/sys/src/cmd/cc/cc.y   (you might need the Plan 9 yacc)
the lexical analyser is hand written.
/sys/src/cmd/cc/lex.c
/sys/src/cmd/cc/mac.c (includes /sys/src/cmd/cc/macbody) for the built-in preprocessor

in fact, lex.c includes the compiler main program and some other little functions,
so the lexical analyser proper is smaller