A Lexer for Haskell in Haskell - Tempting paths for an implementor

Tempting paths for an implementor

Can a lexer be split up into a number of simpler passes?

Can nested comments be removed first?: If so, the rest could be specified using regular expressions, and perhaps implemented easily using a standard tool.
Can qualified identifiers be recognized in a separate pass?: If so, the DFA produced by a lexer generator could be much smaller.
Can keywords and reserved operators be recognized in a separate pass?: If so, the DFA produced by a lexer generator could be much smaller.