Created a mask for the Ruby version by avoiding regexp altogether.

luhn.rb

@@ -150,12 +150,63 @@ def mask_with_gsub s
     end
   end
 
+  def mask_with_case s
+    masked = nil
+    i = 0
+    digit_count = 0
+    digit_offset = 0
+    digits = []
+    match_from = -1
+    max_len = 0
+    len = s.length
+    c = ''
+    while i < len
+      c = s[i]
+      case c
+      when '0', '1', '2', '3', '4', '5', '6', '7', '8', '9'
+        match_from = i if match_from < 0
+        digit_count += 1
+        digits << c.to_i
+        max_len = digit_count - digit_offset
+        if max_len >= 14
+          found = test_it(digits, digit_offset, max_len)
+          if found
+            masked = s[0..-1] if not masked
+            j = i
+            while j >= match_from
+              case s[j]
+              when '0', '1', '2', '3', '4', '5', '6', '7', '8', '9'
+                masked[j] = 'X'
+              end
+              j -= 1
+            end
+          end
+        end
+        if max_len >= 16
+          digit_offset += 1
+          match_from += 1 
+        end
+      when '-', ' '
+        # Keep going.
+      else
+        if digit_count > 0
+          digit_count = 0
+          digits = []
+          match_from = -1
+          digit_offset = 0
+        end
+      end
+      i += 1
+    end
+    masked || s
+  end
+
   def tap_stdin
     n_repeats = ARGV[0] ? ARGV[0].to_i : 1
     lines = STDIN.readlines
     n_repeats.times do
       lines.each do |s|
-        puts mask(s)
+        puts mask_with_case(s)
       end
     end
   end

luhny.rb

@@ -6,7 +6,7 @@
 
 
 def sample_test
-  [ '56613959932537',
+  [ #'56613959932537',
 #    '508733740140655',
 #    '6853371389452376',
 #    '49536290423965',
@@ -20,14 +20,14 @@ def sample_test
 #    '0' * 1000,
 #    '4352 7211 4223 5131',
 #    '7288-8379-3639-2755',
-#    'java.lang.FakeException: 7230 3161 3748 4124 is a card #.',
+    'java.lang.FakeException: 7230 3161 3748 4124 is a card #.',
 #    "4111 1111 1111 111 doesn't have enough digits.",
 #    '56613959932535089 has too many digits.',
   #'0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000',
   #'5212843322137064296101908962963186202766554788983704435814703972605368416396055678877682488862234546927406211307521284332213706429610190896296318620276655478898370443581470397260536841639605567887768248886223454692740621130752128433221370642961019089629631862027665547889837044358147039726053684163960556788776824888622345469274062113075212843322137064296101908962963186202766554788983704435814703972605368416396055678877682488862234546927406211307521284332213706429610190896296318620276655478898370443581470397260536841639605567887768248886223454692740621130752128433221370642961019089629631862027665547889837044358147039726053684163960556788776824888622345469274062113075212843322137064296101908962963186202766554788983704435814703972605368416396055678877682488862234546927406211307521284332213706429610190896296318620276655478898370443581470397260536841639605567887768248886223454692740621130752128433221370642961019089629631862027665547889837044358147039726053684163960556788776824888622345469274',
   #'5451496852732996063216961135925002811586537152199011985874232493633063047918301881385483284586533476253043731721256291647129524137724321728426184434461211703740649863341542579718271551110706936707319896126135944655506777360650140073402696573847382312143994860950153547889826890506187544774005026327396239056283010290981735778560515623251759619833225650753259593746554508212002384743816147220901767420098517594528110348433559626620298669171000062321471778438988210772771125375553564585320157635817785646893772472227467874437527001732836456864256454316370375336790286880557855773092293464498480234269658315323895080609167720971257548045968322939533404152970558280322501801922337656360557826932953501196791996392141409716242024358132638936168824328539157595191336754405365165167449323818836023303930252215414173373588852488517005006682091203000533131570369087503070508446066122223326556254899598241462739083519446495374791938334509806165682356091342533772992235963750210739883141610937988149208758059239',
   ].each do |s|
-    puts Luhn.new.mask(s)
+    puts Luhn.new.mask_with_case(s)
   end
 end