Solution to http://www.therubygame.com/challenges/6 relying on Ruby Regexp

morse_regexp.rb

require 'singleton'

# This is a parser pattern based on regexp as lexer.
# Express your tokens as parenthesized regexps separated by
# alternation (|). Erroneous input causes immediate return.
module Parse
  def parse( s, p, &block)
    i = 0
    while i < s.length do
      # could pass i to match and use block form in newer lib
      m = s[i..-1].match(p)
      if m
        block.call(m) # call block with match data
        i += m.end(0)
      else
        # unexpected input terminates with no error
        i = s.length # extend pattern here to support recovery!
      end
    end
  end
end

class MorseRegexp
  include Parse
  include Singleton # only build the lookup once
  # def self.instance
  #   self.new
  # end
  def decode(s)
    result = ''
    parse(s, @tokenizer) do |m|
      if m[3] # between letters
        # no-op
      elsif m[2] # between words
        result <<  ' '
      else # letter
        result << self[m[1]]
      end
    end
    result
  end
    
  def initialize
    @lookup = Array.new( 3**4 - 1 ) # addressable via a four-digit ternary number
    codes = <<END # http://www.therubygame.com/challenges/6
A	.-	N	-.
B	-...	O	---
C	-.-.	P	.--.
D	-..	Q	--.-
E	.	R	.-.
F	..-.	S	...
G	--.	T	-
H	....	U	..-
I	..	V	...-
J	.---	W	.--
K	-.-	X	-..-
L	.-..	Y	-.--
M	--	Z	--..
END
    parse(codes, /\A([[:alpha:]])\s+([\.-]{1,4})\s*/) do |m|
      self[m[2]] = m[1]
    end
    @tokenizer = /\A([\.-]{1,4})|(\s{3})|(\s)/
  end
  def [](m)
    @lookup[idx(m)]
  end

  def []=(m,c)
    @lookup[idx(m)] = c
  end

  private

  def idx(m) # convert morse code string to four-digit ternary number
    m.rjust(4).tr(' .-', '012').to_i(3)
  end
end

MorseRegexp.instance.decode(morse)