but was great fun back then.

The first thing I'd usually do when starting on a new architecture was write a disassembler. In the late 70s, I was mostly programming in 6809 assembly, usually in Microware's OS-9 operating system. Microware released a Pascal compiler, so I ran my disassembler over that. Turns out the compiler was self-hosting, written in Pascal, and the compiler output was almost completely unoptimized. So I decompiled the compiler back to Pascal by hand, just to see how it worked. I obviously had waaaaay more free-time back then.

I eventually read the green Dragon book (Aho & Ullman, Principles of Compiler Design), and ended up at Microsoft on the C/C++ team in 1991. Over the years, I've written several domain-specific languages (DSLs), small languages for some limited purpose. That early exposure to lexing, parsing, and error detection was invaluable.