find octets in a string with no periods

octet_finder.rb

#!/bin/env ruby

def octet?(str)
  (0...256) === str.to_i
end

def octets(str, current = [], output = [])

  # bail if there's nothing left to get
  if str.nil? || str.empty?
    output << current.join(".") if current.length == 4
    return
  end

  # bail early if we've gone past the end
  if current.length >= 4 and not str.empty?
    return
  end
  
  # go 3 characters in, and check for octet
  #   yes: recurse for the next octet
  #   no : go back 1 character and check for octet
  max = [str.length, 3].min
  max.downto(1).each_with_object(output) do |length|
    substr = str[0, length]
    if octet?(substr)
      octets(str[length..-1], current + [substr], output)
    end
  end

end

$stdin.each_line do |line|
  puts "#{line.strip}: #{octets(line.strip)}"
end

[jonathan@beep octets]$ ruby octets.rb 
738493992
738493992: []
34987
34987: ["34.9.8.7", "3.49.8.7", "3.4.98.7", "3.4.9.87"]
8397958
8397958: ["83.97.95.8", "83.97.9.58", "83.9.79.58", "8.39.79.58"]
255355266
255355266: ["255.35.52.66"]
255255255255
255255255255: ["255.255.255.255"]
1111
1111: ["1.1.1.1"]
987654321
987654321: []
87654321
87654321: ["87.65.43.21"]
654321
654321: ["65.43.2.1", "65.4.32.1", "65.4.3.21", "6.54.32.1", "6.54.3.21", "6.5.43.21"]
54321
54321: ["54.3.2.1", "5.43.2.1", "5.4.32.1", "5.4.3.21"]
4321
4321: ["4.3.2.1"]
321
321: []