Ruby: All Combinations / Permutations up to length n-1 of a String

What?
In writing a Scrabble like game, I needed a quick fix to get all possible combinations of a string (of length 1..n). I would then look these combinations up in the dictionary to determine if they are words valid for placement.

Why?
It would be nice to able to do the following:

'abc'.combinations.each do |combo|
dictionary.lookup(combo)
end

So I hacked up the following:

class String
# Get all the combinations of a string.
# Returns an array of all combinations of a string
# If a block is given, then that block is called once for each combination
def combinations &action
  ret = []
  0.upto length-1 do |n|
    #rest = self.split('') # regular
    rest = self.split(//u) # UTF-8
    picked = rest.delete_at n
    action.call picked if action
    ret << picked
    rest.join.combinations.each do |perm|
      genperm = picked+perm
      ret << genperm
      action.call genperm if action
    end
  end
  ret
end
end

Which does the following:

p 'abc'.combinations # => ["a", "ab", "abc", "ac", "acb", "b", "ba", "bac", "bc", "bca", "c", "ca", "cab", "cb", "cba"]

'abc'.combinations do |combination|
 puts combination
end
# => 
#   a
#   ab
#   abc
#   ac
#   acb
#   b
#   ba
#   bac
#   bc
#   bca
#   c
#   ca
#   cab
#   cb
#   cba

Just what I wanted!