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Decode the string
This is my first challenge on ppcg!
Input
A string consisting of two different ascii characters. For example
ABAABBAAAAAABBAAABAABBAABA
Challenge
The task is to decode this string following these rules:
Skip the first two characters
Split the rest of the string into groups of 8 characters
In each group, replace each character with 0 if that character is the same as the first character of the original string, and with 1 otherwise
Now each group represents a byte. Convert each group to character from byte char code
Concatenate all characters
Example
Let's decode the above string.
AB AABBAAAA AABBAAAB AABBAABA
^^ ^ ^ ^
   
 \/
 
Skip Convert to binary
Notice that A is the first character in the original string and B is the second. Therefore, replace each A with 0 and each B with 1. Now we obtain:
00110000 00110001 00110010
which is [0x30, 0x31, 0x32] in bina

J, 17 Bytes
u:_8#.\2&({.i.}.)
Explanation:
u:_8#.\2&({.i.}.)
2&({.i.}.)  Index of each item of the rest of the string in the first two items
_8#.\  Convert nonintersecting subarrays of length 8 from binary
u:  Convert to characters
Examples:
2&({. ; }.) 'ABAABBAAAAAABBAAABAABBAABA'
┌──┬────────────────────────┐
│AB│AABBAAAAAABBAAABAABBAABA│
└──┴────────────────────────┘
2&({.i.}.) 'ABAABBAAAAAABBAAABAABBAABA'
0 0 1 1 0 0 0 0 0 0 1 1 0 0 0 1 0 0 1 1 0 0 1 0
_8#.\2&({.i.}.) 'ABAABBAAAAAABBAAABAABBAABA'
48 49 50
u:_8#.\2&({.i.}.) 'ABAABBAAAAAABBAAABAABBAABA'
012
20180423 00:49:14 
Python 2, 88 bytes
lambda s:[chr(int(''.join('01'[c==s[1]]for c in t),2))for t in(s[2:10],s[10:18],s[18:])]
Try it online!
20180423 01:02:15