What's the file extension?

Question

Your challenge is to find the file extension of a provided filename:

hi.txt -> txt or .txt
carrot.meme -> meme or .meme
lol (undefined behavior)
what..is..this..file -> file or .file
.bashrc -> bashrc or .bashrc
T00M@n3KaPZ.h0wC[]h -> h0wC[]h or .h0wC[]h
agent.000 -> 000 or .000

You must get the text from the last . or after the last . to the end of the string. The first capturing group match of the regular expression /\.([^.]+)$/ works, and so does splitting the input on .s and returning the last one.

The file name will always contain at least one ., but it may contain multiple .. (see examples)

The input will always match ^[.a-zA-Z0-9^!\[\]{}@$%+=]+$.

Answer 1

Retina, 5 bytes

.*\.

Replaces everything and a dot with nothing at all.

Try it online!

Answer 2

JavaScript (ES6), 19 bytes

Returns the full filename when there's no file extension. I suppose this is an acceptable undefined behavior.

let f =

s=>s.split`.`.pop()

console.log(f("hi.txt"))               // -> txt
console.log(f("carrot.meme"))          // -> meme
console.log(f("lol"))                  // -> undefined behavior
console.log(f("what..is..this..file")) // -> file
console.log(f("T00M@n3KaPZ.h0wC[]h")) // -> h0wC[]h
console.log(f(".bashrc")) // -> bashrc

Answer 3

Mathematica, 13 22 bytes

Edit: Not sure how I missed the ".bashrc" test case. Thanks to Artyer for keeping me honest.

FileExtension["a"<>#]&

If the input matches \.[^.]+, then FileExtension just returns the empty string, so we prepend the letter a. In any other case, prepending a doesn't affect output of FileExtension.

Answer 4

Pure bash, 13

echo ${1##*.}

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Answer 5

05AB1E, 4 bytes

'.¡¤

Try it online! or Try All Tests

'.   # Push '.'
  ¡  # Split on occurrences of '.'
   ¤ # Tail

Answer 6

Japt, 3 bytes

2 bytes of code, +1 for the h flag.

q.

Explanation:

q.       Split the input by `.`
   -h    Return the last item

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Answer 7

c function, 21

• 1 byte saved thanks to @Dennis.
• 3 bytes saved thanks to @JohanduToit.
• 2 bytes saved thanks to @Neil.
• 1 byte saved thanks to @algmyr.

f(s){s=rindex(s,46);}

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Answer 8

PHP, 21 Bytes

<?=pathinfo($argn,4);

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is a shorter expression for

<?=pathinfo($argn)[extension];

pathinfo

PHP, 27 Bytes

<?=end(explode(".",$argn));

explode

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PHP <7.0, 26 Bytes

<?=end(split("\.",$argn));

deprecated split

Answer 9

V, 5, 3 bytes

Since more of this answer is unprintable than printable, here is a hexdump:

00000000: cd81 ae                                  ...

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This uses Jan Dvorak's algorithm, it just happens to be a more efficient encoding of it.

Explanation:

Í       " Remove all occurrences of:
 0x81   "   Anything (greedy)
     ®  "   Followed by a dot 

Old solution:

$T.d|

Answer 10

Neo4j Cypher, 24 bytes

return split($i,".")[-1]

Input is in a param (i), probably the only way for cypher to take input. Pretty straightforward. Cypher has never been used on PPCG before

Answer 11

GNU Make, 12 bytes

$(suffix $1)

Not using a builtin, 27 bytes:

$(lastword $(subst ., ,$1))

Answer 12

Batch, 10 bytes

@echo %~x1

Strangely competitive for once.

Answer 13

JavaScript (ES6), 33 31 28 bytes

s=>s.slice(s.lastIndexOf`.`)

Spec change in comments removes the need for +1.

-3 bytes thanks to nderscore

Answer 14

Jelly, 4 bytes

ṣ”.Ṫ

A monadic link taking the file name and returning the extension with no leading ..

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How?

Literally does what was asked...

ṣ”.Ṫ - Main link: list of characters, f
 ”.  - literal '.'
ṣ    - split f at occurrences of '.'
   Ṫ - tail (get the last chunk)

Answer 15

sed, 8 bytes

s:.*\.::

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Answer 16

Go, 85 bytes

Go is... troublesome.

import(."fmt"
."os"
."strings")
func main(){s:=Split(Args[1],".");Print(s[len(s)-1])}

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A sample of parentheses from the code:

()(){([])([()])}

Answer 17

Javascript (ES5), 38 bytes

function(s){return s.split(".").pop()}

Answer 18

Common Lisp, 57 bytes

(lambda(s)(#1=reverse(subseq #2=(#1# s)0(search"."#2#))))

Try it online (added some bytes to call this anonymous function and display returned string)

Explanation

(#1=reverse ...)     ;reverse is now accessible with #1# - saves 1 byte. I 
                     ;also need to reverse output of function inside to 
                     ;get extension in correct order
#2=(#1# s)           ;reverse of input string is now accessible with #2#
(search"."#2#)       ;I take reversed string and search for "." to get position of 
                     ;first instance of "." in string from the end of it
(subseq ... 0 ...)   ;get part of reversed string, 
                     ;starting from first character and ending just 
                     ;before first occurance of "."
                     ;this gives reversed extension

I get substring of reversed string, starting from 0, ending on this

Answer 19

C#, 33 41 bytes

a=>a.Split('.').Last();

Edit as suggested:

using System.Linq;a=>a.Split('.').Last();

Answer 20

Java 8, 52 27 bytes

s->s.replaceAll(".*\\.","")

Try it here.

Replace everything before the last dot (and the dot itself) with nothing.

This is shorter than using split (s->s.split("\\.")[s.split("\\.").length-1];) or substring (s->s.substring(s.lastIndexOf('.'));).

Answer 21

Gema, 3 characters

*.=

Sample run:

bash-4.4$ gema '*.=' <<< 'what..is..this..file'
file

Answer 22

MATL, 8 7 bytes

46&YbO)

Try it at MATL Online!

Explanation

        % Implicitly grab input as string
46      % ASCII for '.'
&Yb     % Split the input string at the '.' characters
O)      % Retrieve just the last part
        % Implicitly print the result

Answer 23

Awk, 14 13 characters

(10 9 characters code + 4 characters command line option.)

{$0=$NF}1

Thanks to:

• Robert Benson for spotting the unnecessary semicolon (-1 character)

Sample run:

bash-4.4$ awk -F. '{$0=$NF}1' <<< $'hi.txt\ncarrot.meme\nlol\nwhat..is..this..file\n.bashrc\nT00M@n3KaPZ.h0wC[]h'
txt
meme
lol
file
bashrc
h0wC[]h

Answer 24

Lua, 53 30 bytes

Replaces everything upto the last . with the empty string ''.

Double parenthesis to select only the first return value of gsub.

Any golfing tips are welcome, I'm rusty in lua...

print(((...):gsub('.*%.','')))

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Answer 25

Vim, 5 bytes

$F.d0

Explanation: find last . in line, delete everything before it

Another, longer, but in my opinion still interesting approach with 9 bytes (notice the trailing new line)

d/.*\./e

This one works similarly, 5 bytes (again, trailing new line):

d?\.

Answer 26

Taxi, 1397 bytes

Go to Post Office:w 1 l 1 r 1 l.Pickup a passenger going to Chop Suey.Go to Chop Suey:n 1 r 1 l 4 r 1 l.[a]Pickup a passenger going to Narrow Path Park.Go to Narrow Path Park:n 1 l 1 r 1 l.Go to Chop Suey:e 1 r 1 l 1 r.Switch to plan "b" if no one is waiting.Switch to plan "a".[b]Go to The Babelfishery:n 1 l 1 l.[c]Go to Fueler Up:n.Go to Joyless Park:n 2 r.Go to Narrow Path Park:w 1 r 3 l.Pickup a passenger going to Cyclone.Go to Cyclone:w 1 l 1 r 2 l.Pickup a passenger going to Crime Lab.Pickup a passenger going to Joyless Park.'.' is waiting at Writer's Depot.Go to Writer's Depot:s.Pickup a passenger going to Crime Lab.Go to Crime Lab:n 1 r 2 r 2 l.Switch to plan "c" if no one is waiting.Go to Narrow Path Park:n 5 l.[d]Pickup a passenger going to Chop Suey.Go to Chop Suey:e 1 r 1 l 1 r.Go to Narrow Path Park:n 1 l 1 r 1 l.Switch to plan "e" if no one is waiting.Switch to plan "d".[e]Go to Joyless Park:e 1 r 3 l.Switch to plan "f" if no one is waiting.Pickup a passenger going to Narrow Path Park.Go to Fueler Up:w 1 l.Go to Narrow Path Park:n 4 l.Switch to plan "e".[f]Go to Narrow Path Park:w 1 r 3 l.[g]Switch to plan "h" if no one is waiting.Pickup a passenger going to KonKat's.Go to KonKat's:e 1 r.Pickup a passenger going to KonKat's.Go to Narrow Path Park:n 2 l.Switch to plan "g".[h]Go to KonKat's:e 1 r.Pickup a passenger going to Post Office.Go to Post Office:s 3 r 1 l.

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Taxi doesn't have a reverse function so this ballooned pretty quickly. The logic is:

1. Break the string into characters
2. Reverse the array
3. Iterate through each until a period is found, storing each in a FIFO array
4. Empty the array (because there's only one LIFO array available)
5. Dump the FIFO array into the LIFO array
6. Concatenate the LIFO array and output

Answer 27

JS (ES6), 26 25 bytes

x=>/\.([^.]+)$/.exec(x)[1]

-1 byte thanks to Shaggy

document.querySelector('pre').innerText = (x=>/\.([^.]+)$/.exec(x)[1])("example.txt")

<input oninput = "document.querySelector('pre').innerText = (x=>x.match(/\.([^.]+)$/)[1])(this.value)" value = "example.txt">
<pre></pre>

Alternate, 29 bytes:

x=>(y=x.split`.`)[y.length-1]

Answer 28

Brain-Flak, 84 bytes

Includes +2 for -rc

(()){{}([((((()()()){}())()){}{}){}]({}<>)<>)({()(<{}>)}{})}{}{{}}<>{}{({}<>)<>}<>

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# Push 1 to start the loop
(())

# Start loop
{{}

  # If TOS == 46 i.e. '.'
  ([((((()()()){}())()){}{}){}]({}<>)<>)({()(<{}>)}{})
  # ^------------------------^ ^-------^ 
  #           This is 46         Also, copy TOS to other stack

# End loop after the first '.'
}{}

# Delete everything from this stack
{{}}

# Delete the '.' that got copied
<>{}

# Copy everything back to reverse it to the correct order
{({}<>)<>}<>

Answer 29

Japt, 6 5 bytes

q'. o

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Explanation

 q'. o
Uq'. o
Uq'.    # Split the input at "."
     o # Return the last item

Answer 30

Charcoal, 6 4 bytes

-2 bytes thanks to Erik the Outgolfer

⊟⪪Ｓ.

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Explanation

⊟      Pop
  ⪪ .  Split on "."
   Ｓ  Next input as string
       Implicit print of value

Answer 31

jq, 15 14 characters

(11 10 characters code + 4 characters command line options.)

./"."|last

Sample run:

bash-4.4$ jq -Rr './"."|last' <<< 'what..is..this..file'
file

On-line test

Answer 32

Octave, 24 bytes

@(x)strsplit(x,'.'){end}

Creates an anonymous function named ans which can accept a string as input

Online Demo

Answer 33

C#, 47 34 27 bytes

System.IO.Path.GetExtension

Saved 6 bytes thanks to @totallyhuman.

Try it online!

Answer 34

Pyth, 5 bytes

ecz\.

try it here

Answer 35

REXX 53

parse value reverse(arg(1)) with e "." 
say reverse(e)

Answer 36

C#, 35 bytes

s=>s.Substring(s.LastIndexOf("."));

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Answer 37

Haskell, 32 30 29 bytes

-1 byte thanks to Laikoni

r=reverse
r.fst.span(/='.').r

self-explanatory

Answer 38

Cheddar, 16 bytes

@.sub(/.*\./,"")

simple regex solution. Alternative solution using split:

@.split('.')[-1]

Answer 39

K (oK), 6 bytes

Solution:

*|"."\

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Examples:

*|"."\"whats.up.txt"
"txt"
  *|"."\"T00M@n3KaPZ.h0wC[]h"
"h0wC[]h"

Explanation:

Split \ the input on ".", reverse | this list, and then take the first * one. K is interpreted right-to-left:

*|"."\ / the solution
  "."\ / "." split (\),
 |     / reverse
*      / first

Answer 40

Ohm v2, 5 bytes

I..ï⁾

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Answer 41

Perl 5, 8 bytes

7 bytes code + 1 for -p.

s;.*\.;

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Answer 42

JavaScript, 21 bytes

x=>x.split('.').pop()

Explanation:

This code takes x.txt and turns it into a array. The last element of the array is the file type. This code uses the pop function to remove the last element. In the process, the pop function returns the last element.

Answer 43

Python 3, 46 bytes

-8 bytes thanks to Wheat Wizard and Scrooble

import sys
print(sys.argv[1].split('.')[-1])

EDIT: I fixed the code, and the filename is a command-line argument

Answer 44

C# (Visual C# Interactive Compiler), 22 bytes

s=>s.Split('.').Last()

Implicit using System.Linq; FTW!

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Answer 45

FORTRAN 90, 87 bytes

CHARACTER(99)F,L
READ*,F;DO I=1,LEN(F)
IF(F(I:I)=='.')L=F(I:LEN(F))
ENDDO
PRINT*,L
END

A shorter version in FORTRAN 90.

---

FORTRAN 77, 137 bytes

      PROGRAMC;IMPLICITCHARACTER*99(F)
      READ*,F;DOI=LEN(F),1,-1;IF(F(I:I).EQ.'.')THEN
      PRINT*,F(I:LEN(F));EXIT;ENDIF;ENDDO;END

There is no space in PROGRAM C, nor in IMPLICIT CHARACTER. It works (!) in gfortran, but I'm not sure it works in others compilers. The program takes input from stdin and outputs the extension with the period. The total length of the file name is limited by 99.

Answer 46

SmileBASIC, 45 bytes

INPUT S$@L
E$=POP(S$)+E$ON"."==E$[0]GOTO@L?E$

Outputs extension with the period.

Answer 47

Kotlin, 17 bytes

split(".").last()

Beautified

split(".").last()

Test

data class Test(val input: String, val output: String)

val test = listOf(
        Test("hi.txt", "txt"),
        Test("carrot.meme", "meme"),
        Test("what..is..this..file", "file"),
        Test(".bashrc", "bashrc"),
        Test("T00M@n3KaPZ.h0wC[]h", "h0wC[]h"),
        Test("agent.000", "000")
)

fun String.f() =
split(".").last()

fun main(args: Array<String>) {
    for ((i, o) in test) {
        if (o != i.f()) {
            throw AssertionError()
        }
    }
}

TIO

TryItOnline

Answer 48

V, 4 bytes

òdt.

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Recursively delete everything up to, but not including the next .

Answer 49

C++, 73 bytes

I'm a bit surprised that no one tried C++, since it's an easy one :

#include<string>
auto e=[](std::string s){return s.substr(s.rfind(46));};

And the code to test ( may have to iostream, initializer_list, and exception ) :

std::initializer_list<std::string> test{
    "hi.txt", // .txt
    "carrot.meme", // .meme
    "lol", // invalid string position
    "what..is..this..file", // .file
    ".bashrc", // .bashrc
    "T00M@n3KaPZ.h0wC[]h", // .h0wC[]h
    "agent.000" // .000
};

for (const auto& a : test) {
    try {
        std::cout << e(a) << '\n';
    }
    catch (std::out_of_range& r) {
        std::cout << "out of range exception : " << r.what() << '\n';
    }
}

And, as you may expect, like compilers, if there's undefined behavior, there's no undefined behavior. If there's no file extension, the function will throw a std::out_of_range exception, as said in cppreference

Answer 50

PowerShell, 34 26 bytes

-8 bytes thanks to @mazzy

(Read-Host).Split('.')[-1]

Takes input from STDIN. Actually managed to golf it down shorter than the built-in.

PowerShell, 20 bytes

($args-split'.')[-1]

Takes input from commandline arguments. Big thanks to @mazzy again!

PowerShell (with built-in), 36 bytes

[IO.Path]::GetExtension((Read-Host))

Answer 51

Python 2, 25 bytes

lambda s:s.split('.')[-1]

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Answer 52

Groovy, 37 bytes

{it.substring(it.lastIndexOf(".")+1)}

Answer 53

Ruby, 21 bytes

->f{f.split('.')[-1]}

12 bytes

File.extname

Answer 54

C (gcc), 77 60 52 bytes

char*f(char*s){for(s+=strlen(s);*--s-46;);return s;}

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-17 bytes from Jonathan Frech

-8 bytes by removing i and doing arithmetic on s directly

Ungolfed (same strategy):

char *extension(char *original) {
    original = original + strlen(original);
    while(original[0] != '.') --original;
    return original;
}

Answer 55

Tcl, 21 bytes

puts [file ext $argv]

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Answer 56

Julia, 22 bytes

g(n)=split(n,'.')[end]

Answer 57

Ruby, 16 bytes

->s{s[/[^.]+$/]}

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Answer 58

Excel, 58 bytes

=TRIM(RIGHT(SUBSTITUTE(A1,".",REPT(" ",LEN(A1))),LEN(A1)))

Answer 59

Excel VBA, 29 bytes

An anonymous VBE immediate window function that takes input via cell [A1] and outputs to the console. Return string is of the form .Extension.

?Mid([A1],InStrRev([A1],"."))

Answer 60

Yabasic, 34 bytes

A basic answer

Takes input as a string from STDIN, and outputs of the form .Extension to STDOUT.

Input""s$
?Mid$(s$,RInStr(s$,"."))

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Answer 61

Google Sheets, 25 bytes

An anonymous worksheet function that takes input from cell A1 and outputs the detected file extension of the form Extension to the calling cell.

=RegexExtract(A1,"[^\.]+$

Answer 62

><>, 50 bytes

i:0( ?v
v[:<2~<
r  ^]+1r<
>:"."=?v^
v?l<r~r<;
>o ^

Try it online!

Answer 63

ActionScript 2.0, 42 bytes

function a(b){trace(b.split(".").pop());};

Technically the ;s aren't required for it to compile, at least in JPEXS, but it's good practice. Call:

a("a.b");

(traces "b")

Answer 64

CJam, 6 bytes

q'./W=

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        Print
    W     the last
     =    element
   /      of the result of splitting
q         the input
 '.       on the character '.'

Answer 65

Add++, 9 bytes

L,"."$tbU

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Answer 66

Aceto, 6 bytes

r'.:Qp

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r       grabs input as string
 '.     literal period
   :    split string on period
    Q   grap bottom item
     p  print it

Answer 67

><>, 18 bytes

i:0(6$.:"."=?]
ro|

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How It Works:

i:(6$.  Jump to the second line if out of input
      :"."=  Else check if the character is a .
           ?[ And create a new stack if it is
              Loop back to the beginning of the line
If it is end of input
ro|  Reverse the current stack once and output, erroring on the EOF (-1)

Answer 68

Stax, 4 bytes

'./H

Run and debug online!

Explanation

Split on ., take last part.

Answer 69

T-SQL, 48 bytes

SELECT RIGHT(F,CHARINDEX('.',REVERSE(F))) FROM T

SQL Fiddle

Answer 70

SNOBOL4 (CSNOBOL4), 46 bytes

	I =INPUT
S	I '.' REM . I	:S(S)
	OUTPUT =I
END

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Takes the input, then repeatedly replaces it with all the text following the first . until no .s remain, then outputs the value.

Answer 71

Funky, 26 bytes

s=>(k=s::split".")[(#k)-1]

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Answer 72

Jotlin, 17 bytes

split(".").last()

Full file:

data class Test(val input: String, val output: String)

val test = listOf(
        Test("hi.txt", "txt"),
        Test("carrot.meme", "meme"),
        Test("what..is..this..file", "file"),
        Test(".bashrc", "bashrc"),
        Test("T00M@n3KaPZ.h0wC[]h", "h0wC[]h"),
        Test("agent.000", "000")
)

fun String.f() = split(".").last()

for ((i, o) in test) {
    if (o != i.f()) {
        throw AssertionError()
    }
}

Answer 73

R, 40 26 bytes

-14 bytes thanks to J.Doe

sub("^.*[.]","",scan(,""))

Try it online!

Answer 74

MBASIC, 129 bytes

1 INPUT F$:F=INSTR(F$,"."):IF F=0 THEN END
2 FOR I=LEN(F$) TO 1 STEP -1:P$=MID$(F$,I,1):O$=P$+O$:IF P$="." THEN PRINT O$:END
3 NEXT

Explanation

Get a filename. If it doesn't contain a period, bail out. Otherwise, collect letters from right to left to build an output string. When we see a period, print the string.

Output

? hi.txt
.txt

? .bashrc
.bashrc

? T00M@n3KaPZ.h0wC[]h
.h0wC[]h

Answer 75

Red 9 bytes

Suffix? f

Assumes file is is in word 'f

Answer 76

Pepe, 48 bytes

rEeeEeEEEeREEeREEEerEEREEEEEEEreererEEEEeEeereee

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Answer 77

Turing Machine But Way Worse, 1315 1287 bytes

0 0 0 1 1 0 0
1 0 1 1 8 0 0
0 1 0 1 2 0 0
1 1 1 1 9 0 0
0 2 0 1 3 0 0
1 2 1 1 a 0 0
0 3 0 1 4 0 0
1 3 1 1 b 0 0
0 4 0 1 5 0 0
1 4 1 1 c 0 0
0 5 0 1 6 0 0
1 5 1 1 d 0 0
0 6 0 1 7 0 0
1 6 1 1 e 0 0
0 7 0 1 f 0 0
1 7 1 1 0 0 0
0 8 0 1 9 0 0
1 8 1 1 9 0 0
0 9 0 1 a 0 0
1 9 1 1 a 0 0
0 a 0 1 b 0 0
1 a 1 1 b 0 0
0 b 0 1 c 0 0
1 b 1 1 c 0 0
0 c 0 1 d 0 0
1 c 1 1 d 0 0
0 d 0 1 e 0 0
1 d 1 1 e 0 0
0 e 0 1 0 0 0
1 e 1 1 0 0 0
0 f 0 0 g 0 0
1 f 1 0 g 0 0
0 g 0 0 h 0 0
1 g 1 0 p 0 0
0 h 0 0 q 0 0
1 h 1 0 i 0 0
0 i 0 0 r 0 0
1 i 1 0 j 0 0
0 j 0 0 s 0 0
1 j 1 0 k 0 0
0 k 0 0 l 0 0
1 k 1 0 t 0 0
0 l 0 0 u 0 0
1 l 1 0 m 0 0
0 m 0 0 n 0 0
1 m 1 0 v 0 0
0 n 0 0 o 0 0
1 n 1 0 g 0 0
0 p 0 0 q 0 0
1 p 1 0 q 0 0
0 q 0 0 r 0 0
1 q 1 0 r 0 0
0 r 0 0 s 0 0
1 r 1 0 s 0 0
0 s 0 0 t 0 0
1 s 1 0 t 0 0
0 t 0 0 u 0 0
1 t 1 0 u 0 0
0 u 0 0 f 0 0
1 u 1 0 f 0 0
0 o 0 1 w 0 0
1 o 1 1 w 0 0
0 w 0 1 x 0 0
1 w 1 1 E 0 0
0 x 0 1 y 0 0
1 x 1 1 F 0 0
0 y 0 1 z 0 0
1 y 1 1 G 0 0
0 z 0 1 A 0 0
1 z 1 1 H 0 0
0 A 0 1 B 0 0
1 A 1 1 I 0 0
0 B 0 1 C 0 0
1 B 1 1 J 0 0
0 C 0 1 D 0 0
1 C 1 1 K 0 0
0 D 0 1 D 0 1
1 D 1 1 w 1 0
0 E 0 1 F 0 0
1 E 1 1 F 0 0
0 F 0 1 G 0 0
1 F 1 1 G 0 0
0 G 0 1 H 0 0
1 G 1 1 H 0 0
0 H 0 1 I 0 0
1 H 1 1 I 0 0
0 I 0 1 J 0 0
1 I 1 1 J 0 0
0 J 0 1 K 0 0
1 J 1 1 K 0 0
0 K 0 1 w 1 0
1 K 1 1 w 1 0

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Wow, this is big.

Answer 78

Emacs, 6 bytes

The cursor needs to be at the start of the line containing the string.

This will delete the entire line if there's no extension.

C-<SPC> C-e C-r . <RET> <BACKSPACE>

Explanation:

C-<SPC>      start a selection
C-e          go to the end of the line
C-r . <RET>  search for "." backward
<BACKSPACE>  delete the selected text (which should be the text before the last ".")

Answer 79

Pushy, 13 bytes

K46-$v;F@46+"

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               \ Implicit: string on stack as character codes
K46-           \ Subtract 46 from each code point, mapping '.' to 0
    $v;        \ While the top of stack is non-zero, move to auxiliary stack
       F       \ Copy auxiliary stack onto main stack
        @      \ Reverse (to obtain original order)
         46+   \ Add 46 (to obtain original characters)
            "  \ Print as a string.  

Answer 80

Brain-Flak, 76 bytes

{((((([()()()]){}){}){}()){}{}<>)<>}<>{({}(((()()()()()){}()){}()){}<>)<>}<>

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Explanation:

{          loop over stack
  (                            push
    (((([()()()]){}){}){}()){} -46
  {}                           plus the top of the stack
  <>)                          to the other side
<>}        retuen for more

<>      on the other side...

{       loop until 0
  (                             push
     {}                         the top of the stack
     (((()()()()()){}()){}()){} +46
  <>)                           to the first side
<>}       return for more

<> print the first side

6 bytes less than the other Brain-Flack solution.

Answer 81

Perl 6, 11 bytes

{m/\.\w+$/}

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Answer 82

Befunge-98 (FBBI), 50 bytes

{v
 >~:a`!#v_
--2*86:$<:u-10_v#
02-u0}>:#,_@   >00

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Puts string onto stack, then iterates through it in reverse, putting each new character onto new stack. Once a . is encountered, discard original stack and output new stack in 0gnirts format.

Answer 83

Forth (gforth), 45 bytes

: f begin 1 -1 d+ s" ."search 0= until type ;

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Code Explanation

: f              \ start a new word definition
  begin          \ start an indefinite loop
    1 -1 d+      \ remove the first character from the string
    s" ."search  \ find the length and starting address of the first substring that starts with '.'
    0=           \ check if '.' was found in the string
  until          \ end the loop if not
  type           \ output the result
;                \ end the word definition

Answer 84

33 v1.0, 13 bytes

Gets the file name passed as an argument and prints the extension (without leading '.') to standard output.

1bt'.'ywmcbtp

Explanation:

1b            (Gets the second item in argv, the filename input)
  t'.'y       (Splits that by '.')
       wmcb   (Gets the last element in the list)
           tp (Prints to standard output)

Answer 85

brainfuck, 45 bytes

,[>,]>+[[-]++[<->------]<---<[->+>+<<]>]>[.>]

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Answer 86

Python & JavaScript, 22 bytes

'x.h'.split('.').pop()

Explanation

I did split the string 'x.h', which is the full file name at the dot and then I did remove the last index in the list (the extension) and this happens to return the value of the removed list item which in my case would be the extension.

Note: This could run on either JavaScript or Python without adaptations