# Swap the Alphabet

In this challenge, you will be "reversing" the alphabet or swapping a-z with z-a. This is commonly known as the Atbash cypher.

Because this transformation makes the output look like some foreign language, your code will need to be as short as possible.

## Examples

abcdefghijklmnopqrstuvwxyz
zyxwvutsrqponmlkjihgfedcba

Programming Puzzles & Code Golf
Kiltiznnrmt Kfaaovh & Xlwv Tlou

Hello, World!
Svool, Dliow!

## Specification

• The input may contain multiple lines, and will be ASCII-only
• Case must be preserved

This is so shortest code in bytes wins

• +1 for the justification "Because this transformation makes the output look like some foreign language, your code will need to be as short as possible."
– cat
Jan 3 '16 at 19:31
• Some Trivia: this is Atbash, a known cipher as old as the bible. Jan 4 '16 at 9:21
• "Because this transformation makes the output look like some foreign language, your code will need to be as short as possible." What kind of logic is that?? Because your description raises about as much question marks as if it was written in some foreign language your answer to this question should be in intelligible natural language and some verbosity is appreciated.
– Bart
Jan 5 '16 at 22:03
• Kiltiznnrmt Kfaaovh & Xlwv Tlou should be the new site for "Programming Trivials & Code Bowling"... Aug 3 '16 at 17:09
• Why don't you allow additional whitespace? That makes it impossible in some languages that always have a trailing newline. I can't see it adding anything to the challenge. May 7 '17 at 5:32

# C, 59 bytes

Sorry for bringing up C again, but I was a bit disappointed to see only C functions here. I was under the impression OP was looking for a usable product.

main(c){while(~(c=getchar()))putchar(isalpha(c)?c+4^31:c);}

Compiled on Ubuntu 14.04 with a simple:

cc swalpha.c

The resulting executable reads any number of lines from stdin, and writes the result to stdout.

Thanks to so many of the other posters for the XOR trick.

• This was the most extreme compact program that does something remarkably amazing like this! @Ruud Helderman this was extremely well done! Feb 17 '21 at 17:45

## Pyth, 8 bytes

XXzG)rG1

@xnor suggested this simpler approach on @FryAmTheEggman's Pyth answer, then I translated it to Pyth.

This uses the handy behavior of X (translate) when given only two arguments: it translates from the second argument to the reversed second argument. We do this first with the lowercase alphabet (G), and then with uppercased G.

## CJam, 17 bytes

I wanted to help GamrCorps golf his CJam solution, but the result ended up so different that I decided to make a separate answer.

q'[,_el^_W%32f^er

Try it online.

### Explanation

'[,   e# Get a character range from the null byte up to and including "Z".
_el   e# Duplicate and convert to lowercase.
^     e# Symmetric set difference. Due to the lowercase operation only letters will not
e# appear in both sets, and so we get a string with all uppercase letters followed
e# by all lowercase letters, i.e "ABC...XYZabc...xyz".
_W%   e# Duplicate and reverse. Gives: "zyx...cbaZYX...CBA".
32f^  e# Take each character XOR 32 which swaps the case, so now we have:
e#                               "ZYX...CBAzyx...cba"
er    e# Transliterate: substitute each character in the first string with the correspoding
e# character in the second string.
• Well, I'm glad you were trying to help! That is the solution that I was trying to make, I just couldn't figure out the XOR. Nice Job! Jan 3 '16 at 20:19
• SE's syntax highlighting hates esolangs. Jan 5 '16 at 6:58
• @Cyoce They probably don't have time; given how many languages are on this site. That said, CJam is probably one of the easier ones, as it's not self-modifying and has a pretty clear structure. I'd like to see them try to add syntax-highlighting to a self-modifying language like ///. May 7 '17 at 5:22
• @Challenger5 afaik, the syntax highlighting just uses Google's prettify, do support for various esolangs would have to be added there. May 7 '17 at 5:24

# JavaScript (ES6), 69 67 bytes

x=>x.replace(/[A-Z]/gi,c=>String.fromCharCode(c.charCodeAt()+4^31))

Uses the same strategy as my Japt answer:

x=>x.replace(/[A-Z]/gi,C=>   // Replace each letter C with
String.fromCharCode(        //  the character with char code
C.charCodeAt()+4^31))      //   the char code of C, plus 4, with the last 5 bits flipped.

Curse your incredibly long property names, JS...

• +1 for mentioning JS's excruciatingly long property names. My favorite is still the infamous document.getElementById Jan 5 '16 at 7:04
• @Cyoce Try document.getElementsByClassName or, on Firefox and Chrome, document.DOCUMENT_POSITION_IMPLEMENTATION_SPECIFIC Jan 6 '16 at 10:28
• This seems to only be 66 bytes not 67 Jan 6 '16 at 23:28
• @Doᴡɴɢᴏᴀᴛ You're right about that, thanks! Jan 6 '16 at 23:28
• It looks like you are missing a closing bracket on the end of the golfed code. Jan 7 '16 at 6:56

# Retina, 1714 13 bytes

Code:

\Tw_dZ-Az-a

Explanation:

\             # This suppresses the trailing linefeed
T            # Switches to transliterate mode
w          # w is short for _0-9A-Za-z
_d       # d is short for 0-9
Z-Az-a # Z-Az-a

This does some magic stuff and completes the task.

Try it here.

• Lucky you that I recently added \. Unlucky you that I didn't get around to adding character classes for letters and reversed character classes yet. Jan 3 '16 at 20:19
• @MartinBüttner Very lucky indeed, that almost invalidated my answer :) Jan 3 '16 at 20:36
• For reference, as of Retina 0.7.2 you could do \TlLRlRL. Jan 7 '16 at 11:11

## Pyth, 10 9

uXGr;H)2z

Thanks to Jakube for saving a byte with the new feature of ;!

Test Suite

A quick explanation: reduce starting with the input over the numbers 0 and 1. The operation to be performed is translate the lower case alphabet with either r...0 or r...1 which are the lower and upper functions from python, respectively, applied to it, and then reversed.

• 9 bytes due to a new feature (not even a day old): uXGr;H)2z Jan 3 '16 at 20:05
• Can you add an explanation? Jan 3 '16 at 20:20
• Can't you use X with the alphabet and then with the capitalized alphabet?
– xnor
Jan 3 '16 at 20:49
• @xnor XXzG)rG1? That seems to work. Jan 3 '16 at 21:03
• Or, maybe do X on the lowercase and uppercase alphabets concatenated, then swap case? That's probably longer, actually.
– xnor
Jan 3 '16 at 21:05

# Julia, 7461 47 bytes

s->replace(s,r"[a-z]"i,t->Char(31$Int(t[1])-4)) This is a lambda function that accepts a string and returns a string. To call it, assign it to a variable. We match each letter using a regular expression and replace each letter with the ASCII character corresponding to 31 XOR the ASCII code for the letter, minus 4. • Wow, this is very elegant. I get a deprecation warning for$ so you might want to update that to . Didn't know that you can use a function in replace. Apr 13 '18 at 14:33

## C, 150 129 Bytes

void rev(char*s){int i,t;for(i=0;i<strlen(s);i++){t=s[i]+25;t=t<116?180-t:244-t;isalpha(s[i])?printf("%c",t):printf("%c",s[i]);}}

This function just converts char to int and adds the appropriate offset to the int before printing. I know it's not the shortest but I didn't see a C implementation.

Example usage

#include<stdio.h>
#include<string.h>
#include<ctype.h>

void rev(char*s){int i,temp;for(i=0;i<strlen(s);i++){temp=s[i]+25;temp=temp<116?180-temp:244-temp;isalpha(s[i])?printf("%c",temp):printf("%c",s[i]);}}

int main(){
char *s = "hello, there";
rev(s);
return 0;
}

UPDATE: shortened a variable name.

• Welcome to Programming Puzzles and Code Golf Stack Exchange! This is a very good approach. It could be golfed more by making all variables (including temp) exactly one character long. Jan 4 '16 at 10:33
• Ahh you are right! I will regolf this one Jan 4 '16 at 10:35
• Do you think you could have a go at my challenge? Analyse your Chair Jan 4 '16 at 10:54
• Yes it will work without the headers you just get a compiler warning @edc65 I figured since it compiles and runs it wouldn't be considered cheating but in real world use you should always include the headers. Jan 4 '16 at 16:05
• 107 bytes: i,t;r(char*s){for(;i<strlen(s);i++){t=s[i]+25;t=t<116?180-t:244-t;isalpha(s[i])?putchar(t):putchar(s[i]);}} Jan 4 '16 at 16:29

## Python, 61 bytes

lambda x:''.join([c,chr(ord(c)+4^31)][c.isalpha()]for c in x)

An anonymous function. On letters, does the reversing operation on the bit representation by adding 4, then flipping the last five bits, similar to ETHproductions' Javascript answer.

• Just wrote the exact same thing haha May 7 '17 at 5:25

# Japt, 23 22 bytes

Ur"[A-Za-z]"_c +4^31 d

Try it online!

### How it works

Ur"[A-Za-z]"_  // Take the input and replace each letter with:
c +4          //  Take its char code and add 4. This results in
//  the string      "ABC...XYZabc...xyz"
//  becoming        "EFG...\]^efg...|}~".
^31           //  XOR the result by 31. This flips its last five 5 bits.
//  We now have     "ZYX...CBAzyx...cba".
d             //  Convert back from a char code.
// Implicit: output last expression
• Nice idea (which I borrowed...) to use XOR for this Jan 4 '16 at 1:12

# C, 64

A void function that modify the string in place.

t(char*p){for(int c;c=*p;)*p++=c>64&c<91|c>96&c<123?(c^31)-4:c;}

Test: ideone

• c;t(char*p){for(;c=*p;)*p++=c>64&c<91|c>96&c<123?(c^31)-4:c;} Jan 4 '16 at 12:33
• @CoolGuy right, but I don't like the global inside a function... it's just me Jan 4 '16 at 12:59
• Isn't this 64 bytes? Jan 7 '16 at 2:37
• @Doᴡɴɢᴏᴀᴛ yes it is .. thanks .. Jan 7 '16 at 7:05

# R, 69 61 bytes

Thanks to @Giuseppe for shaving off some extra bytes:

function(s)cat(chartr("a-zA-Z",intToUtf8(c(122:97,90:65)),s))

Previous version:

function(s)cat(chartr("a-zA-Z",rawToChar(as.raw(c(122:97,90:65))),s))

This is an anonymous function. Usage:

> f=function(s)cat(chartr("a-zA-Z",rawToChar(as.raw(c(122:97,90:65))),s))
> f("Hello, World!")
Svool, Dliow!
> f("Programming Puzzles & Code Golf")
Kiltiznnrmt Kfaaovh & Xlwv Tlou
> f("This is
+ a multiline
+ example.")
Gsrh rh
z nfogrormv
vcznkov.
• intToUtf8(c(122:97,90:65)) instead of the raw conversions, and you can get rid of the cat as well. I'm making it a bit of a habit to come golf 1+ year old answers of yours... Sep 7 '17 at 14:31

# Ruby, 40 bytes

New solution: Stole that bit flipping magic from some of the other posts here:

->s{s.gsub(/[a-z]/i){($&.ord+4^31).chr}} # Ruby, 55 46 bytes ->s{s.tr'a-zA-Z',[*?A..?Z,*?a..?z].reverse*''} 9 bytes off thanks to @manatwork test run: ->s{s.gsub(/[a-z]/i){($&.ord+4^31).chr}}["Kiltiznnrmt Kfaaovh & Xlwv Tlou"]
=> "Programming Puzzles & Code Golf"

# Jolf, 15 bytes

~Ai+plpu+_pl_pu
~A              I don't know what to call this, besides "dictionary replace"
i              the input
+plpu         previous dictionary: lower + upper alphabet
+_p1_pu  new dictionary: reversed lower + reversed upper
• The word for this is "transliteration," FYI. Jan 4 '16 at 18:43

## Seriously, 31 bytes (non-competing)

úúû+╗úRúûR+╝,;╜íuWD╛E(X0WXMεj

Hex Dump:

a3a3962bbba352a396522bbc2c603bbda1755744be452858305758604dee6a

Try It Online

Expl:

úúû+╗                             Put UPPERCASElowercase in reg0
úRúûR+╝                      Put ESACREPPUesacrewol in reg1
,                     Fetch input.
             Mεj   Map over the characters in string as list, joining result
;╜íu                Find 1-index of character in UPPERCASElowercase
W     0WX       If it is positive (present):
D              Convert back to 0-index
╛E            Look it up in ESACREPPUesacrewol
(X          Delete the original character.
(Else just leave the original character unchanged.)

I just realized the spec say no additional whitespace, but there is no way to suppress trailing newlines in Seriously output, so there is no Seriously solution.

• What does ESACREPPUesacrewol mean? Jan 3 '16 at 20:56
• @Doᴡɴɢᴏᴀᴛ UPPERCASE reversed LOWERCASE reversed Jan 3 '16 at 21:01
• the reversed uppercase alphabet prepended to the reversed lowercase alphabet Jan 3 '16 at 21:01

# 𝔼𝕊𝕄𝕚𝕟 2, 12 chars / 26 bytes (non-competitive)

ïĪ(ᶐ+ᶛ,ᶐᴙ+ᶛᴙ

Try it here (Firefox only).

Added transliterate function after the challenge was posted.

# Explanation

ïĪ(ᶐ+ᶛ,ᶐᴙ+ᶛᴙ // implicit: ï=input
ïĪ(           // transliterate ï...
ᶐ+ᶛ,       // from uppercase+lowercase alphabet...
ᶐᴙ+ᶛᴙ  // ... to reversed uppercase+reversed lowercase alphabet
// implicit output
• I thought 𝔼𝕊𝕄𝕚𝕟 2 was going to be called 𝔼𝕊 ∞? Jan 3 '16 at 22:01
• No, I decided to save that for a complementary (but different) language for 𝔼𝕊𝕄𝕚𝕟. Jan 3 '16 at 22:02
• IIRC There is a blackboard bold 2. Jan 4 '16 at 18:46
• @ՊՓԼՃՐՊՃՈԲՍԼ Call it ESMax (in doublestruck). Jan 4 '16 at 20:35

## CJam, 21 bytes

q'[,65>__el_@+W%@@+er

Not an optimal solution... yet... Try it online

Its hard to explain without grouping things, so here is a general explanation: gets input, pushes uppercase alphabet twice and lowercase twice, rotates things around, combines uppercase and lowercase strings, reverses one, and uses transliteration (similar to the Retina answer).

• Does this output a trailing newline? Jan 4 '16 at 0:26
• @LegionMammal978 It shouldn't unless aditsu changed how the stack is outputted. Jan 4 '16 at 0:29
• @LegionMammal978 I doesn't. Jan 4 '16 at 10:05

# C (function), 50

f(char*s){for(;*s;s++)*s=isalpha(*s)?*s+4^31:*s;}

This builds on all three previous C answers, so credit to @Ruud, @Danwakeem and @edc65.

This function modifies a char array in place.

My understanding is function entries are allowed unless explicitly banned in the question.

Try it online.

# PostgreSQL, 118 125 bytes

SELECT s,TRANSLATE(s,t||UPPER(t),REVERSE(t)||REVERSE(UPPER(t)))
FROM(SELECT text'Programming Puzzles & Code Golf's,text'abcdefghijklmnopqrstuvwxyz't)r

SqlFiddleDemo

Output:

╔══════════════════════════════════╦═════════════════════════════════╗
║                s                 ║            translate            ║
╠══════════════════════════════════╬═════════════════════════════════╣
║ Programming Puzzles & Code Golf  ║ Kiltiznnrmt Kfaaovh & Xlwv Tlou ║
╚══════════════════════════════════╩═════════════════════════════════╝

Input: SELECT text'...'s

EDIT:

Input as table:

SELECT s,TRANSLATE(s,t||UPPER(t),REVERSE(t)||REVERSE(UPPER(t)))
FROM i,(SELECT text'abcdefghijklmnopqrstuvwxyz't)r
GROUP BY s,t

SqlFiddleDemo

Output:

╔══════════════════════════════════╦═════════════════════════════════╗
║                s                 ║            translate            ║
╠══════════════════════════════════╬═════════════════════════════════╣
║ Hello, World!                    ║ Svool, Dliow!                   ║
║ Programming Puzzles & Code Golf  ║ Kiltiznnrmt Kfaaovh & Xlwv Tlou ║
║ abcdefghijklmnopqrstuvwxyz       ║ zyxwvutsrqponmlkjihgfedcba      ║
╚══════════════════════════════════╩═════════════════════════════════╝
• Hey lad2025, I think this is shorter than 118, but I am no expert in the counting rules May 5 '16 at 11:04

# Python 3, 195169168 166 bytes

Thanks to @TrangOul for -2 bytes!

How didn't I see that I could have golfed that down before?

x=__import__('string').ascii_letters;y,z=x[26:],x[:26];a,b=y[::-1],z[::-1];print(''.join(b[z.index(i)]if i in b else a[y.index(i)]if i in a else i for i in input()))

(sorta) ungolfed:

x = __import__('string').ascii_letters;
y, z = x[26: ], x[: 26];
a, b = y[::-1], z[::-1];
print(''.join(b[z.index(i)]
if i in b
else a[y.index(i)]
if i in a
else i
for i in input()
))

Try it on Ideone!

• You can save 2 bytes by removing the outermost [] from join. Jan 4 '16 at 13:03
• @TrangOul really? It's a list comprehension so I thought it needs to be a list or else you get generator object <genexpr> at..., but I will test it
– cat
Jan 4 '16 at 13:25
• @TrangOul Nevermind, I learned something, thanks!
– cat
Jan 4 '16 at 13:26
• Any iterable [containing str values] can be passed to join() function. Jan 4 '16 at 13:42

Saved 15 bytes thanks to @nimi.

c=fromEnum;s=toEnum;f[]="";f(x:y)|64<c x&&c x<91=s(155-c x):f y|96<c x&&c x<123=s(219-c x):f y|0<1=x:f y

Usage:

f "abcdefghijklmnopqrstuvwxyz"
"zyxwvutsrqponmlkjihgfedcba"

f "Programming Puzzles & Code Golf"
"Kiltiznnrmt Kfaaovh & Xlwv Tlou"

f "Hello, World!"
"Svool, Dliow!"

Explanation

let c=fromEnum;s=toEnum;--wrap names for later use, fromEnum gets ascii code from char, toEnum gets char from ascii code
f[]=[];                 --feed empty list (of chars in this case), get empty list
f(x:y)                  --feed a list, separate the first element and...
|64<c x&&c x<91=        --if its an uppercase char (using ascii code range)...
s(c x*(-1)+155)         --  inverse its ascii code value, move it to the range of uppercase and get the new char
--  (like rotating half turn a ruler by the side and then sliding it to the space it previously occupied)
:f y                    --  feed the rest of the list and stick the new char in front of the result
|96<c x&&c x<123=       --if its a lowercase char (using ascii code range)...
s(c x*(-1)+219)         --  inverse its ascii code value, move it to the range of lowercase and get the new char
:f y                    --  feed the rest of the list and stick the new char in front of the result
|True=x:f y             --otherwise feed the rest of the list and stick the char in front of the result

I'm new to Haskell... to functional programming... and to the site, and i know there are (a lot of) better answers to this question, but bear with me.

• Some golfing tips: a) no need to use let. Start directly with c=fromEnum. Use (155-c x) and (219-c x). c) Truecan be replaced by 1<2. -- The code fails to load for me with the "ambigious type variable" error for functions c and s (ghci 7.10.2), but this can easily be fixed with f[]="" instead of f[]=[].
– nimi
Jan 7 '16 at 19:13
• Thanks!, i was actually wondering if it was valid without "let", since it doesnt run directly on GHC. Jan 22 '16 at 6:34

# Perl 6, 28 bytes

{S:g/\w/{chr $/.ord+4+^31}/} ### Usage: # give it a lexical name my &swap = { … } say swap 'Programming Puzzles & Code Golf'; # Kiltiznnrmt Kfaaovh & Xlwv Tlou say swap ('a'..'z').join # zyxwvutsrqponmlkjihgfedcba • When I run this it says: Warning: Use of "ord" without parentheses is ambiguous at (eval 8)[/System/Library/Perl/5.18/perl5db.pl:732] line 2. I know nothing about perl so am I doing something wrong? How would I run this? Feb 5 '16 at 6:31 • @Doᴡɴɢᴏᴀᴛ If you notice it says this is written in Perl 6 You can go to #perl6 on freenode and type m: my &swap = {S:g/\w/{chr$/.ord+4+^31}/}; say swap ('a'..'z').join Feb 5 '16 at 6:34
• Ah okay, apparently I can't read :P Feb 5 '16 at 6:35
• @Doᴡɴɢᴏᴀᴛ I added a note that you can test it on #perl6 on freenode Feb 5 '16 at 6:36

## Java, 136 bytes

void x(String i){for(char c:i.toCharArray()){if(Character.isLetter(c))c=c<91?(char)(90-(c-65)):(char)(122-(c-97));System.out.print(c);}}

Example usage:

class Test {
static void x(String i){for(char c:i.toCharArray()){if(Character.isLetter(c))c=c<91?(char)(90-(c-65)):(char)(122-(c-97));System.out.print(c);}}
public static void main(String[] args) {
x("Programming Puzzles & Code Golf");
// produces "Kiltiznnrmt Kfaaovh & Xlwv Tlou"
}
}

Probably the worst commonly-used language in terms of byte size.

• Java's is a meme here actually Feb 5 '16 at 19:55
• I know this has been posted about 1.5 year ago, but you can golf quite a few things like this: void x(String i){for(Character c:i.toCharArray())System.out.print(c.isLetter(c)?(char)(c<91?90-(c-65):122-(c-97)):c);} (118 bytes) Sep 7 '17 at 14:14
• Or even shorter: void y(String i){for(int c:i.getBytes())System.out.print((char)(c>65&c<91|c>96&c<123?c<91?90-(c-65):122-(c-97):c));} (116 bytes) Sep 7 '17 at 14:15
• @KevinCruijssen You can also rearrange the calculations to negate the need for parentheses by making 90-(c-65) into -c+65+90 and 122-(‌​c-97) into -c+97+122, which saves a byte each. Sep 9 '17 at 5:03

## Unix shell + tr + printf, 35 bytes

tr A-Za-z printf %s {Z..A} {z..a}

Here you are, a canonical answer in tr. I thought how could a question to transliterate the alphabet go without a canonical answer to transliterate the alphabet?

tr by itself does not even do a "Hello, World!" and as such isn't a programming language, so I marked the answer as noncompeting[1].

[1]: Edit: Actually, Unix shell is the language and tr is the standard library. Thanks to Downgoat and Digital Trauma for helping me spot this out.

• I think its fine to claim shell or bash as a language, and common utilities (tr is in coreutils) as the standard library. See countless of my answers ;-) Jan 4 '16 at 15:54
• But it doesn't work: tr: range-endpoints of 'Z-A' are in reverse collating sequence order. I think you have to do something like tr A-Za-z $(printf %s {Z..A} {z..a}) (and replace$( ) with backticks) Jan 4 '16 at 15:55
• @DigitalTrauma but not everyone has printf on their system. Jan 4 '16 at 16:35
• I think you'd have quite a hard time finding a Unix system that doesn't have printf - after all, its specified by Posix. Also its a builtin in bash which is pretty ubiquitous these days. Regardless, having something pre-installed on everyone's system is not a prerequisite for code-golf - not everyone has CJam on their system either ;-) Jan 4 '16 at 20:20
• The space between } and { is required - otherwise the expansion becomes something like Zz Zy Zx ... Za Yz ... Aa Jan 7 '16 at 19:06

# 05AB1E, 5 bytes [non-competing]

žnžo‡

Uses CP-1252 character set.

Try it online!

Explanation:

žn    - Push [A-Za-z]
žo  - Push [Z-Az-a]
‡ - Transliterate.
• Why is this noncompeting? May 7 '17 at 5:24

# Perl 5, 28 bytes

27 bytes of code + 1 for -p

s/[a-z]/chr(4+ord\$&^31)/gie

Try it online!

# Vyxal, 5 bytes

kBklĿ

Try it Online!

Explanation:

kB     # Constant 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz'
kl   # Constant 'ZYXWVUTSRQPONMLKJIHGFEDCBAzyxwvutsrqponmlkjihgfedcba'
Ŀ  # Transliterate a(b -> c)
# Implicit output

# MATL, 21 28 bytes

Uses version 6.0.0, which is earlier than this challenge. The code runs in Octave.

jttk2Y2mXK)o31Z~4-cK(

### Example

>> matl
> jttk2Y2mXK)o31Z~4-cK(
>
> Hello, World!
Svool, Dliow!

### Explanation

j             % input string
t             % duplicate
tk            % duplicate and convert to lowercase
2Y2           % string 'abc...xyz'
m             % "ismember" function: produces logical index of letter positions
XK            % copy to clipboard K
)             % index into string to get its letters
o31Z~4-       % bitwise XOR with 31 and subtract 4
c             % convert to char
K             % paste logical index from clipboard K
(             % put back modified letters into original string

### Old approach, 28 bytes

j1Y2!tkh"t@2#m@0v27b-)wXK)K(

# MATLAB, 61 bytes

@(x)[abs(-x+ismember(x,65:90)*155+ismember(x,97:122)*219),'']

I tried f=@ismember, and making the range a=65:90 a variable and do 32+a in the second ismember-call. All this shortened the code, but would result in a program and thus require both disp and input.

This gives:

ans('Hello, World!')
ans =
Svool, Dliow!

ans('Programming Puzzles & Code Golf')
ans =
Kiltiznnrmt Kfaaovh & Xlwv Tlou

# Brachylog, 66 bytes

_|hH(:64>:91<,77-H+78=X;H:96>:123<,109-H+110=X;HX),[X]:"~c"w,?b:0&

The lack of built-ins really hurts here, so we have to resort to good old ASCII codes computations.

The predicate brachylog_main expects a character codes string as input and no output, e.g. brachylog_main(Hello, World!,_).

### Explanation

_                                                                  § If the input is empty,
§ return true
§
|                                                                 § Else
§
hH(                                            ),[X]:"~c"w       § Print variable X which
§ depends on the head of
§ the input as a char code
§
,?b:0& § Recursive call on the
§ tail of the input
§
:64>:91<,77-H+78=X                                            § If H is capital, X =
§ 77 - H + 78
;                                           § Else if H is non-cap, X=
H:96>:123<,109-H+110=X                     § 109 - H + 110
;                    § Else (not a letter)
HX                  § H = X