Lexer/parser generators are simply not needed and not particularly helpful. Two areas where they are particularly inferior:

1. Error messages. Since the generator doesn't know what you're doing, error messages tend to be generic and very unhelpful to the user. With a hand-built one, context can be supplied to the user in the language of the language(!). Suggestions for corrections for the most common mistakes can be given.

2. Error recovery. Doing this well relies on guessing at what the most likely AST was intended by the user. But a generator can't know that.

By the time you've expended effort trying to overcome (1) and (2), you'd have been better off just writing a custom one.

Here are D's lexer and parser:

https://github.com/dlang/dmd/blob/master/src/dmd/lexer.d

https://github.com/dlang/dmd/blob/master/src/dmd/parse.d

For a complex language like D, they are pretty simple. (The lexer even comes with some builtin unittests!)

Probably the most complex part is the arbitrary lookahead scheme. Of course, this wouldn't be necessary if the grammar didn't need lookahead.

P.S. The initial versions of these were written 20 years ago in C, so the coding style is a bit archaic and could use improvement.