Notice that this operates on a lexer stream to transform a source file into the destination without any macros/macro-substitutions remaining. It's not the same

Notice that this operates on a lexer stream to transform a source file into the destination without any macros/macro-substitutions remaining. It's not the same lexer as that in the C compiler, which doesn't know anything about macros. Also, on the C compiler-side, people might be interested in the lexer hack because C isn't a CFG. https://en.wikipedia.org/wiki/Lexer_hack https://en.wikipedia.org/wiki/Lexer_hack