A non-empty encoded string consisting of printable ASCII characters (in the range 32-126), where some missing letters have been replaced with _.


A decoded string of the same length with all letters in lowercase, including the missing ones.


Edit: As mentioned by @Deusovi in the comments, this is a variant of Bacon's cipher.

  • Gather all letters in the original string and group them by 5. Additional letters that do not fit in a full group of 5 are ignored.
  • Convert each group into binary: lowercase = 0, uppercase = 1. This leads to a list of integers.
  • Use each value N in this list to replace each _ in the original string with the N-th letter of the alphabet (0-indexed), in order of appearance.

Example: prOGraMMIng PuZZleS & cOde ____

prOGr --> 00110 -->  6 -->  7th letter = 'g'
aMMIn --> 01110 --> 14 --> 15th letter = 'o'
gPuZZ --> 01011 --> 11 --> 12th letter = 'l'
leScO --> 00101 -->  5 -->  6th letter = 'f'

By replacing the missing letters and converting everything back to lowercase, the original string is unveiled:

programming puzzles & code golf

This is the expected output.

Clarifications and rules

  • The missing letters are guaranteed to appear at the end of the string. More formally: there will never be any letter after the first _ in the input string. However, there may be other printable ASCII characters such as spaces and punctuation marks.
  • The input is guaranteed not to contain any useless capital letter: all capital letters are bits set to 1 which are required to decode the missing letters. Everything else is in lowercase.
  • The input string is guaranteed to be valid. Especially:
    • It will always contain enough full groups of 5 letters to decode the underscores.
    • The binary-encoded integers are guaranteed to be in the range [0-25].
  • There may be no _ at all in the input string, in which case you just have to return the input.
  • This is , so the shortest answer in bytes wins!

Test cases

Input : hello!
Output: hello!

Input : helLO, worl_!
Output: hello, world!

Input : i aM yoUr faTh__.
Output: i am your father.

Input : prOGraMMIng PuZZleS & cOde ____
Output: programming puzzles & code golf

Input : Can YOu gUesS tHE ENd oF This ____?
Output: can you guess the end of this text?

Input : THe qUICk brown FOx JUMps oVEr the la__ ___.
Output: the quick brown fox jumps over the lazy dog.

Input : RoadS? wHERe we're goinG WE doN't need _____.
Output: roads? where we're going we don't need roads.

Input : thE greatESt Trick thE DeVIl EVer PUllEd wAs CONvInciNg tHe WorLD h_ ____'_ _____.
Output: the greatest trick the devil ever pulled was convincing the world he didn't exist.

Some extra test-cases:

Input : BInar_
Output: binary

Input : 12 MonKey_
Output: 12 monkeys

Input : hyPerbolIZ__
Output: hyperbolized

Input : {[One Last Test ca__]}
Output: {[one last test case]}
  • \$\begingroup\$ Should we only consider as many groups of 5 as there are underscores in the input? \$\endgroup\$ Commented May 12, 2018 at 18:10
  • \$\begingroup\$ In that case, the rule when there's no _ in the input string is a bit of a special case. \$\endgroup\$ Commented May 12, 2018 at 18:17
  • 1
    \$\begingroup\$ Ooh, Bacon cipher! \$\endgroup\$
    – Deusovi
    Commented May 12, 2018 at 19:38
  • 1
    \$\begingroup\$ @SztupY As The input is guaranteed not to contain any useless capital letter, in case there is no underscore there also won't be a capital letter. \$\endgroup\$
    – Laikoni
    Commented May 14, 2018 at 11:03
  • 1
    \$\begingroup\$ @KirillL. Yes, anything in [32-126]. I've added another test case. \$\endgroup\$
    – Arnauld
    Commented May 14, 2018 at 12:16

31 Answers 31


GolfScript, 94 84 82 72 bytes


Try it online!

This is probably not optimal but I can't find anything shorter.


  • {.[65-\97-]{26,?)}/+},

    Filter by:

    • For each of [inp - 65, inp - 97]:
      • Index of this in [0, 1, ..., 25] (-1 if not found)
      • Add one to get 0 for not found, nonzero otherwise
    • Take the sum of the result, giving 0 iff if both results were 0.
  • 5/ Divide into chunks of length 5.

  • {{91<}%2base}%:s;

    Map each chunk of 5 to:

    • Map each of the 5 to:
      • Is it less than 91? (is it uppercase?)
    • Convert this result from binary to decimal. Assign this to the variable s.
  • 0:l; Assign the variable l to 0.

  • { ... }% Map each character of the input to:

    • 32| Convert to lowercase by bitwise or-ing with 32. This will give 127 for an underscore.

    • .127={ ... }@if Return that unless it is 127 (for us, underscore), in which case...

      • l):l Increment l

      • (s= Index into s

      • 97+ Add 97 to make it a lowercase letter.


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