Upside down words!

output

dod -> pop
dollop -> dollop
dot -> top
dots -> stop
loot -> tool
loots -> stool
los -> sol
nu -> nu
otto -> otto
plot -> told
pod -> pod
pop -> dod
sol -> los
solos -> solos
stool -> loots
stop -> dots
stunts -> stunts
suns -> suns
tn -> ut
told -> plot
tool -> loot
toot -> toot
top -> dot
tot -> tot
un -> un
usn -> usn
ut -> tn

upside-down.rb

require 'set'

UPSIDE_DOWN = {
  ?b => ?q,
  ?d => ?p,
  ?n => ?u,

  ?l => ?l,
  ?o => ?o,
  ?s => ?s,
  ?t => ?t,
  ?x => ?x,
}
UPSIDE_DOWN.merge!(UPSIDE_DOWN.invert)
UPSIDE_DOWN.freeze

regex = /^[#{UPSIDE_DOWN.keys.join}]{2,}$/

word_list = File.readlines('/usr/share/dict/cracklib-small').map(&:chomp).grep(regex)
word_set = Set.new(word_list)

def upside_down(w)
  w.chars.map { |c| UPSIDE_DOWN[c] }.join.reverse
end

word_list.select! { |w| word_set.include?(upside_down(w)) }
puts word_list.map { |w| "#{w} -> #{upside_down(w)}" }