What's the second non-repeating character?
MATL, 11 bytes
tk&=s1=)FT)
This exits with an error (allowed by default) if there is no second non-repeated character.
Try it online!
Explanation
t % Implicitly take input string. Duplicate
k % Convert to lowercase
&= % 2D array of equality comparisons
s % Sum of each column
1= % True for entries that equal 1
) % Apply logical index to the input string to keep non-repeated characters
TF) % Apply logical index to take 2nd element if it exists. Implicitly display
Retina, 25 bytes
i!2=`(.)(?<!\1.+)(?!.*\1)
Try it online! (The first line enables running the code on a test suite of several inputs.)
Explanation
This is just a single regex match, the regex being:
(.)(?<!\1.+)(?!.*\1)
That is, match a character and ensure it doesn't appear anywhere else in the input. The rest is configuration:
i
activates case insensitivity.!
tells Retina to print the matches as opposed to counting them.2=
tells Retina to print only the second match as opposed to all of them.
05AB1E, 15 12 bytes
l©v®y¢iy}}1@
Explained
l© # store lower case string in register
v } # for each char in lower case string
®y¢iy # if it occurs once in string, push it to stack
} # end if
1@ # push the 2nd element from stack and implicitly display
Try it online
Saved 3 bytes thanks to @Adnan