[mst]

If you find the code wants factoring out (if you don't, just leave it be, parsers are wonderful things to have written but IME writing them is generally a labour of love) then that split tends to be very natural and works well in the vast majority of cases.

It's also worth having a poke around at some of the Racket community's Language Oriented Programming efforts, they usually have a split they call 'reader' versus 'expander' that I found helped me get my head around how the dividing line can be drawn and why you'd want to.

(Racket's approach isn't quite lexer vs. parser AFAICT but while I at least -think- I've understood it well enough to use ideas, I'm not going to pretend I understand it well enough to provide a correct explanation, let alone a well written one)