class class class
              class class class
          A < class class class AA self end::AB self end::CC self end
self end::B < class class class BA self end::BB self end::BC self end
self end::C < class class class CA self end::CB self end::CC self end
self end::D < class class class DA self end::DB self end::DC self end
self end::E < class class class EA self end::EB self end::EC self end
self end::F < class class class FA self end::FB self end::FC self end
self end

ObjectSpace
  .each_object(Class)
  .select { |c| c.name =~ /\b[A-F][A-C]?$/ }
  .sort_by(&:name)
# => [A,
#     A::B,
#     A::B::C,
#     A::B::C::D,
#     A::B::C::D::E,
#     A::B::C::D::E::F,
#     AA,
#     AA::AB,
#     AA::AB::CC,
#     BA,
#     BA::BB,
#     BA::BB::BC,
#     CA,
#     CA::CB,
#     CA::CB::CC,
#     DA,
#     DA::DB,
#     DA::DB::DC,
#     EA,
#     EA::EB,
#     EA::EB::EC,
#     FA,
#     FA::FB,
#     FA::FB::FC]