The problem with hand-written lexers/parsers (and some generated parsers, notably PEG these days) is that you have no idea if you can actually trust it. There

The problem with hand-written lexers/parsers (and some generated parsers, notably PEG these days) is that you have no idea if you can actually trust it. There are so many weird edge cases for lexing and parsing that I would never dare to do it without machine verification.