ASCII bit trick to convert lowercase to uppercase and back

I just remembered that I had forgotten the ASCII trick to convert lowercase to uppercase and back. The trick is super simple, you xor the 6th bit (or 5th depending on how you count bits) and that changes the case!

Check this out:

a = 01100001
A = 01000001

See, just the 6th bit changed.

Why is it this way? Simply because the people who invented ASCII thought it was a great idea. If you look at characters a..z, you'll see that all of them have the 6th bit set to 1. The ASCII inventors though, hey, let's set 6th bit to 0 for upper case letters A..Z, then it will be super easy to change case. So they did.

a = 01100001    A = 01000001 
b = 01100010    B = 01000010 
c = 01100011    C = 01000011 
d = 01100100    D = 01000100 
e = 01100101    E = 01000101 
f = 01100110    F = 01000110 
g = 01100111    G = 01000111 
h = 01101000    H = 01001000 
i = 01101001    I = 01001001 
j = 01101010    J = 01001010 
k = 01101011    K = 01001011 
l = 01101100    L = 01001100 
m = 01101101    M = 01001101 
n = 01101110    N = 01001110 
o = 01101111    O = 01001111 
p = 01110000    P = 01010000 
q = 01110001    Q = 01010001 
r = 01110010    R = 01010010 
s = 01110011    S = 01010011 
t = 01110100    T = 01010100 
u = 01110101    U = 01010101 
v = 01110110    V = 01010110 
w = 01110111    W = 01010111 
x = 01111000    X = 01011000 
y = 01111001    Y = 01011001 
z = 01111010    Z = 01011010 

Also check this out, if you xor a character with a space, you invert the case:

$ perl -le 'print "a"^" "'   
A
$ perl -le 'print "A"^" "' 
a

Why is that? Because a space ' ' has the value 32, which is 1<<5, which is the 6th bit, which swaps the case!

Bonus

Here are the commands I used for this blog post:

$ perl -e 'printf "%08b\n", ord("K")'
$ perl -e 'printf "%s = %08b    %s = %08b\n", $_, ord, uc, ord uc for a..z' 

Simple articles best articles.