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 

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Saved 3 bytes thanks to @Adnan

Jelly, 11 bytes

Œlµḟœ-Q$Ḋḣ1 

Try it online! or verify all test cases.

How it works

Œlµḟœ-Q$Ḋḣ1 Main link. Argument: s (string) Œl Convert s to lowercase. µ Begin a new, monadic chain. Argument: s (lowercase string) $ Combine the two links to the left into a monadic chain. Q Unique; yield the first occurrence of each character. œ- Perform multiset subtraction, removing the last occurrence of each character. ḟ Filterfalse; keep characters that do not appear in the difference. Ḋ Dequeue; remove the first character. ḣ1 Head 1; remove everything but the first character. 

Python 2, 59 58 bytes

Returns a list of a single character, or an empty list if no output. (Stupid case-insensitivity...)

s=input().lower();print[c for c in s if s.count(c)<2][1:2] 

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Batch, 171 bytes

@echo off set a=. set s=%~1 :l if "%s%"=="" exit/b set c=%s:~0,1% call set t=%%s:%c%=%% if "%s:~1%"=="%t%" set a=%a%%c% set s=%t% if "%a:~2%"=="" goto l echo %c% 

Alternative formulation, also 171 bytes:

@echo off set a=. set s=%~1 :l if "%s%"=="" exit/b set c=%s:~0,1% set t=%s:~1% call set s=%%s:%c%=%% if "%s%"=="%t%" set a=%a%%c% if "%a:~2%"=="" goto l echo %c% 

Pyth, 16 15 bytes

1 byte thanks to @mbomb007

 =rz1.xhtfq1/zTzk =rz1:fq1/zTz1 2 

Test suite.

Actually, 19 bytes

;╗`ù╜ùc1=`░ε;(qq1@E 

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Explanation:

;╗`ù╜ùc1=`░ε;(qq1@E ;╗ push a copy of input to reg0 `ù╜ùc1=`░ [v for v in s if ù╜ùc1= s.lower().count(v.lower()) == 1] ε;(qq append two empty strings to the list 1@E element at index 1 (second element) 

C#, 129 128 bytes

char c(string i){var s=i.Where((n,m)=>i.ToLower().Where(o=>o==Char.ToLower(n)).Count()<2).ToArray();return s.Length>1?s[1]:' ';} 

works fine. I wish i didnt need to lowercase everything

Mathematica, 49 bytes

Cases[Tally@ToUpperCase@#,{_,1}][[2,1]]~Check~""& 

Anonymous function. Takes a list of characters as input. Ignore any errors that are generated.

J, 25 bytes

(1{2{.]-.]#~1-~:)@tolower 

Usage

 f =: (1{2{.]-.]#~1-~:)@tolower f 'DEFD' f f 'FEED' d f 'This is an example input sentence.' x f '...,,,..,,!@' @ f 'ABCDefgHijklMNOPqrsTuVWxyz' b f 'AAAAAABBBBB' f 'Thisxthis' f 'This this.' . 

Explanation

(1{2{.]-.]#~1-~:)@tolower Input: s tolower Converts the string s to lowercase ~: Mark the indices where the first time a char appears 1- Complement it ] Identity function to get s #~ Copy only the chars appearing more than once ] Identity function to get s -. Remove all the chars from s appearing more than once 2{. Take the first 2 chars from the result (pad with empty string) 1{ Take the second char at index 1 and return it 

C# lambda with Linq, 63 bytes

s=>(s=s.ToUpper()).Where(c=>s.Count(C=>c==C)<2).Skip(1).First() 

C#, 141 bytes

void p(){var x=Console.ReadLine().ToLower();var c=0;foreach(char i in x){if(x.Split(i).Length-1<2){if(++c==2){Console.WriteLine(i);break;}}}} 

Without break(smallest), 135 bytes

void p(){var x=Console.ReadLine().ToLower();var c=0;foreach(char i in x){if(x.Split(i).Length-1<2){if(++c==2){Console.WriteLine(i);}}}} 

With for(;;), 150 bytes

void p(){for(;;){var x=Console.ReadLine().ToLower();var c=0;foreach(char i in x){if(x.Split(i).Length-1<2){if(++c==2){Console.WriteLine(i);break;}}}}} 

Ungolfed with comments

void p() { var x=Console.ReadLine().ToLower();//Get lowercase version of input from STDIN var c=0; //Create "count" integer foreach(char i in x){//For each char in input from STDIN if(x.Split(i).Length-1<2)//If current char occurs once in input from STDIN { if(++c==2){ //Add 1 to count and if count is 2 Console.WriteLine(i); //Print result to STDOUT break; //Exit foreach } //End of IF } //End of IF } //End of FOREACH } //End of VOID 

12 bytes saved by TuukkaX(change count to c).

3 bytes saved by TuukkaX(change string to var).

4 bytes saved by TuukkaX in "With for(;;)"(changed while(true) to for(;;)).

2 bytes saved by TuukkaX(changed c++;if(c==2) to if(++c==2)).

14 bytes saved by Bryce Wagner(changed x.ToCharArray() to x).

x86 machine code, 43 bytes

In hex:

FC31C031C95641AC84C0740E3C6172F63C7A77F28066FFDFEBEC5EAC49740B89F751F2AE5974F44A77F1C3 

Function takes a pointer to the input string in (E)SI and an integer in (E)DX and returns the (E)DX-th non-repeating character or zero if there's no such character. As a side-effect it converts the string to upper case.

Disassembly:

fc cld 31 c0 xor eax,eax 31 c9 xor ecx,ecx 56 push esi _loop0: ;Search for the NULL char, 41 inc ecx ;counting the length in the process ac lodsb 84 c0 test al,al 74 0e je _break0 ;NULL found, break 3c 61 cmp al,0x61 ;If char is 72 f6 jb _loop0 ;between 'a' and 'z' 3c 7a cmp al,0x7a ;convert this char 77 f2 ja _loop0 ;to uppercase in-place 80 66 ff df and byte ptr [esi-0x1],0xdf eb ec jmp _loop0 _break0: 5e pop esi ;Reset pointer to the string _loop: ;ECX=string length with NULL ac lodsb ;Load next char to AL 49 dec ecx 74 0b je _ret ;End of string found, break (AL==0) 89 f7 mov edi,esi ;EDI points to the next char 51 push ecx f2 ae repnz scasb ;Search for AL in the rest of the string 59 pop ecx 74 f4 je _loop ;ZF==1 <=> another instance found, continue 4a dec edx 77 f1 ja _loop ;If not yet the EDX-th non-rep char, continue _ret: c3 ret 

APL, 32 bytes

{⊃1↓⍵/⍨1=+/∘.=⍨(⎕UCS ⍵)+32×⍵∊⎕A} 

Try it || All test cases

Explanation:

 (⎕UCS ⍵)+32×⍵∊⎕A Add 32 to uppercase letters ∘.=⍨ Make an equality matrix +/ Check how many matches ⍵/⍨1= Keep elements with 1 match 1↓ Drop the first one ⊃ Return the second one 

I was about to post it with 16 bytes, but the I realized it had to be case-insensitive...

05AB1E, 9 6 bytes

lТϦн 

Input as a list of characters.

Try it online or verify all test cases.

Explanation:

l # Convert the characters in the (implicit) input-list to lowercase Ð # Triplicate this lowercase list ¢ # Pop two, and get the count of each character in the list Ï # Pop the counts and remaining list, and only keep the characters at the truthy # (count==1) positions ¦ # Remove the first character н # Pop and leave the new first character # (after which it is output implicitly as result) 

Unfortunately the .m (least frequent character(s) builtin) doesn't retain its order in the legacy version of 05AB1E and will only keep the first character instead of a list in the new 05AB1E version, otherwise this could have been 5 bytes with l.m¦¬.

Retina, 43 36 bytes

iM!`(.)(?<!\1.+)(?!.*\1) !`(?<=^.¶). 

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JavaScript (Firefox 48 or earlier), 60 bytes

f=s=>(m=s.match(/(.).*\1/i))?f(s.replace(m[1],"","gi")):s[1] 

Returns undefined if there are only zero or one non-repeating characters. Works by case-insensitively deleting all occurrences of characters that appear more than once in the string. Relies on a non-standard Firefox extension that was removed in Firefox 49. 119 91 byte ES6 version:

f=s=>(m=s.match(/(.).*?(\1)(.*\1)?/i))?f((m[3]?s:s.replace(m[2],"")).replace(m[1],"")):s[1] 

Recursively searches for all characters that appear at least twice in the string. If the character appears exactly twice then both occurrences are deleted otherwise only the first occurrence is deleted (the other occurrences will be deleted later). This allows the occurrences to have a difference case.

Bash, 58 bytes

tr A-Z a-z>t tr -dc "`fold -1<t|sort|uniq -u`"<t|cut -c2 

Caution: This creates a temporary file named t. If it already exists, it will be overwritten.

C, 174 bytes

int c(char*s){int y=128,z=256,c[384],t;memset(c,0,z*6);for(;t=toupper(*s);s++){c[t]++?c[t]-2?0:c[z+(c[y+c[z+t]]=c[y+t])]=c[z+t]:c[z]=c[y+(c[z+t]=c[z])]=t;}return c[y+c[y]];} 

This is not the most short, but quite efficient implementation. In essence it uses double-linked list to maintain ordered set of candidate characters and scans input string just once. Returns character code or zero if none found.

A little bit ungolfed version:

int c(char*s) { int y=128,z=256,c[384],t; //It's basically c[3][128], but with linear array the code is shorter memset(c,0,z*6); for(;t=toupper(*s);s++) { c[t]++ ? // c[0][x] - number of char x's occurrence c[t] - 2 ? // > 0 0 // > 1 - nothing to do : c[z + (c[y + c[z + t]] = c[y + t])] = c[z + t] // == 1 - remove char from the list : c[z] = c[y + (c[z + t] = c[z])] = t; // == 0 - add char to the end of the list } return c[y + c[y]]; } 

C#, 143 bytes

char c(string s){var l=s.Select(o=>Char.ToLower(o)).GroupBy(x=>x).Where(n=>n.Count()<2).Select(m=>m.Key).ToList();return l.Count()>1?l[1]:' ';} 

TSQL, 128 bytes

Golfed:

DECLARE @ varchar(99)=',,zzzbb@kkkkkkJgg' ,@i INT=99WHILE @i>1SELECT @i-=1,@=IIF(LEN(@)>LEN(x)+1,x,@)FROM(SELECT REPLACE(@,SUBSTRING(@,@i,1),'')x)x PRINT SUBSTRING(@,2,1) 

Ungolfed:

DECLARE @ varchar(99)=',,zzzbb@kkkkkkJgg' ,@i INT=99 WHILE @i>1 SELECT @i-=1,@=IIF(LEN(@)>LEN(x)+1,x,@) FROM (SELECT REPLACE(@,SUBSTRING(@,@i,1),'')x )x PRINT SUBSTRING(@,2,1) 

Fiddle

Ruby, 53 bytes

Input is STDIN, output is STDOUT. In Ruby, out-of-index positions in an array or string return nil, which is not printed.

String#count is a strange function in Ruby because instead of counting the number of occurrences for the string that was passed in, it counts the number of occurrences for each letter in that string. It's usually annoying but we can use it to our advantage this time. String#swapcase swaps upper and lower case letters.

$><<gets.chars.reject{|c|$_.count(c+c.swapcase)>1}[1] 

Old version that wasn't safe against special characters like . - 46 bytes

$><<gets.chars.reject{|c|$_=~/#{c}.*#{c}/i}[1] 

Java 8, 172 157 bytes

(String s)->{s=s.toLowerCase();for(char i=0,c;s.length()>0;s=s.replace(c+"","")){c=s.charAt(0);if(!s.matches(".*"+c+".*"+c+".*")&&++i>1)return c;}return' ';} 

-15 bytes.. Dang I was bad at golfing back then. ;)

Explanation:

Try it here.

(String s)->{ // Method with String parameter and character return-type s=s.toLowerCase(); // Make the input-String lowercase for(char i=0,c;s.length()>0; // Loop over the characters of `s` s=s.replace(c+"","")){ // And after every iteration, remove all occurrences of the previous iteration c=s.charAt(0); // Get the current first character if(!s.matches(".*"+c+".*"+c+".*") // If it doesn't occur more than once &&++i>1) // And this was the second one we've found return c; // Return this second characters } // End of loop return' '; // Else: return an empty character/nothing } // End of method 

R, 79 bytes

function(z){y=tolower(el(strsplit(z,"")));x=table(y);y[y%in%names(x[x==1])][2]} 

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I definitely feel like something can be golfed out here. But I really enjoyed this challenge.

This answer splits the string into a vector of characters, changes them all to lower case, and tables them (counts them). Characters that occur once are selected and compared to characters within the aforementioned vector, then the second value that is true is returned as output. An empty string, or a string with no repeating characters outputs NA.

Perl 6, 38 32 bytes

-6 bytes thanks to nwellnhof by changing .comb to a case-insensitive regex

{~grep({2>m:g:i/$^a/},.comb)[1]} 

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K (oK) / K4, 11 bytes

Solution:

*1_&1=#:'=_ 

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Explanation:

*1_&1=#:'=_ / the solution _ / convert input to lowercase = / group alike characters #:' / count (#:) each group 1= / 1 equal to length of the group? & / where true 1_ / drop the first * / take the first 

Thunno 2 h, 6 bytes

LDcḅịḣ 

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Explanation

LDcḅịḣ # Implicit input ị # Filter the input by: L c # The count of the character lowercased D # In the input lowercased ḅ # Equals one? ḣ # Remove the first character # Implicit output of next character # (or the empty string if it doesn't exist) 

Japt -g, 8 bytes

k@oX ÅÃÅ 

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k@v èXv)É 

Vyxal, 9 bytes

⇩)ġ~₃f∑Ḣh 

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-3 thanks to @lyxal

Explanation

⇩)ġ~₃f∑Ḣh # Implicit input ⇩)ġ # Group by lowercasing ~₃ # Filter by length == 1 f∑ # Flatten and join Ḣ # Remove the first character h # Then take the next one # (or an empty string # if it doesn't exist) # Implicit output