That example you gave could certainly be done in Lex/Flex and I assume other lexers/tokenizers as well, for instance, you would probably use states and have "$x" in the initial state evaluate to a different token type than "$x" in the string state.

But I do get your meaning, I've written a lot of tokenizers by hand as well, sometimes they can be easier to follow the hand written code. Config files for grammars can get convoluted fast.

But again, I was not meaning it as criticism. But your talk title does start with "How to write a programming language and shell in Go" so given the title I think Lexers / Tokenizers are worth noting.