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.

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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

Pure bash, 13

echo ${1##*.}

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Answer 4

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 5

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 6

05AB1E, 4 bytes

'.¡¤

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

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

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 10

GNU Make, 12 bytes

$(suffix $1)

Not using a builtin, 27 bytes:

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

Answer 11

Batch, 10 bytes

@echo %~x1

Strangely competitive for once.

Answer 12

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 13

sed, 8 bytes

s:.*\.::

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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

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 16

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 17

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 18

Gema, 3 characters

*.=

Sample run:

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

Answer 19

MATL, 8 7 bytes

46&YbO)

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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 20

Javascript (ES5), 38 bytes

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

Answer 21

C#, 33 41 bytes

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

Edit as suggested:

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

Answer 22

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 23

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 24

Java 8, 52 27 bytes

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

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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 25

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 26

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 27

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 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

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