Extract file extension

From Rosetta Code
Extract file extension is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

Filename extensions are a rudimentary but commonly used way of identifying files types.


Task

Write a function or program that

  • takes one string argument representing the path/URL to a file
  • returns the filename extension according to the below specification, or an empty string if the filename has no extension.


If your programming language  (or standard library)  has built-in functionality for extracting a filename extension,
show how it would be used and how exactly its behavior differs from this specification.


Specification

For the purposes of this task, a filename extension:

  • occurs at the very end of the filename
  • consists of a period, followed solely by one or more ASCII letters or digits (A-Z, a-z, 0-9)


Test cases
Input Output Comment
http://example.com/download.tar.gz .gz
CharacterModel.3DS .3DS
.desktop .desktop
document empty string
document.txt_backup empty string, because _ is not a letter or number
/etc/pam.d/login empty string, as the period is in the parent directory name rather than the filename


Other tasks related to string operations:
Metrics
Counting
Remove/replace
Anagrams/Derangements/shuffling
Find/Search/Determine
Formatting
Song lyrics/poems/Mad Libs/phrases
Tokenize
Sequences



11l

Translation of: Python
F extract_ext(path)
   V m = re:‘\.[A-Za-z0-9]+$’.search(path)
   R I m {m.group(0)} E ‘’

V paths = [‘http://example.com/download.tar.gz’,
           ‘CharacterModel.3DS’,
           ‘.desktop’,
           ‘document’,
           ‘document.txt_backup’,
           ‘/etc/pam.d/login’]

L(path) paths
   print(path.rjust(max(paths.map(p -> p.len)))‘ -> ’extract_ext(path))
Output:
http://example.com/download.tar.gz -> .gz
                CharacterModel.3DS -> .3DS
                          .desktop -> .desktop
                          document ->
               document.txt_backup ->
                  /etc/pam.d/login ->

Action!

INCLUDE "D2:CHARTEST.ACT" ;from the Action! Tool Kit

PROC FileExt(CHAR ARRAY path,ext)
  BYTE pos,c

  pos=path(0)
  ext(0)=0
  WHILE pos>0
  DO
    c=path(pos)
    IF c='. THEN
      EXIT
    ELSEIF IsDigit(c)=0 AND IsAlpha(c)=0 THEN
      RETURN
    FI
    pos==-1
  OD
  IF pos=0 THEN
    RETURN
  FI
  SCopyS(ext,path,pos,path(0))
RETURN

PROC Test(CHAR ARRAY path)
  CHAR ARRAY ext(10)

  FileExt(path,ext)
  PrintF("""%S"":%E""%S""%E%E",path,ext)
RETURN

PROC Main()
  Put(125) PutE() ;clear the screen

  Test("http://example.com/download.tar.gz")
  Test("CharacterModel.3DS")
  Test(".desktop")
  Test("document")
  Test("document.txt_backup")
  Test("/etc/pam.d/login")
RETURN
Output:

Screenshot from Atari 8-bit computer

"http://example.com/download.tar.gz":
".gz"

"CharacterModel.3DS":
".3DS"

".desktop":
".desktop"

"document":
""

"document.txt_backup":
""

"/etc/pam.d/login":
""

Ada

As originally specified

with Ada.Text_IO;       use Ada.Text_IO;
with Ada.Strings.Fixed; use Ada.Strings.Fixed;
use Ada.Strings;
with Ada.Characters.Handling; use Ada.Characters.Handling;

procedure Main is
   function extension (S : in String) return String is
      P_Index : Natural;
   begin
      P_Index :=
        Index (Source => S, Pattern => ".", From => S'Last, Going => Backward);
      if P_Index = 0 then
         return "";
      else
         for C of S (P_Index + 1 .. S'Last) loop
            if not Is_Alphanumeric (C) then
               return "";
            end if;
         end loop;
         return S (P_Index .. S'Last);
      end if;
   end extension;
   F1 : String := "http://example.com/download.tar.gz";
   F2 : String := "CharacterModel.3DS";
   F3 : String := ".desktop";
   F4 : String := "document";
   F5 : String := "document.txt_backup";
   F6 : String := "/etc/pam.d/login:";
begin
   Put_Line (F1 & " -> " & extension (F1));
   Put_Line (F2 & " -> " & extension (F2));
   Put_Line (F3 & " -> " & extension (F3));
   Put_Line (F4 & " -> " & extension (F4));
   Put_Line (F5 & " -> " & extension (F5));
   Put_Line (F6 & " -> " & extension (F6));
end Main;
Output:
http://example.com/download.tar.gz -> .gz
CharacterModel.3DS -> .3DS
.desktop -> .desktop
document -> 
document.txt_backup -> 
/etc/pam.d/login: -> 

In response to problem discussions

with Ada.Text_IO;       use Ada.Text_IO;
with Ada.Strings.Fixed; use Ada.Strings.Fixed;
use Ada.Strings;
with Ada.Characters.Handling; use Ada.Characters.Handling;

procedure Main is
   function extension (S : in String) return String is
      P_Index : Natural;
   begin
      P_Index :=
        Index (Source => S, Pattern => ".", From => S'Last, Going => Backward);
      if P_Index < 2 or else P_Index = S'Last then
         return "";
      else
         for C of S (P_Index + 1 .. S'Last) loop
            if not Is_Alphanumeric (C) then
               return "";
            end if;
         end loop;
         return S (P_Index .. S'Last);
      end if;
   end extension;
   F1 : String := "http://example.com/download.tar.gz";
   F2 : String := "CharacterModel.3DS";
   F3 : String := ".desktop";
   F4 : String := "document";
   F5 : String := "document.txt_backup";
   F6 : String := "/etc/pam.d/login:";
   F7 : String := "filename.";
   F8 : String := ".";
begin
   Put_Line (F1 & " -> " & extension (F1));
   Put_Line (F2 & " -> " & extension (F2));
   Put_Line (F3 & " -> " & extension (F3));
   Put_Line (F4 & " -> " & extension (F4));
   Put_Line (F5 & " -> " & extension (F5));
   Put_Line (F6 & " -> " & extension (F6));
   Put_Line (F7 & " -> " & extension (F7));
   Put_Line (F8 & " -> " & extension (F8));
end Main;
Output:
http://example.com/download.tar.gz -> .gz
CharacterModel.3DS -> .3DS
.desktop -> 
document -> 
document.txt_backup -> 
/etc/pam.d/login: -> 
filename. -> 
. -> 

ALGOL 68

Works with: ALGOL 68G version Any - tested with release 2.8.win32
# extracts a file-extension from the end of a pathname. The file extension is #
# defined as a dot followed by one or more letters or digits                  #
OP  EXTENSION = ( STRING pathname )STRING:
    IF LWB pathname >= UPB pathname THEN
        # the pathname has 0 or 1 characters and so has no extension          #
        ""
    ELIF NOT isalnum( pathname[ UPB pathname ] ) THEN
        # the final character is not a letter or digit - no extension         #
        ""
    ELSE
        # could have an extension                                             #
        INT    pos := UPB pathname;
        WHILE pos > LWB pathname AND isalnum( pathname[ pos ] ) DO
            pos -:= 1
        OD;
        IF pathname[ pos ] = "." THEN
            # the character before the letters and digits was a "."           #
            pathname[ pos : ]
        ELSE
            # no "." before the letters and digits - no extension             #
            ""
        FI
    FI ; # EXTENSION #

# test the EXTENSION operator                                                 #
PROC test extension = ( STRING pathname, STRING expected extension )VOID:
    BEGIN
        STRING extension = EXTENSION pathname;
        write( ( ( pathname
                 + " got extension: ("
                 + extension
                 + ") "
                 + IF extension = expected extension THEN "" ELSE "NOT" FI
                 + " as expected"
                 )
               , newline
               )
             )
    END ; # text extension #
                                  
               
main:
( test extension( "http://example.com/download.tar.gz", ".gz"      )
; test extension( "CharacterModel.3DS",                 ".3DS"     )
; test extension( ".desktop",                           ".desktop" )
; test extension( "document",                           ""         )
; test extension( "document.txt_backup",                ""         )
; test extension( "/etc/pam.d/login",                   ""         )
)
Output:
http://example.com/download.tar.gz got extension: (.gz)  as expected
CharacterModel.3DS got extension: (.3DS)  as expected
.desktop got extension: (.desktop)  as expected
document got extension: ()  as expected
document.txt_backup got extension: ()  as expected
/etc/pam.d/login got extension: ()  as expected

ALGOL W

begin
    % extracts a file-extension from the end of a pathname.                   %
    % The file extension is defined as a dot followed by one or more letters  %
    % or digits. As Algol W only has fixed length strings we limit the        %
    % extension to 32 characters and the pathname to 256 (the longest string  %
    % allowed by Algol W)                                                     %
    string(32) procedure extension( string(256) value pathname ) ;
    begin

        integer pathPos;

        % position to the previous character in the pathname                  %
        procedure prev         ; pathPos := pathPos - 1;
        % get the character as pathPos from pathname                          %
        string(1) procedure ch ; pathname( pathPos // 1 );
        % checks for a letter or digit - assumes the letters are contiguous   %
        % in the character set - not true for EBCDIC                          %
        logical   procedure isLetterOrDigit( string(1) value c ) ;
                  ( c <= "z" and c >= "a" ) or ( c <= "Z" and c >= "A" )
                                            or ( c <= "9" and c >= "0" ) ;

        % find the length of the pathname with trailing blanks removed        %
        pathPos := 255;
        while pathPos >= 0 and ch = " " do prev;

        % extract the extension if possible                                   %
        if pathPos <= 0
        then ""       % no extension: 0 or 1 character pathname               %
        else if not isLetterOrDigit( ch )
        then ""       % no extension: last character not a letter/digit       %
        else begin
            while pathPos > 0 and isLetterOrDigit( ch ) do prev;
            if ch not = "."
            then ""   % no extension: letters/digits not preceeded by "."     %
            else begin
                % have an extension                                           %
                string(32) ext;
                ext := " ";
                % algol W substring lengths must be compile-time constants    %
                % hence the loop to copy the extension characters             %
                for charPos := 0 until 31 do begin
                    if pathPos <= 255 then begin
                        ext( charPos // 1 ) := pathname( pathPos // 1 );
                        pathPos := pathPos + 1
                    end
                end for_charPos ;
                ext
            end
        end

    end extension ;


    % test the extension procedure                                            %
    procedure testExtension( string(256) value pathname
                           ; string(32)  value expectedExtension
                           ) ;
    begin
        string(32) ext;
        ext := extension( pathname );
        write( pathname( 0 // 40 )
             , " -> ("
             , ext( 0 // 16 )
             , ") "
             , if ext = expectedExtension then "" else "NOT"
             , " as expected"
             )
    end ; % text extension %
               
    testExtension( "http://example.com/download.tar.gz", ".gz"      );
    testExtension( "CharacterModel.3DS",                 ".3DS"     );
    testExtension( ".desktop",                           ".desktop" );
    testExtension( "document",                           ""         );
    testExtension( "document.txt_backup",                ""         );
    testExtension( "/etc/pam.d/login",                   ""         );

end.
Output:
http://example.com/download.tar.gz       -> (.gz             )  as expected
CharacterModel.3DS                       -> (.3DS            )  as expected
.desktop                                 -> (.desktop        )  as expected
document                                 -> (                )  as expected
document.txt_backup                      -> (                )  as expected
/etc/pam.d/login                         -> (                )  as expected

AppleScript

AppleScript paths can have either of two formats, depending on the system used to access the items on a disk or network. The current task specification implies that the slash-separated "POSIX" format is intended in all cases. Some macOS "files" are actually directories called "bundles" or "packages". Their paths may or may not end with separators. Underscores are valid extension characters in macOS and extensions are returned without leading dots. When extracting extensions in AppleScript, one would normally follow the rules for macOS, but variations are possible. The task specification is taken at its word that the input strings do represent paths to files.

Vanilla

on getFileNameExtension from txt given underscores:keepingUnderscores : true, dot:includingDot : false
    set astid to AppleScript's text item delimiters
    -- Extract the file or bundle name from the path. 
    set AppleScript's text item delimiters to "/"
    if (txt ends with "/") then
        set itemName to text item -2 of txt
    else
        set itemName to text item -1 of txt
    end if
    -- Extract the extension.
    if (itemName contains ".") then
        set AppleScript's text item delimiters to "."
        set extn to text item -1 of itemName
        if ((not keepingUnderscores) and (extn contains "_")) then set extn to ""
        if ((includingDot) and (extn > "")) then set extn to "." & extn
    else
        set extn to ""
    end if
    set AppleScript's text item delimiters to astid
    
    return extn
end getFileNameExtension

set output to {}
repeat with thisString in {"http://example.com/download.tar.gz", "CharacterModel.3DS", ".desktop", "document", "document.txt_backup", "/etc/pam.d/login"}
    set end of output to {thisString's contents, getFileNameExtension from thisString with dot without underscores}
end repeat

return output
Output:
{{"http://example.com/download.tar.gz", ".gz"}, {"CharacterModel.3DS", ".3DS"}, {".desktop", ".desktop"}, {"document", ""}, {"document.txt_backup", ""}, {"/etc/pam.d/login", ""}}

ASObjC

AppleScriptObjectiveC makes the task a little easier, but not necessarily more efficient.

use AppleScript version "2.4" -- Mac OS X 10.10 (Yosemite) or later.
use framework "Foundation"

on getFileNameExtension from txt given underscores:keepingUnderscores : true, dot:includingDot : false
    -- Get an NSString version of the text and extract the 'pathExtension' from that as AppleScript text.
    set txt to current application's class "NSString"'s stringWithString:(txt)
    set extn to txt's pathExtension() as text
    if ((not keepingUnderscores) and (extn contains "_")) then set extn to ""
    if ((includingDot) and (extn > "")) then set extn to "." & extn
    
    return extn
end getFileNameExtension

set output to {}
repeat with thisString in {"http://example.com/download.tar.gz", "CharacterModel.3DS", ".desktop", "document", "document.txt_backup", "/etc/pam.d/login"}
    set end of output to {thisString's contents, getFileNameExtension from thisString with dot without underscores}
end repeat

return output
Output:
{{"http://example.com/download.tar.gz", ".gz"}, {"CharacterModel.3DS", ".3DS"}, {".desktop", ".desktop"}, {"document", ""}, {"document.txt_backup", ""}, {"/etc/pam.d/login", ""}}

AutoHotkey

data := ["http://example.com/download.tar.gz"
		,"CharacterModel.3DS"
		,".desktop"
		,"document"
		,"document.txt_backup"
		,"/etc/pam.d/login"]

for i, file in data{
	RegExMatch(file, "`am)\.\K[a-zA-Z0-9]+$", ext)
	result .= file " --> " ext "`n"
}
MsgBox % result
Output:
http://example.com/download.tar.gz --> gz
CharacterModel.3DS --> 3DS
.desktop --> desktop
document --> 
document.txt_backup --> 
/etc/pam.d/login --> 

AWK

The following code shows two methods.

The first one was provided by an earlier contributor and shows a little more awk syntax and builtins (albeit with a bug fixed: it was testing for underscores in the extension but not other characters such as hyphens). It can be adjusted to allow any character in the extension other than /, \, : or . by replacing [^a-zA-Z0-9] with [\\/\\\\:\\.].

BEGIN {
    arr[++i] = "picture.jpg"
    arr[++i] = "http://mywebsite.com/picture/image.png"
    arr[++i] = "myuniquefile.longextension"
    arr[++i] = "IAmAFileWithoutExtension"
    arr[++i] = "/path/to.my/file"
    arr[++i] = "file.odd_one"

    for (j=1; j<=i; j++) {
      printf("%-40s '%s'\n",arr[j],extract_ext(arr[j]))
    }
    exit(0)
}
function extract_ext(fn,  sep1,sep2,tmp) {
    while (fn ~ (sep1 = ":|\\\\|\\/")) { # ":" or "\" or "/"
      fn = substr(fn,match(fn,sep1)+1)
    }
    while (fn ~ (sep2 = "\\.")) { # "."
      fn = substr(fn,match(fn,sep2)+1)
      tmp = 1
    }
    if (fn ~ /[^a-zA-Z0-9]/ || tmp == 0) {
    return("")
    }
    return(fn)
}

The second method is shorter and dispenses with the need to search for and remove the path components first. It too can be modified to allow all valid extensions (not just those described in the specification), by replacing \\.[A-Za-z0-9]+$ with \\.[^\\/\\\\:\\.]+$.

BEGIN {
    arr[++i] = "picture.jpg"
    arr[++i] = "http://mywebsite.com/picture/image.png"
    arr[++i] = "myuniquefile.longextension"
    arr[++i] = "IAmAFileWithoutExtension"
    arr[++i] = "/path/to.my/file"
    arr[++i] = "file.odd_one"

    for (j=1; j<=i; j++) {
      printf("%-40s '%s'\n",arr[j],extract_ext(arr[j]))
    }
    exit(0)
}
function extract_ext(fn,  pos) {
	pos = match(fn, "\\.[^\\/\\\\:\\.]+$")
	if (pos == 0) {
		return ("")
	} else {
		return (substr(fn,pos+1))
	}
}

Both examples give the output:

picture.jpg                              'jpg'
http://mywebsite.com/picture/image.png   'png'
myuniquefile.longextension               'longextension'
IAmAFileWithoutExtension                 ''
/path/to.my/file                         ''
file.odd_one                             ''

Batch File

@echo off

:loop
if "%~1"=="" exit /b
echo File Path: "%~1" ^| File Extension "%~x1"
shift
goto loop
Output:
File Path: "http://example.com/download.tar.gz" | File Extension ".gz"
File Path: "CharacterModel.3DS" | File Extension ".3DS"
File Path: ".desktop" | File Extension ".desktop"
File Path: "document" | File Extension ""
File Path: "document.txt_backup" | File Extension ".txt_backup"
File Path: "/etc/pam.d/login" | File Extension ""

BCPL

get "libhdr"

// Find filename extension, store at `v'
let extension(s, v) = valof
$(  let loc = valof
    $(  for i=s%0 to 1 by -1
        $(  let c = s%i
            if c = '.'
                resultis i
            unless 'A'<=c<='Z' | 'a'<=c<='z' | '0'<=c<='9' 
                resultis 0
        $)
        resultis 0
    $)
    test loc=0 do 
        v%0 := 0
    or
    $(  v%0 := s%0-loc+1
        for i=1 to v%0 do
            v%i := s%(i+loc-1)
    $)
    resultis v
$)

let show(s) be
$(  let v = vec 32
    writef("*"%S*": *"%S*"*N", s, extension(s, v))
$)

let start() be
$(  show("http://example.com/download.tar.gz")
    show("CharacterModel.3DS")
    show(".desktop")
    show("document")
    show("document.txt_backup")
    show("/etc/pam.d/login")
$)
Output:
"http://example.com/download.tar.gz": ".gz"
"CharacterModel.3DS": ".3DS"
".desktop": ".desktop"
"document": ""
"document.txt_backup": ""
"/etc/pam.d/login": ""

C

#include <assert.h>
#include <ctype.h>
#include <string.h>
#include <stdio.h>

/* Returns a pointer to the extension of 'string'.
 * If no extension is found, returns a pointer to the end of 'string'. */
char* file_ext(const char *string)
{
    assert(string != NULL);
    char *ext = strrchr(string, '.');

    if (ext == NULL)
        return (char*) string + strlen(string);

    for (char *iter = ext + 1; *iter != '\0'; iter++) {
        if (!isalnum((unsigned char)*iter))
            return (char*) string + strlen(string);
    }

    return ext;
}

int main(void)
{
    const char *testcases[][2] = {
        {"http://example.com/download.tar.gz", ".gz"},
        {"CharacterModel.3DS", ".3DS"},
        {".desktop", ".desktop"},
        {"document", ""},
        {"document.txt_backup", ""},
        {"/etc/pam.d/login", ""}
    };

    int exitcode = 0;
    for (size_t i = 0; i < sizeof(testcases) / sizeof(testcases[0]); i++) {
        const char *ext = file_ext(testcases[i][0]);
        if (strcmp(ext, testcases[i][1]) != 0) {
            fprintf(stderr, "expected '%s' for '%s', got '%s'\n",
                testcases[i][1], testcases[i][0], ext);
            exitcode = 1;
        }
    }
    return exitcode;
}

C#

public static string FindExtension(string filename) {
    int indexOfDot = filename.Length;
    for (int i = filename.Length - 1; i >= 0; i--) {
        char c = filename[i];
        if (c == '.') {
            indexOfDot = i;
            break;
        }
        if (c >= '0' && c <= '9') continue;
        if (c >= 'A' && c <= 'Z') continue;
        if (c >= 'a' && c <= 'z') continue;
        break;
    }
    //The dot must be followed by at least one other character,
    //so if the last character is a dot, return the empty string
    return indexOfDot + 1 == filename.Length ? "" : filename.Substring(indexOfDot);
}

Using regular expressions (C# 6)

public static string FindExtension(string filename) => Regex.Match(filename, @"\.[A-Za-z0-9]+$").Value;

C++

#include <iostream>
#include <filesystem>

int main() {
    for (std::filesystem::path file : { "picture.jpg", 
                                        "http://mywebsite.com/picture/image.png",
                                        "myuniquefile.longextension",
                                        "IAmAFileWithoutExtension",
                                        "/path/to.my/file",
                                        "file.odd_one",
                                        "thisismine." }) {
        std::cout << file << " has extension : " << file.extension() << '\n' ;
    }
}
Output:
"picture.jpg" has extension : ".jpg"
"http://mywebsite.com/picture/image.png" has extension : ".png"
"myuniquefile.longextension" has extension : ".longextension"
"IAmAFileWithoutExtension" has extension : ""
"/path/to.my/file" has extension : ""
"file.odd_one" has extension : ".odd_one"
"thisismine." has extension : "."

Clojure

(defn file-extension [s]
  (second (re-find #"(\.[a-zA-Z0-9]+)$" s)))
Output:
(map file-extension ["http://example.com/download.tar.gz"
                     "CharacterModel.3DS"
                     ".desktop"
                     "document"
                     "document.txt_backup"
                     "/etc/pam.d/login"])

(".gz" ".3DS" ".desktop" nil nil nil)

CLU

CLU contains a built-in filename parser, which behaves slightly differently than the task specification. It returns the first, rather than last dotted part, and also accepts non-alphanumeric characters in the extension. Furthermore, it does not include the dot itself in its output.

% Find the extension of a filename, according to the task specification
extension = proc (s: string) returns (string)
    for i: int in int$from_to_by(string$size(s), 1, -1) do
        c: char := s[i]
        if c>='A' & c<='Z' 
         | c>='a' & c<='z' 
         | c>='0' & c<='9' then continue end
        if c='.' then return(string$rest(s,i)) end
        break
    end
    return("")
end extension
        
        
% For each test case, show both the extension according to the task,
% and the extension that the built-in function returns.
start_up = proc ()
    po: stream := stream$primary_output()
    
    tests: sequence[string] := sequence[string]$[
        "http://example.com/download.tar.gz",
        "CharacterModel.3DS",
        ".desktop",
        "document",
        "document.txt_backup",
        "/etc/pam.d/login"
    ]
    
    stream$putleft(po, "Input", 36)
    stream$putleft(po, "Output", 10)
    stream$putl(po, "Built-in")
    stream$putl(po, "---------------------------------------------------------")
    
    for test: string in sequence[string]$elements(tests) do
        stream$putleft(po, test, 36)
        stream$putleft(po, extension(test), 10)
        
        % Using the built-in filename parser
        stream$putl(po, file_name$parse(test).suffix)
        except when bad_format:
            stream$putl(po, "[bad_format signaled]")
        end
    end 
end start_up
Output:
Input                               Output    Built-in
---------------------------------------------------------
http://example.com/download.tar.gz  .gz       tar
CharacterModel.3DS                  .3DS      3DS
.desktop                            .desktop  desktop
document
document.txt_backup                           txt_backup
/etc/pam.d/login

Common Lisp

(pathname-type "foo.txt")
=>
"txt"

D

Variant 1

import std.stdio;
import std.path;

void main()
{
  auto filenames = ["http://example.com/download.tar.gz",
                    "CharacterModel.3DS",
                    ".desktop",
                    "document",
                    "document.txt_backup",
                    "/etc/pam.d/login"]

  foreach(filename; filenames)
    writeln(filename, " -> ", filename.extension);
    
}
Output:
http://example.com/download.tar.gz -> .gz
CharacterModel.3DS -> .3DS
.desktop ->
document -> 
document.txt_backup -> .txt_backup
/etc/pam.d/login ->

Variant 2

import std.stdio;
import std.string;
import std.range;
import std.algorithm;

void main()
{
  auto filenames = ["http://example.com/download.tar.gz",
                    "CharacterModel.3DS",
                    ".desktop",
                    "document",
                    "document.txt_backup",
                    "/etc/pam.d/login"]

  foreach(filename; filenames)
  {
    string ext;
    auto idx = filename.lastIndexOf(".");
    if(idx >= 0)
    {
      auto tmp = filename.drop(idx);
      if(!tmp.canFind("/", "\\", "_", "*");
        ext = tmp;
    }
    writeln(filename, " -> ", ext);
  }
    
}
Output:
http://example.com/download.tar.gz -> .gz
CharacterModel.3DS -> .3DS
.desktop -> .desktop
document -> 
document.txt_backup ->
/etc/pam.d/login ->

Delphi

program Extract_file_extension;

{$APPTYPE CONSOLE}

uses
  System.SysUtils,
  System.Character;

const
  TEST_CASES: array[0..5] of string = ('http://example.com/download.tar.gz',
    'CharacterModel.3DS', '.desktop', 'document', 'document.txt_backup',
    '/etc/pam.d/login');

function GetExt(path: string): string;
var
  c: char;
begin
  // Built-in functionality, just extract substring after dot char
  Result := ExtractFileExt(path);

  // Fix ext for dot in subdir
  while (Result.IndexOf('/') > -1) do
  begin
    Result := Result.Substring(Result.IndexOf('/'), MaxInt);
    Result := ExtractFileExt(Result);
  end;

  // Ignore empty or "." ext
  if length(result) < 2 then
    exit('');

  // Ignore ext with not alphanumeric char (except the first dot)
  for var i := 2 to length(result) do
  begin
    c := result[i];
    if not c.IsLetterOrDigit then
      exit('');
  end;
end;

begin
  for var path in TEST_CASES do
    Writeln(path.PadRight(40), GetExt(path));

  {$IFNDEF UNIX} readln; {$ENDIF}
end.
Output:
http://example.com/download.tar.gz      .gz
CharacterModel.3DS                      .3DS
.desktop                                .desktop
document
document.txt_backup
/etc/pam.d/login

EasyLang

func isalphanum c$ .
   c = strcode c$
   return if c >= 65 and c <= 90 or c >= 97 and c <= 122 or c >= 48 and c <= 57
.
func$ exext path$ .
   for i = len path$ downto 1
      c$ = substr path$ i 1
      if isalphanum c$ = 1
         ex$ = c$ & ex$
      elif c$ = "."
         return ex$
      else
         break 1
      .
   .
.
for s$ in [ "http://example.com/download.tar.gz" "CharacterModel.3DS" ".desktop" "document" "document.txt_backup" "/etc/pam.d/login" ]
   print s$ & " -> " & exext s$
.

Ed

Prints empty line for extension-less/extension-invalid names.

H
,p
g/.*/s/^.*(\.[[:alnum:]]+)$/\1/g
v/^\.[[:alnum:]]+$/s/.*//
,p
Q
Output:
$ cat extension.ed | ed -E extension.input 
Newline appended
109
http://example.com/download.tar.gz
CharacterModel.3DS
.desktop
document
document.txt_backup
/etc/pam.d/login
.gz
.3DS
.desktop



?
Warning: buffer modified

Emacs Lisp

(file-name-extension "foo.txt")
=>
"txt"

No extension is distinguished from empty extension but an (or ... "") can give "" for both if desired

(file-name-extension "foo.") => ""
(file-name-extension "foo")  => nil

An Emacs backup ~ or .~NUM~ are not part of the extension, but otherwise any characters are allowed.

(file-name-extension "foo.txt~")        => "txt"
(file-name-extension "foo.txt.~1.234~") => "txt"

Factor

Factor's file-extension word allows symbols to be in the extension and omits the dot from its output.

USING: assocs formatting kernel io io.pathnames math qw
sequences ;
IN: rosetta-code.file-extension

qw{
  http://example.com/download.tar.gz
  CharacterModel.3DS
  .desktop
  document
  document.txt_backup
  /etc/pam.d/login
}

dup [ file-extension ] map zip
"Path" "| Extension" "%-35s%s\n" printf
47 [ "-" write ] times nl
[ "%-35s| %s\n" vprintf ] each
Output:
Path                               | Extension
-----------------------------------------------
http://example.com/download.tar.gz | gz
CharacterModel.3DS                 | 3DS
.desktop                           | desktop
document                           |
document.txt_backup                | txt_backup
/etc/pam.d/login                   |

Forth

: invalid? ( c -- f )
   toupper dup [char] A [char] Z 1+ within
   swap [char] 0 [char] 9 1+ within or 0= ;
: extension ( addr1 u1 -- addr2 u2 )
   dup 0= if exit then
   2dup over +
   begin 1- 2dup <= while dup c@ invalid? until then
   \ no '.' found
   2dup - 0> if 2drop dup /string exit then
   \ invalid char
   dup c@ [char] . <> if 2drop dup /string exit then
   swap -
   \ '.' is last char
   2dup 1+ = if drop dup then
   /string ;

: type.quoted ( addr u -- )
   [char] ' emit type [char] ' emit ;
: test ( addr u -- )
   2dup type.quoted ."  => " extension type.quoted cr ;
: tests
   s" http://example.com/download.tar.gz" test
   s" CharacterModel.3DS" test
   s" .desktop" test
   s" document" test
   s" document.txt_backup" test
   s" /etc/pam.d/login" test ;
Output:
cr tests 
'http://example.com/download.tar.gz' => '.gz'
'CharacterModel.3DS' => '.3DS'
'.desktop' => '.desktop'
'document' => ''
'document.txt_backup' => ''
'/etc/pam.d/login' => ''
 ok

Fortran

The plan is to scan backwards from the end of the text until a non-extensionish character is encountered. If it is a period, then a valid file extension has been spanned. Otherwise, no extension. Yet again the "no specification" on the possibility of shortcut evaluation of compound logical expressions prevents the structured use of a DO WHILE(L1 > 0 & TEXT(L1:L1)etc) loop because the possible evaluation of both parts of the expression means that the second part may attempt to access character zero of a text. So, the compound expression has to be broken into two separate parts.

The source incorporates a collection of character characterisations via suitable spans of a single sequence of characters. Unfortunately, the PARAMETER statement does not allow its constants to appear in EQUIVALENCE statements, so the text is initialised by DATA statements, and thus loses the protection of read-only given to constants defined via PARAMETER statements. The statements are from a rather more complex text scanning scheme, as all that are needed here are the symbols of GOODEXT.

The text scan could instead check for a valid character via something like ("a" <= C & C <= "z") | ("A" <= C & C <= "Z") | (0 <= C & C <= "9") but this is not just messy but unreliable - in EBCDIC for example there are gaps in the sequence of letters that are occupied by other symbols. So instead, a test via INDEX into a sequence of all the valid symbols. If one was in a hurry, for eight-bit character codes, an array GOODEXT of 256 logical values could be indexed by the numerical value of the character.

      MODULE TEXTGNASH	!Some text inspection.
       CHARACTER*10 DIGITS		!Integer only.
       CHARACTER*11 DDIGITS		!With a full stop masquerading as a decimal point.
       CHARACTER*13 SDDIGITS		!Signed decimal digits.
       CHARACTER*4  EXPONENTISH		!With exponent parts.
       CHARACTER*17 NUMBERISH		!The complete mix.
       CHARACTER*16 HEXLETTERS		!Extended for base sixteen.
       CHARACTER*62 DIGILETTERS		!File nameish but no .
       CHARACTER*26 LITTLELETTERS,BIGLETTERS	!These are well-known.
       CHARACTER*52 LETTERS		!The union thereof.
       CHARACTER*66 NAMEISH		!Allowing digits and . and _ as well.
       CHARACTER*3  ODDITIES		!And allow these in names also.
       CHARACTER*1 CHARACTER(72)	!Prepare a work area.
       EQUIVALENCE			!Whose components can be fingered.
     1  (CHARACTER( 1),EXPONENTISH,NUMBERISH),	!Start with numberish symbols that are not nameish.
     2  (CHARACTER( 5),SDDIGITS),		!Since the sign symbols are not nameish.
     3  (CHARACTER( 7),DDIGITS,NAMEISH),	!Computerish names might incorporate digits and a .
     4  (CHARACTER( 8),DIGITS,HEXLETTERS,DIGILETTERS),	!A proper name doesn't start with a digit.
     5  (CHARACTER(18),BIGLETTERS,LETTERS),	!Just with a letter.
     6  (CHARACTER(44),LITTLELETTERS),		!The second set.
     7  (CHARACTER(70),ODDITIES)		!Tack this on the end.
       DATA EXPONENTISH /"eEdD"/	!These on the front.
       DATA SDDIGITS /"+-.0123456789"/	!Any of these can appear in a floating point number.
       DATA BIGLETTERS    /"ABCDEFGHIJKLMNOPQRSTUVWXYZ"/	!Simple.
       DATA LITTLELETTERS /"abcdefghijklmnopqrstuvwxyz"/	!Subtly different.
       DATA ODDITIES /"_:#"/		!Allow these in names also. This strains := usage!

       CHARACTER*62 GOODEXT	!These are all the characters allowed
       EQUIVALENCE (CHARACTER(8),GOODEXT)
c       PARAMETER (GOODEXT = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"	!for an approved
c     1                    //"abcdefghijklmnopqrstuvwxyz"	!file "extension" part
c     2                    //"0123456789")			!Of a file name.
       INTEGER MEXT		!A fixed bound.
       PARAMETER (MEXT = 28)	!This should do.
       CONTAINS
        CHARACTER*(MEXT) FUNCTION FEXT(FNAME)	!Return the file extension part.
         CHARACTER*(*) FNAME	!May start with the file's path name blather.
         INTEGER L1,L2		!Fingers to the text.
          L2 = LEN(FNAME)	!The last character of the file name.
          L1 = L2		!Starting at the end...
   10     IF (L1.GT.0) THEN	!Damnit, can't rely on DO WHILE(safe & test)
            IF (INDEX(GOODEXT,FNAME(L1:L1)).GT.0) THEN	!So do the two parts explicitly.
              L1 = L1 - 1		!Well, that was a valid character for an extension.
              GO TO 10			!So, move back one and try again.
            END IF		!Until the end of valid stuff.
            IF (FNAME(L1:L1).EQ.".") THEN	!Stopped here. A proper introduction?
              L1 = L1 - 1			!Yes. Include the period.
              GO TO 20				!And escape.
            END IF		!Otherwise, not valid stuff.
          END IF		!Keep on moving back.
          L1 = L2		!If we're here, no period was found.
   20     FEXT = FNAME(L1 + 1:L2)	!The text of the extension.
        END FUNCTION FEXT	!Possibly, blank.
      END MODULE TEXTGNASH	!Enough for this.

      PROGRAM POKE
      USE TEXTGNASH

      WRITE (6,*) FEXT("Picture.jpg")
      WRITE (6,*) FEXT("http://mywebsite.com/picture/image.png")
      WRITE (6,*) FEXT("myuniquefile.longextension")
      WRITE (6,*) FEXT("IAmAFileWithoutExtension")
      WRITE (6,*) FEXT("/path/to.my/file")
      WRITE (6,*) FEXT("file.odd_one")
      WRITE (6,*)
      WRITE (6,*) "Now for the new test collection..."
      WRITE (6,*) FEXT("http://example.com/download.tar.gz")
      WRITE (6,*) FEXT("CharacterModel.3DS")
      WRITE (6,*) FEXT(".desktop")
      WRITE (6,*) FEXT("document")
      WRITE (6,*) FEXT("document.txt_backup")
      WRITE (6,*) FEXT("/etc/pam.d/login")
      WRITE (6,*) "Approved characters: ",GOODEXT
      END

The output cheats a little, in that trailing spaces appear just as blankly as no spaces. The result of FEXT could be presented to TRIM (if that function is available), or the last non-blank could be found. With F2003, a scheme to enable character variables to be redefined to take on a current length is available, and so trailing spaces could no longer appear. This facility would also solve the endlessly annoying question of "how long is long enough", manifested in parameter MEXT being what might be a perfect solution. Once, three was the maximum extension length (not counting the period), then perhaps six, but now, what?

 .jpg
 .png
 .longextension




 Now for the new test collection...
 .gz
 .3DS
 .desktop



 Approved characters:
 0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz

Note that if FEXT were presented with a file name containing trailing spaces, it would declare no extension to be present.

FreeBASIC

' FB 1.05.0 Win64

Function isAlphaNum(s As String) As Boolean
  Return ("a" <= s AndAlso s <= "z") OrElse ("A" <= s AndAlso s <= "Z") OrElse("0" <= s AndAlso s <= "9")
End Function

Function extractFileExt(filePath As String) As String
  If filePath = "" Then Return ""
  Dim index As Integer = InstrRev(filePath, ".")
  If index = 0 Then Return ""
  Dim ext As String  = Mid(filePath, index + 1)
  If ext = "" Then Return ""
  For i As Integer = 1 To Len(ext)
    If Not isAlphaNum(Mid(ext, i, 1)) Then Return ""
  Next
  Return ext
End Function

Dim filePaths(1 To 6) As String = _
{ _
  "http://example.com/download.tar.gz", _	
  "CharacterModel.3DS", _	
  ".desktop", _	
  "document", _
  "document.txt_backup", _
  "/etc/pam.d/login" _
}

Print "File path"; Tab(40); "Extension"
Print "========="; Tab(40); "========="
Print
For i As Integer = 1 To 6
  Print filePaths(i); Tab(40); 
  Dim ext As String = extractFileExt(filePaths(i))
  If ext = "" Then
    Print "(empty string)"
  Else
    Print ext
  End If
Next
Print
Print "Press any key to quit"
Sleep
Output:
File path                              Extension
=========                              =========

http://example.com/download.tar.gz     gz
CharacterModel.3DS                     3DS
.desktop                               desktop
document                               (empty string)
document.txt_backup                    (empty string)
/etc/pam.d/login                       (empty string)


Frink

fileExtension[str] :=
{
   if [ext] = str =~ %r/(\.[A-Za-z0-9]+)$/
      return ext
   else
      return ""
}

files = ["http://example.com/download.tar.gz",
         "CharacterModel.3DS",
         ".desktop",
         "document",
         "document.txt_backup",
         "/etc/pam.d/login"]

r = new array
for f = files
   r.push[[f, "->", fileExtension[f]]]

println[formatTable[r, "right"]]
Output:
http://example.com/download.tar.gz ->      .gz
                CharacterModel.3DS ->     .3DS
                          .desktop -> .desktop
                          document ->         
               document.txt_backup ->         
                  /etc/pam.d/login ->         

FutureBasic

The underscores is a valid extension character in macOS and extensions are returned without leading dots.

include "NSLog.incl"

void local fn DoIt
  CFArrayRef paths = @[@"http://example.com/download.tar.gz",@"CharacterModel.3DS",@".desktop",@"document",@"document.txt_backup",@"/etc/pam.d/login"]
  CFStringRef path
  for path in paths
    NSLog(@"%@",fn StringPathExtension( path ))
  next
end fn

fn DoIt

HandleEvents
Output:
gz
3DS
desktop
null
txt_backup
null

Gambas

As Gambas has its own tools for file extension extraction I have used those rather than complicate the code to match the requested criteria.

Click this link to run this code

Public Sub Main()
Dim sDir As String = "/sbin"
Dim sFileList As String[] = Dir(sDir)
Dim sTemp As String
Dim sFile As String

For Each sTemp In sFileList
  sFile = sDir &/ sTemp
  Print File.Name(sFile) & Space(25 - Len(File.Name(sFile)));
  Print File.Ext(sFile)
Next

End

Output:

....
mount.ntfs               ntfs
iptables-save            
mkfs.minix               minix
exfatlabel               
modprobe                 
vgrename                 
mkfs.ext2                ext2
lsmod                    
umount.ecryptfs_private  ecryptfs_private
fstab-decode             
mount.ecryptfs           ecryptfs
....

Go

package main

import "fmt"

func Ext(path string) string {
	for i := len(path) - 1; i >= 0; i-- {
		c := path[i]
		switch {
		case c == '.':
			return path[i:]
		case '0' <= c && c <= '9':
		case 'A' <= c && c <= 'Z':
		case 'a' <= c && c <= 'z':
		default:
			return ""
		}
	}
	return ""
}

func main() {
	type testcase struct {
		input  string
		output string
	}

	tests := []testcase{
		{"http://example.com/download.tar.gz", ".gz"},
		{"CharacterModel.3DS", ".3DS"},
		{".desktop", ".desktop"},
		{"document", ""},
		{"document.txt_backup", ""},
		{"/etc/pam.d/login", ""},
	}

	for _, testcase := range tests {
		ext := Ext(testcase.input)
		if ext != testcase.output {
			panic(fmt.Sprintf("expected %q for %q, got %q",
				testcase.output, testcase.input, ext))
		}
	}
}

Haskell

module FileExtension
   where

myextension :: String -> String
myextension s
   |not $ elem '.' s = ""
   |elem '/' extension || elem '_' extension = ""
   |otherwise = '.' : extension
      where
	 extension = reverse ( takeWhile ( /= '.' ) $ reverse s )
Output:
map myextension ["http://example.com/download.tar.gz", "CharacterModel.3DS", ".desktop", "document", "document.txt_backup", "/etc/pam.d/login"]
[".gz",".3DS",".desktop","","",""]

Posix compliant

On Unix systems, the penultimate file extension would be recognised, so using the Haskell library function takeExtension:

import System.FilePath.Posix (FilePath, takeExtension)

fps :: [FilePath]
fps =
  [ "http://example.com/download.tar.gz",
    "CharacterModel.3DS",
    ".desktop",
    "document",
    "document.txt_backup",
    "/etc/pam.d/login"
  ]

main :: IO ()
main = mapM_ (print . takeExtension) fps
Output:
".gz"
".3DS"
".desktop"
""
".txt_backup"
""

J

Implementation:

require'regex'
ext=: '[.][a-zA-Z0-9]+$'&rxmatch ;@rxfrom ]

Obviously most of the work here is done by the regex implementation (pcre, if that matters - and this particular kind of expression tends to be a bit more concise expressed in perl than in J...).

Perhaps of interest is that this is an example of a J fork - here we have three verbs separated by spaces. Unlike a unix system fork (which spins up child process which is an almost exact clone of the currently running process), a J fork is three independently defined verbs. The two verbs on the edge get the fork's argument and the verb in the middle combines those two results.

The left verb uses rxmatch to find the beginning position of the match and its length. The right verb is the identity function. The middle verb extracts the desired characters from the original argument. (For a non-match, the length of the "match" is zero so the empty string is extracted.)


Alternative non-regex Implementation

ext=: (}.~ i:&'.')@(#~ [: -. [: +./\. -.@e.&('.',AlphaNum_j_)


Task examples:

   ext 'http://example.com/download/tar.gz'
.gz
   ext 'CharacterModel.3DS'
.3DS

   Examples=: 'http://example.com/download.tar.gz';'CharacterModel.3DS';'.desktop';'document';'document.txt_backup';'/etc/pam.d/login'
   ext each Examples
┌───┬────┬────────┬┬┬┐
.gz.3DS│.desktop││││
└───┴────┴────────┴┴┴┘

Java

import java.io.File;
public static void main(String[] args) {
    String[] strings = {
        "http://example.com/download.tar.gz",
        "CharacterModel.3DS",
        ".desktop",
        "document",
        "document.txt_backup",
        "/etc/pam.d/login",
    };
    for (String string : strings)
        System.out.println(extractExtension(string));
}

static String extractExtension(String string) {
    /* we can use the 'File' class to extract the file-name */
    File file = new File(string);
    String filename = file.getName();
    int indexOf = filename.lastIndexOf('.');
    if (indexOf != -1) {
        String extension = filename.substring(indexOf);
        /* and use a regex to match only valid extensions */
        if (extension.matches("\\.[A-Za-z\\d]+"))
            return extension;
    }
    return "";
}
Output:
.gz
.3DS
.desktop


JavaScript

let filenames = ["http://example.com/download.tar.gz", "CharacterModel.3DS", ".desktop", "document", "document.txt_backup", "/etc/pam.d/login"];
let r = /\.[a-zA-Z0-9]+$/;
filenames.forEach((e) => console.log(e + " -> " + (r.test(e) ? r.exec(e)[0] : "")));
Output:
http://example.com/download.tar.gz -> .gz
CharacterModel.3DS -> .3DS
.desktop -> .desktop
document -> 
document.txt_backup -> 
/etc/pam.d/login -> 


With JS embedded in browsers and other applications across most or all operating systems, we need some flexibility in any reusable takeExtension function.

One approach is to define a more general curried function, from which we can obtain various simpler and OS-specific functions by specialisation:

(() => {
    'use strict';

    // OS-INDEPENDENT CURRIED FUNCTION --------------------

    // takeExtension :: Regex String -> FilePath -> String
    const takeExtension = charSet => fp => {
        const
            rgx = new RegExp('^[' + charSet + ']+$'),
            xs = fp.split('/').slice(-1)[0].split('.'),
            ext = 1 < xs.length ? (
                xs.slice(-1)[0]
            ) : '';
        return rgx.test(ext) ? (
            '.' + ext
        ) : '';
    };

    // OS-SPECIFIC SPECIALIZED FUNCTIONS ------------------

    // takePosixExtension :: FilePath -> String
    const takePosixExtension = takeExtension('A-Za-z0-9\_\-');

    // takeWindowsExtension :: FilePath -> String
    const takeWindowsExtension = takeExtension('A-Za-z0-9');


    // TEST -------------------------------------------
    // main :: IO()
    const main = () => {
        [
            ['Posix', takePosixExtension],
            ['Windows', takeWindowsExtension]
        ].forEach(
            ([osName, f]) => console.log(
                tabulated(
                    '\n\ntake' + osName +
                    'Extension :: FilePath -> String:\n',
                    x => x.toString(),
                    x => "'" + x.toString() + "'",
                    f,
                    [
                        "http://example.com/download.tar.gz",
                        "CharacterModel.3DS",
                        ".desktop",
                        "document",
                        "document.txt_backup",
                        "/etc/pam.d/login"
                    ]
                ),
                '\n'
            )
        )

    };

    // GENERIC FUNCTIONS FOR TESTING AND DISPLAY OF RESULTS

    // comparing :: (a -> b) -> (a -> a -> Ordering)
    const comparing = f =>
        (x, y) => {
            const
                a = f(x),
                b = f(y);
            return a < b ? -1 : (a > b ? 1 : 0);
            i
        };

    // compose (<<<) :: (b -> c) -> (a -> b) -> a -> c
    const compose = (f, g) => x => f(g(x));

    // justifyRight :: Int -> Char -> String -> String
    const justifyRight = (n, cFiller, s) =>
        n > s.length ? (
            s.padStart(n, cFiller)
        ) : s;

    // Returns Infinity over objects without finite length.
    // This enables zip and zipWith to choose the shorter
    // argument when one is non-finite, like cycle, repeat etc

    // length :: [a] -> Int
    const length = xs =>
        (Array.isArray(xs) || 'string' === typeof xs) ? (
            xs.length
        ) : Infinity;

    // Map over lists or strings

    // map :: (a -> b) -> [a] -> [b]
    const map = (f, xs) =>
        (Array.isArray(xs) ? (
            xs
        ) : xs.split('')).map(f);


    // maximumBy :: (a -> a -> Ordering) -> [a] -> a
    const maximumBy = (f, xs) =>
        0 < xs.length ? (
            xs.slice(1)
            .reduce((a, x) => 0 < f(x, a) ? x : a, xs[0])
        ) : undefined;

    // tabulated :: String -> (a -> String) ->
    //                        (b -> String) ->
    //           (a -> b) -> [a] -> String
    const tabulated = (s, xShow, fxShow, f, xs) => {
        // Heading -> x display function ->
        //           fx display function ->
        //    f -> values -> tabular string
        const
            ys = map(xShow, xs),
            w = maximumBy(comparing(x => x.length), ys).length,
            rows = zipWith(
                (a, b) => justifyRight(w, ' ', a) + ' -> ' + b,
                ys,
                map(compose(fxShow, f), xs)
            );
        return s + '\n' + unlines(rows);
    };

    // take :: Int -> [a] -> [a]
    // take :: Int -> String -> String
    const take = (n, xs) =>
        'GeneratorFunction' !== xs.constructor.constructor.name ? (
            xs.slice(0, n)
        ) : [].concat.apply([], Array.from({
            length: n
        }, () => {
            const x = xs.next();
            return x.done ? [] : [x.value];
        }));


    // unlines :: [String] -> String
    const unlines = xs => xs.join('\n');

    // Use of `take` and `length` here allows zipping with non-finite lists
    // i.e. generators like cycle, repeat, iterate.

    // zipWith :: (a -> b -> c) -> [a] -> [b] -> [c]
    const zipWith = (f, xs, ys) => {
        const
            lng = Math.min(length(xs), length(ys)),
            as = take(lng, xs),
            bs = take(lng, ys);
        return Array.from({
            length: lng
        }, (_, i) => f(as[i], bs[i], i));
    };

    // MAIN ---
    return main();
})();
Output:
takePosixExtension :: FilePath -> String:

http://example.com/download.tar.gz -> '.gz'
                CharacterModel.3DS -> '.3DS'
                          .desktop -> '.desktop'
                          document -> ''
               document.txt_backup -> '.txt_backup'
                  /etc/pam.d/login -> '' 


takeWindowsExtension :: FilePath -> String:

http://example.com/download.tar.gz -> '.gz'
                CharacterModel.3DS -> '.3DS'
                          .desktop -> '.desktop'
                          document -> ''
               document.txt_backup -> ''
                  /etc/pam.d/login -> '' 

jq

The following definitions include the delimiting period.

In the first section, a version intended for jq version 1.4 is presented. A simpler definition using "match", a regex feature of subsequent versions of jq, is then given.

Works with: jq version 1.4
def file_extension:
  def alphanumeric: explode | unique
  | reduce .[] as $i
      (true;
       if . then $i | (97 <= . and . <= 122) or (65 <= . and . <= 90) or (48 <= . and . <= 57)
       else false
       end );
  rindex(".") as $ix
  | if $ix then .[1+$ix:] as $ext
    | if $ext|alphanumeric then ".\($ext)" # include the period
      else ""
      end
    else ""
    end;
Works with: jq version 1.5
def file_extension:
  (match( "(\\.[a-zA-Z0-9]*$)" ) | .captures[0].string) 
  // "" ;

Examples:

Using either version above gives the same results.

"http://example.com/download.tar.gz",
"CharacterModel.3DS",
".desktop",
"document",
"document.txt_backup",
"/etc/pam.d/login"
| "\(.) has extension: \(file_extension)"
$ jq -r -n -f Extract_file_extension.jq
Output:
http://example.com/download.tar.gz has extension: .gz
CharacterModel.3DS has extension: .3DS
.desktop has extension: .desktop
document has extension: 
document.txt_backup has extension: 
/etc/pam.d/login has extension: 

Jsish

#!/usr/bin/env jsish
/* Extract filename extension (for a limited subset of possible extensions) in Jsish */
function extractExtension(filename) {
    var extPat = /\.[a-z0-9]+$/i;
    var ext = filename.match(extPat);
    return ext ? ext[0] : '';
}

if (Interp.conf('unitTest')) {
    var files = ["http://example.com/download.tar.gz", "CharacterModel.3DS",
                 ".desktop", "document", "document.txt_backup", "/etc/pam.d/login"];
    for (var fn of files) puts(fn, quote(extractExtension(fn)));
}

/*
=!EXPECTSTART!=
http://example.com/download.tar.gz ".gz"
CharacterModel.3DS ".3DS"
.desktop ".desktop"
document ""
document.txt_backup ""
/etc/pam.d/login ""
=!EXPECTEND!=
*/
Output:
prompt$ jsish --U extractExtension.jsi
http://example.com/download.tar.gz ".gz"
CharacterModel.3DS ".3DS"
.desktop ".desktop"
document ""
document.txt_backup ""
/etc/pam.d/login ""

prompt$ jsish -u extractExtension.jsi
[PASS] extractExtension.jsi

Julia

extension(url::String) = try match(r"\.[A-Za-z0-9]+$", url).match catch "" end

@show extension("http://example.com/download.tar.gz")
@show extension("CharacterModel.3DS")
@show extension(".desktop")
@show extension("document")
@show extension("document.txt_backup")
@show extension("/etc/pam.d/login")
Output:
extension("http://example.com/download.tar.gz") = ".gz"
extension("CharacterModel.3DS") = ".3DS"
extension(".desktop") = ".desktop"
extension("document") = ""
extension("document.txt_backup") = ""
extension("/etc/pam.d/login") = ""

Kotlin

// version 1.0.6

val r = Regex("[^a-zA-Z0-9]") // matches any non-alphanumeric character

fun extractFileExtension(path: String): String {
    if (path.isEmpty()) return ""
    var fileName = path.substringAfterLast('/')
    if (path == fileName) fileName = path.substringAfterLast('\\')
    val splits = fileName.split('.')
    if (splits.size == 1) return ""
    val ext = splits.last()
    return if (r.containsMatchIn(ext)) "" else "." + ext
}

fun main(args: Array<String>) {
    val paths = arrayOf(
        "http://example.com/download.tar.gz",
        "CharacterModel.3DS",
        ".desktop",
        "document",
        "document.txt_backup",
        "/etc/pam.d/login",
        "c:\\programs\\myprogs\\myprog.exe",          // using back-slash as delimiter
        "c:\\programs\\myprogs\\myprog.exe_backup"    // ditto
    )
    for (path in paths) {
        val ext =  extractFileExtension(path)
        println("${path.padEnd(37)} -> ${if (ext.isEmpty()) "(empty string)" else ext}")
    }
}
Output:
http://example.com/download.tar.gz    -> .gz
CharacterModel.3DS                    -> .3DS
.desktop                              -> .desktop
document                              -> (empty string)
document.txt_backup                   -> (empty string)
/etc/pam.d/login                      -> (empty string)
c:\programs\myprogs\myprog.exe        -> .exe
c:\programs\myprogs\myprog.exe_backup -> (empty string)

Lua

-- Lua pattern docs at http://www.lua.org/manual/5.1/manual.html#5.4.1
function fileExt (filename) return filename:match("(%.%w+)$") or "" end
 
local testCases = {
    "http://example.com/download.tar.gz",
    "CharacterModel.3DS",
    ".desktop",
    "document",
    "document.txt_backup",
    "/etc/pam.d/login"
}
for _, example in pairs(testCases) do
    print(example .. ' -> "' .. fileExt(example) .. '"')
end
Output:
http://example.com/download.tar.gz -> ".gz"
CharacterModel.3DS -> ".3DS"
.desktop -> ".desktop"
document -> ""
document.txt_backup -> ""
/etc/pam.d/login -> ""

Mathematica/Wolfram Language

FileExtension is a built-in function:

FileExtension /@ {"http://example.com/download.tar.gz", "CharacterModel.3DS", ".desktop", "document","document.txt_backup","/etc/pam.d/login"}
Output:

{"gz", "3DS", "", "", "txt_backup", ""}

Nanoquery

The File object type in Nanoquery has a built-in method to extract the file extension from a filename, but it treats all characters as potentially valid in an extension and URLs as not being. As a result, the .txt_backup extension is included in the output.

import Nanoquery.IO

filenames =  {"http://example.com/download.tar.gz", "CharacterModel.3DS"}
filenames += {".desktop", "document", "document.txt_backup", "/etc/pam.d/login"}

for fname in filenames
	println new(File, fname).getExtension()
end
Output:
.gz
.3DS
.desktop

.txt_backup

Nim

As can be seen in the examples, Nim standard library function splitFile detects that a file such as .desktop is a special file. But, on the other hand, it considers that an underscore is a valid character in an extension.

import os, strutils

func extractFileExt(path: string): string =
  var s: seq[char]
  for i in countdown(path.high, 0):
    case path[i]
    of Letters, Digits:
      s.add path[i]
    of '.':
      s.add '.'
      while s.len > 0: result.add s.pop()
      return
    else:
      break
  result = ""

for input in ["http://example.com/download.tar.gz", "CharacterModel.3DS",
              ".desktop", "document", "document.txt_backup", "/etc/pam.d/login"]:
  echo "Input: ", input
  echo "Extracted extension:    ", input.extractFileExt()
  echo "Using standard library: ", input.splitFile()[2]
  echo()
Output:
Input: http://example.com/download.tar.gz
Extracted extension:    .gz
Using standard library: .gz

Input: CharacterModel.3DS
Extracted extension:    .3DS
Using standard library: .3DS

Input: .desktop
Extracted extension:    .desktop
Using standard library: 

Input: document
Extracted extension: 
Using standard library: 

Input: document.txt_backup
Extracted extension: 
Using standard library: .txt_backup

Input: /etc/pam.d/login
Extracted extension: 
Using standard library: 

Objeck

use Query.RegEx;

class FindExtension {
  function : Main(args : String[]) ~ Nil {
    file_names := [
      "http://example.com/download.tar.gz", "CharacterModel.3DS",
      ".desktop", "document",  "document.txt_backup", "/etc/pam.d/login"];

    each(i : file_names) {
      file_name := file_names[i];
      System.IO.Console->Print(file_name)->Print(" has extension: ")->PrintLine(GetExtension(file_name));
    };
  }

  function : GetExtension(file_name : String) ~ String {
    index := file_name->FindLast('.');
    if(index < 0) {
      return "";
    };

    ext := file_name->SubString(index, file_name->Size() - index);
    if(ext->Size() < 1) {
      return "";
    };
    
    if(<>RegEx->New("\\.([a-z]|[A-Z]|[0-9])+")->MatchExact(ext)) {
      return "";
    };

    return ext;
  }
}
Output:
http://example.com/download.tar.gz has extension: .gz
CharacterModel.3DS has extension: .3DS
.desktop has extension: .desktop
document has extension:
document.txt_backup has extension:
/etc/pam.d/login has extension:

OCaml

Since OCaml 4.04 there is a function Filename.extension:

let () =
  let filenames = [
    "http://example.com/download.tar.gz";
    "CharacterModel.3DS";
    ".desktop";
    "document";
    "document.txt_backup";
    "/etc/pam.d/login"]
  in
  List.iter (fun filename ->
    Printf.printf " '%s' => '%s'\n" filename (Filename.extension filename)
  ) filenames

differs a little bit from the specification of this task.

Output:
 'http://example.com/download.tar.gz' => '.gz'
 'CharacterModel.3DS' => '.3DS'
 '.desktop' => ''
 'document' => ''
 'document.txt_backup' => '.txt_backup'
 '/etc/pam.d/login' => ''

Oforth

If extension is not valid, returns null, not "". Easy to change if "" is required.

: fileExt( s -- t ) 
| i |
   s lastIndexOf('.') dup ->i ifNull: [ null return ]
   s extract(i 1+, s size) conform(#isAlpha) ifFalse: [ null return ]
   s extract(i, s size)
;
Output:
>"http://example.com/download.tar.gz" fileExt .
.gz ok
>
ok
>"CharacterModel.3DS" fileExt .
.3DS ok
>
ok
>".desktop" fileExt .
.desktop ok
>"document" fileExt .
null ok
>"document.txt_backup" fileExt .
null ok
>"/etc/pam.d/login" fileExt .
null ok
>

Pascal

Free Pascal

Program Extract_file_extension;

{FreePascal has the built-in function ExtractFileExt which returns the file extension.
* it does need charachters before the period to return the proper extension and it returns 
* the extension including the period}

Uses character,sysutils;

Const arr : array of string = ('http://example.com/download.tar.gz','CharacterModel.3DS','.desktop',
                               'document','document.txt_backup','/etc/pam.d/login');

Function extractextension(fn: String): string;
Var 
  i: integer;
Begin
  fn := 'prefix' + fn; {add charachters before the period}
  fn := ExtractFileExt(fn);
  For i := 2 to length(fn) Do {skip the period}
    If Not IsLetterOrDigit(fn[i]) Then exit('');
 extractextension := fn;
End;

Var i : string;
Begin
  For i In arr Do
    writeln(i:35,' -> ',extractextension(i))
End.
Output:
 http://example.com/download.tar.gz -> gz
                 CharacterModel.3DS -> 3DS
                           .desktop -> desktop
                           document -> 
                document.txt_backup -> 
                   /etc/pam.d/login -> 

Perl

Translation of: Raku
sub extension {
    my $path = shift;
    $path =~ / \. [a-z0-9]+ $ /xi;
    $& // '';
}

Testing:

printf "%-35s %-11s\n", $_, "'".extension($_)."'"
for qw[
    http://example.com/download.tar.gz
    CharacterModel.3DS
    .desktop
    document
    document.txt_backup
    /etc/pam.d/login
];
Output:
http://example.com/download.tar.gz  '.gz'      
CharacterModel.3DS                  '.3DS'     
.desktop                            '.desktop' 
document                            ''         
document.txt_backup                 ''         
/etc/pam.d/login                    ''

Phix

with javascript_semantics
function getExtension(string filename)
    for i=length(filename) to 1 by -1 do
        integer ch = filename[i]
        if ch='.' then return filename[i..$] end if
        if find(ch,"\\/_") then exit end if
    end for
    return ""
end function
 
constant tests = {"mywebsite.com/picture/image.png",
                  "http://mywebsite.com/picture/image.png",
                  "myuniquefile.longextension",
                  "IAmAFileWithoutExtension",
                  "/path/to.my/file",
                  "file.odd_one",
                  "http://example.com/download.tar.gz",
                  "CharacterModel.3DS",
                  ".desktop",
                  "document",
                  "document.txt_backup",
                  "/etc/pam.d/login"}
for i=1 to length(tests) do
    printf(1,"%s ==> %s\n",{tests[i],getExtension(tests[i])})
end for
Output:
mywebsite.com/picture/image.png ==> .png
http://mywebsite.com/picture/image.png ==> .png
myuniquefile.longextension ==> .longextension
IAmAFileWithoutExtension ==>
/path/to.my/file ==>
file.odd_one ==>
http://example.com/download.tar.gz ==> .gz
CharacterModel.3DS ==> .3DS
.desktop ==> .desktop
document ==>
document.txt_backup ==>
/etc/pam.d/login ==>

The builtin get_file_extension() could also be used, however that routine differs from the task description in that "libglfw.so.3.1" => "so", and all results are lowercase even if the input is not.

PHP

$tests = [
    ['input'=>'http://example.com/download.tar.gz', 'expect'=>'.gz'],
    ['input'=>'CharacterModel.3DS', 'expect'=>'.3DS'],
    ['input'=>'.desktop', 'expect'=>'.desktop'],
    ['input'=>'document', 'expect'=>''],
    ['input'=>'document.txt_backup', 'expect'=>''],
    ['input'=>'/etc/pam.d/login', 'expect'=>'']
];

foreach ($tests as $key=>$test) {
    $ext = pathinfo($test['input'], PATHINFO_EXTENSION);
    // in php, pathinfo allows for an underscore in the file extension
    // the following if statement only allows for A-z0-9 in the extension
    if (ctype_alnum($ext)) {
        // pathinfo returns the extension without the preceeding '.' so adding it back on
        $tests[$key]['actual'] = '.'.$ext;
    } else {
        $tests[$key]['actual'] = '';
    }
}
foreach ($tests as $test) {
    printf("%35s -> %s \n", $test['input'],$test['actual']);
}
Output:
 http://example.com/download.tar.gz -> .gz 
                 CharacterModel.3DS -> .3DS 
                           .desktop -> .desktop 
                           document ->  
                document.txt_backup ->  
                   /etc/pam.d/login ->  

PicoLisp

(de extension (F)
   (and
      (fully
         '((C)
            (or
               (>= "Z" C "A")
               (>= "z" C "a")
               (>= "9" C "0") ) )
      (setq F (stem (member "." (chop F)) ".")) )
      (pack F) ) )
(println (extension "http://example.com/download.tar.gz"))
(println (extension "CharacterModel.3DS"))
(println (extension ".desktop"))
(println (extension "document"))
(println (extension "document.txt_backup"))
(println (extension "/etc/pam.d/login"))
Output:
"gz"
"3DS"
"desktop"
NIL
NIL
NIL

Plain English

The 'Extract' imperative extracts parts of a path. When extracting an extension, it starts from the last period (.) in the path string and goes until the end of the string.

To run:
Start up.
Show the file extension of "http://example.com/download.tar.gz".
Show the file extension of "CharacterModel.3DS".
Show the file extension of ".desktop".
Show the file extension of "document".
Show the file extension of "document.txt_backup".
Show the file extension of "/etc/pam.d/login".
Wait for the escape key.
Shut down.

To show the file extension of a path:
Extract an extension from the path.
Write the extension to the console.
Output:
.gz
.3DS
.desktop

.txt_backup
.d/login

PowerShell

function extension($file){
    $ext = [System.IO.Path]::GetExtension($file)
    if (-not [String]::IsNullOrEmpty($ext)) {
        if($ext.IndexOf("_") -ne -1) {$ext = ""}
    }
    $ext
}
extension "http://example.com/download.tar.gz"
extension "CharacterModel.3DS"
extension ".desktop"
extension "document"
extension "document.txt_backup"
extension "/etc/pam.d/login"

Output:

.gz
.3DS
.desktop


Python

Uses re.search.

import re
def extractExt(url):
  m = re.search(r'\.[A-Za-z0-9]+$', url)
  return m.group(0) if m else ""

and one way of allowing for OS-specific variations in the character sets permitted in file extensions is to write a general and reusable curried function, from which we can obtain simpler OS-specific functions by specialisation:

'''Obtaining OS-specific file extensions'''

import os
import re


# OS-INDEPENDENT CURRIED FUNCTION -------------------------

# takeExtension :: Regex String -> FilePath -> String
def takeExtension(charSet):
    '''The extension part (if any) of a file name.
       (Given a regex string representation of the
       character set accepted in extensions by the OS).'''
    def go(fp):
        m = re.search(
            r'\.[' + charSet + ']+$',
            (fp).split(os.sep)[-1]
        )
        return m[0] if m else ''
    return lambda fp: go(fp)


# DERIVED (OS-SPECIFIC) FUNCTIONS -------------------------


# takePosixExtension :: FilePath -> String
def takePosixExtension(fp):
    '''The file extension, if any,
       of a Posix file path.'''
    return takeExtension(r'A-Za-z0-9\-\_')(fp)


# takeWindowsExtension :: FilePath -> String
def takeWindowsExtension(fp):
    '''The file extension, if any,
       of a Windows file path.'''
    return takeExtension(r'A-Za-z0-9')(fp)


# TEST ----------------------------------------------------
def main():
    '''Tests'''

    for f in [takePosixExtension, takeWindowsExtension]:
        print(
            tabulated(f.__name__ + ' :: FilePath -> String:')(
                str
            )(str)(f)([
                "http://example.com/download.tar.gz",
                "CharacterModel.3DS",
                ".desktop",
                "document",
                "document.txt_backup",
                "/etc/pam.d/login"
            ])
        )
        print()


# GENERIC -------------------------------------------------


# tabulated :: String -> (a -> String) ->
#                        (b -> String) ->
#                        (a -> b) -> [a] -> String
def tabulated(s):
    '''Heading -> x display function -> fx display function ->
          number of columns -> f -> value list -> tabular string.'''
    def go(xShow, fxShow, f, xs):
        w = max(map(lambda x: len(xShow(x)), xs))
        return s + '\n' + '\n'.join([
            xShow(x).rjust(w, ' ') + ' -> ' + fxShow(f(x)) for x in xs
        ])
    return lambda xShow: lambda fxShow: (
        lambda f: lambda xs: go(
            xShow, fxShow, f, xs
        )
    )


# MAIN ---
if __name__ == '__main__':
    main()
Output:
takePosixExtension :: FilePath -> String:
http://example.com/download.tar.gz -> .gz
                CharacterModel.3DS -> .3DS
                          .desktop -> .desktop
                          document -> 
               document.txt_backup -> .txt_backup
                  /etc/pam.d/login -> 

takeWindowsExtension :: FilePath -> String:
http://example.com/download.tar.gz -> .gz
                CharacterModel.3DS -> .3DS
                          .desktop -> .desktop
                          document -> 
               document.txt_backup -> 
                  /etc/pam.d/login -> 

Quackery

  [ bit
    [ 0
      $ "abcdefghijklmnopqrstuvwxyz"
      $ "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
      $ "1234567890." join join
      witheach [ bit | ] ] constant 
      & 0 > ]                        is validchar ( c --> b ) 
    
  [ dup $ "" = if done
    dup -1 peek char . = iff
      [ drop $ "" ] done
    $ "" swap 
    reverse witheach 
      [ dup dip join 
        dup validchar iff
          [ char . = if 
             [ reverse conclude ] ]
        else   
          [ 2drop $ "" conclude ] ] 
    dup $ "" = if done
    dup 0 peek char . != if
      [ drop $ "" ] ]                is extension ( $ --> $ )
          
  [ cr dup echo$ say " --> "
    extension 
    dup $ "" = iff
      [ drop say "no extension" ]
    else echo$ 
    cr ]                             is task      ( $ -->   )
    
  $ "http://example.com/download.tar.gz" task
  $ "CharacterModel.3DS"                 task      
  $ ".desktop"                           task      
  $ "document"                           task      
  $ "document.txt_backup"                task      
  $ "/etc/pam.d/login"                   task
Output:
http://example.com/download.tar.gz --> .gz

CharacterModel.3DS --> .3DS

.desktop --> .desktop

document --> no extension

document.txt_backup --> no extension

/etc/pam.d/login --> no extension

Racket

#lang racket

;; Note that for a real implementation, Racket has a
;; `filename-extension` in its standard library, but don't use it here
;; since it requires a proper name (fails on ""), returns a byte-string,
;; and handles path values so might run into problems with unicode
;; string inputs.
 
(define (string-extension x)
  (cadr (regexp-match #px"(\\.[[:alnum:]]+|)$" x)))
 
(define examples '("http://example.com/download.tar.gz"
                   "CharacterModel.3DS"
                   ".desktop"
                   "document"
                   "document.txt_backup"
                   "/etc/pam.d/login"))

(for ([x (in-list examples)])
  (printf "~a | ~a\n" (~a x #:width 34) (string-extension x)))
Output:
http://example.com/download.tar.gz | .gz
CharacterModel.3DS                 | .3DS
.desktop                           | .desktop
document                           | 
document.txt_backup                | 
/etc/pam.d/login                   | 

Raku

(formerly Perl 6)

The built-in IO::Path class has an .extension method:

say $path.IO.extension;

Contrary to this task's specification, it

  • doesn't include the dot in the output
  • doesn't restrict the extension to letters and numbers.


Here's a custom implementation which does satisfy the task requirements:

sub extension (Str $path --> Str) {
    $path.match(/:i ['.' <[a..z0..9]>+]? $ /).Str
}

# Testing:

printf "%-35s %-11s %-12s\n", $_, extension($_).perl, $_.IO.extension.perl
for <
    http://example.com/download.tar.gz
    CharacterModel.3DS
    .desktop
    document
    document.txt_backup
    /etc/pam.d/login
>;
Output:
http://example.com/download.tar.gz  ".gz"       "gz"        
CharacterModel.3DS                  ".3DS"      "3DS"       
.desktop                            ".desktop"  "desktop"   
document                            ""          ""          
document.txt_backup                 ""          "txt_backup"
/etc/pam.d/login                    ""          ""

REXX

Using this paraphrased Rosetta Code task's definition that:

a legal file extension   only   consists of mixed-case Latin letters and/or decimal digits.

/*REXX pgm extracts the file extension (defined above from the RC task) from a file name*/
@.=                                              /*define default value for the @ array.*/
parse arg fID                                    /*obtain any optional arguments from CL*/
if fID\==''  then @.1 = fID                      /*use the  filename  from the C.L.     */
             else do                             /*No filename given? Then use defaults.*/
                  @.1 = 'http://example.com/download.tar.gz'
                  @.2 = 'CharacterModel.3DS'
                  @.3 = '.desktop'
                  @.4 = 'document'
                  @.5 = 'document.txt_backup'
                  @.6 = '/etc/pam.d/login'
                  end

   do j=1  while  @.j\=='';     x=               /*process  (all of)  the file name(s). */
   p=lastpos(., @.j)                             /*find the last position of a period.  */
   if p\==0  then x=substr(@.j, p+1)             /*Found a dot?  Then get stuff after it*/
   if \datatype(x, 'A')   then x=                /*Not upper/lowercase letters | digits?*/
   if x==''  then x= " [null]"                   /*use a better name for a  "null"  ext.*/
             else x= . || x                      /*prefix the extension with a  period. */
   say 'file extension='       left(x, 20)     "for file name="         @.j
   end       /*j*/                               /*stick a fork in it,  we're all done. */

output   when using the default (internal) inputs:

file extension= .gz                  for file name= http://example.com/download.tar.gz
file extension= .3DS                 for file name= CharacterModel.3DS
file extension= .desktop             for file name= .desktop
file extension=  [null]              for file name= document
file extension=  [null]              for file name= document.txt_backup
file extension=  [null]              for file name= /etc/pam.d/login

Ring

# Project : Extract file extension

test = ["http://example.com/download.tar.gz",
           "CharacterModel.3DS",
           ".desktop",
           "document",
           "document.txt_backup",
           "/etc/pam.d/login"]

for n = 1 to len(test)
    flag = 1
    revtest = revstr(test[n])
    nr = substr(revtest, ".")
    if nr > 0
       revtest2 = left(revtest, nr)
       for m = 1 to len(revtest2)
           if (ascii(revtest2[m]) > 64 and ascii(revtest2[m]) < 91) or 
              (ascii(revtest2[m]) > 96 and ascii(revtest2[m]) < 123) or
               isdigit(revtest2[m]) or revtest2[m] = "."
           else
               flag = 0
           ok
       next
    else
       flag = 0
    ok
    if flag = 1
       revtest3 = revstr(revtest2)
       see test[n] + " -> " + revtest3 + nl
    else
       see test[n] + " -> (none)" + nl
    ok    
next

func revstr(cStr)
       cStr2 = ""
       for x = len(cStr) to 1 step -1
           cStr2 += cStr[x]
       next
       return cStr2

Output:

http://example.com/download.tar.gz -> .gz
CharacterModel.3DS -> .3DS
.desktop -> .desktop
document -> (none)
document.txt_backup -> (none)
/etc/pam.d/login -> (none)

Ruby

names = 
%w(http://example.com/download.tar.gz
   CharacterModel.3DS
   .desktop
   document
   /etc/pam.d/login)
names.each{|name| p File.extname(name)}

output

".gz"
".3DS"
""
""
""

Apparently, the built-in method does not consider ".desktop" to be a file extension (on Linux).

Rust

use std::path::Path;

fn main() {
    let filenames = &[
        "http://example.com/download.tar.gz",
        "CharacterModel.3DS",
        ".desktop",
        "document",
        "document.txt_backup",
        "/etc/pam.d/login",
    ];

    for filename in filenames {
        println!(
            "{:34} | {:8} | {:?}",
            filename,
            extension(filename),
            Path::new(filename).extension()
        );
    }
}

fn extension(filename: &str) -> &str {
    filename
        .rfind('.')
        .map(|idx| &filename[idx..])
        .filter(|ext| ext.chars().skip(1).all(|c| c.is_ascii_alphanumeric()))
        .unwrap_or("")
}

The built-in method requires a filename before the extension, allows any non-period character to appear in the extension, and returns None if no extension is found.

Output:
http://example.com/download.tar.gz | .gz      | Some("gz")
CharacterModel.3DS                 | .3DS     | Some("3DS")
.desktop                           | .desktop | None
document                           |          | None
document.txt_backup                |          | Some("txt_backup")
/etc/pam.d/login                   |          | None

Scala

package rosetta

object FileExt {
  
  private val ext = """\.[A-Za-z0-9]+$""".r
   
  def isExt(fileName: String, extensions: List[String]) =
    extensions.map { _.toLowerCase }.exists { fileName.toLowerCase endsWith "." + _ }
  
  def extractExt(url: String) = ext findFirstIn url getOrElse("")
  
}

object FileExtTest extends App {
    val testExtensions: List[String] = List("zip", "rar", "7z", "gz", "archive", "A##", "tar.bz2") 
  
  val isExtTestFiles: Map[String, Boolean] = Map(
      "MyData.a##"          -> true,
      "MyData.tar.Gz"       -> true,
      "MyData.gzip"         -> false,
      "MyData.7z.backup"    -> false,
      "MyData..."           -> false,
      "MyData"              -> false,
      "MyData_v1.0.tar.bz2" -> true,
      "MyData_v1.0.bz2"     -> false
      )
      
  val extractExtTestFiles: Map[String, String] = Map(      
      "http://example.com/download.tar.gz" -> ".gz",           
      "CharacterModel.3DS"                 -> ".3DS",          
      ".desktop"                           -> ".desktop",      
      "document"                           -> "",
      "document.txt_backup"                -> "",
      "/etc/pam.d/login"                   -> "",
      "/etc/pam.d/login.a"                 -> ".a",                  
      "/etc/pam.d/login."                  -> "",              
      "picture.jpg"                        -> ".jpg",          
      "http://mywebsite.com/picture/image.png"-> ".png",       
      "myuniquefile.longextension"         -> ".longextension",
      "IAmAFileWithoutExtension"           -> "",              
      "/path/to.my/file"                   -> "",              
      "file.odd_one"                       -> "",              
      // Extra, with unicode                                   
	    "café.png"                           -> ".png",          
      "file.resumé"                        -> "",              
      // with unicode combining characters                     
	    "cafe\u0301.png"                     -> ".png",        
      "file.resume\u0301"                  -> ""
	    )
	      println("isExt() tests:")

  for ((file, isext) <- isExtTestFiles) {
    assert(FileExt.isExt(file, testExtensions) == isext, "Assertion failed for: " + file)
    println("File: " + file + " -> Extension: " + FileExt.extractExt(file))
  }
  println("\nextractExt() tests:")
  for ((url, ext) <- extractExtTestFiles) {
    assert(FileExt.extractExt(url) == ext, "Assertion failed for: " + url)
    println("Url: " + url + " -> Extension: " + FileExt.extractExt(url))
  }
}

output

Url: picture.jpg -> Extension: .jpg
Url: document.txt_backup -> Extension: 
Url: .desktop -> Extension: .desktop
Url: CharacterModel.3DS -> Extension: .3DS
Url: file.resumé -> Extension: 
Url: document -> Extension: 
Url: café.png -> Extension: .png
Url: /etc/pam.d/login. -> Extension: 
Url: http://mywebsite.com/picture/image.png -> Extension: .png
Url: IAmAFileWithoutExtension -> Extension: 
Url: /etc/pam.d/login -> Extension: 
Url: /etc/pam.d/login.a -> Extension: .a
Url: file.odd_one -> Extension: 
Url: /path/to.my/file -> Extension: 
Url: myuniquefile.longextension -> Extension: .longextension
Url: café.png -> Extension: .png
Url: file.resumé -> Extension: 
Url: http://example.com/download.tar.gz -> Extension: .gz

sed

-Ene 's:.*(\.[A-Za-z0-9]+)$:\1:p'

Example of use:

for F in "http://example.com/download.tar.gz" "CharacterModel.3DS" ".desktop" "document" "document.txt_backup" "/etc/pam.d/login"
do
  EXT=`echo $F | sed -Ene 's:.*(\.[A-Za-z0-9]+)$:\1:p'`
  echo "$F: $EXT"
done
Output:
http://example.com/download.tar.gz: .gz
CharacterModel.3DS: .3DS
.desktop: .desktop
document: 
document.txt_backup: 
/etc/pam.d/login: 

Sidef

func extension(filename) {
    filename.match(/(\.[a-z0-9]+)\z/i).to_s
}

var files = [
    'http://example.com/download.tar.gz',
    'CharacterModel.3DS',
    '.desktop',
    'document',
    'document.txt_backup',
    '/etc/pam.d/login',
]

files.each {|f|
    printf("%-36s -> %-11s\n", f.dump, extension(f).dump)
}
Output:
"http://example.com/download.tar.gz" -> ".gz"      
"CharacterModel.3DS"                 -> ".3DS"     
".desktop"                           -> ".desktop" 
"document"                           -> ""         
"document.txt_backup"                -> ""         
"/etc/pam.d/login"                   -> ""         

Smalltalk

The Filename class has a convenient suffix method for that; so we convert the string to a filename and ask it:

names := #(
    'http://example.com/download.tar.gz'
    'CharacterModel.3DS'
    '.desktop'
    'a.desktop'
    'document'
    'document.txt_backup'
    '/etc/pam.d/login'
).
names do:[:f |
    '%-35s -> %s\n' printf:{ f . f asFilename suffix } on:Stdout
]
Output:
http://example.com/download.tar.gz  -> gz
CharacterModel.3DS                  -> 3DS
.desktop                            -> 
a.desktop                           -> desktop
document                            -> 
document.txt_backup                 -> txt_backup
/etc/pam.d/login                    -> 

Note: the task's description seems wrong to me; on a Unix machine, files beginning with "." are treated as hidden files (eg. in ls) and the suffix can be considered to be empty. As opposed to "a.desktop".

SNOBOL4

* Program: extract_extension.sbl
* To run: sbl extract_extension.sbl
* Description: Extract file extension
* Comment: Tested using the Spitbol for Linux version of SNOBOL4

	filenames =
+	"http://example.com/download.tar.gz,"
+	"CharacterModel.3DS,"
+	".desktop,"
+	"document,"
+	"document.txt_backup,"
+	"/etc/pam.d/login"

	epat = ((span(&lcase &ucase '0123456789') ".") | "") . ext
p0
	filenames ? (break(',') . s ',') | (len(1) rem) . s = ""  :f(end)
	reverse(s) ? epat
	ext = reverse(ext)
	output  = ""
	output = "Extension from file '" s "' is '" ext "'"
	:(p0)
END
Output:
Extension from file 'http://example.com/download.tar.gz' is '.gz'

Extension from file 'CharacterModel.3DS' is '.3DS'

Extension from file '.desktop' is '.desktop'

Extension from file 'document' is ''

Extension from file 'document.txt_backup' is ''

Extension from file '/etc/pam.d/login' is ''

Standard ML

This just demonstrates how to functionally extend the built-in function to the alpha-numeric restriction. Since file names starting with '.' are supposed to be "hidden" files in Unix, they're not considered as an extension.

fun fileExt path : string =
  getOpt (Option.composePartial (Option.filter (CharVector.all Char.isAlphaNum), OS.Path.ext) path, "")

val tests = [
  "http://example.com/download.tar.gz",
  "CharacterModel.3DS",
  ".desktop",
  "document",
  "document.txt_backup",
  "/etc/pam.d/login"
]

val () = app (fn s => print (s ^ " -> \"" ^ fileExt s ^ "\"\n")) tests
Output:
http://example.com/download.tar.gz -> "gz"
CharacterModel.3DS -> "3DS"
.desktop -> ""
document -> ""
document.txt_backup -> ""

Tcl

Tcl's built in file extension command already almost knows how to do this, except it accepts any character after the dot. Just for fun, we'll enhance the builtin with a new subcommand with the limitation specified for this problem.

proc assert {expr} {    ;# for "static" assertions that throw nice errors
    if {![uplevel 1 [list expr $expr]]} {
        set msg "{$expr}"
        catch {append msg " {[uplevel 1 [list subst -noc $expr]]}"}
        tailcall throw {ASSERT ERROR} $msg
    }
}

proc file_ext {file} {
    set res ""
    regexp -nocase {\.[a-z0-9]+$} $file res
    return $res
}

set map [namespace ensemble configure file -map]
dict set map ext ::file_ext
namespace ensemble configure file -map $map

# and a test:
foreach {file ext} {
    http://example.com/download.tar.gz      .gz
    CharacterModel.3DS                      .3DS
    .desktop                                .desktop
    document                                ""
    document.txt_backup                     ""
    /etc/pam.d/login                        ""
} {
    set res ""
    assert {[file ext $file] eq $ext}
}

TUSCRIPT

$$ MODE DATA
$$ testcases=*
http://example.com/download.tar.gz
CharacterModel.3DS
.desktop
document
document.txt_backup
/etc/pam.d/login
picture.jpg
http://mywebsite.com/picture/image.png
myuniquefile.longextension
IamAFileWithoutExtension
path/to.my/file
file.odd_one
thisismine

$$ MODE TUSCRIPT,{}

BUILD C_GROUP A0 = *
DATA {&a}
DATA {\0}

BUILD S_TABLE legaltokens=*
DATA :.{1-00}{C:A0}{]}:

LOOP testcase=testcases
 extension=STRINGS (testcase,legaltokens,0,0)
 IF (extension=="") CYCLE
 PRINT testcase, " has extension ", extension
ENDLOOP

Output:

http://example.com/download.tar.gz has extension .gz
CharacterModel.3DS has extension .3DS
.desktop has extension .desktop
picture.jpg has extension .jpg
http://mywebsite.com/picture/image.png has extension .png
myuniquefile.longextension has extension .longextension

VBScript

Function fileExt(fname)
	Set fso = CreateObject("Scripting.FileSystemObject")
	Set regex = new regExp
        Dim ret

	regex.pattern = "^[A-Za-z0-9]+$"	'Only alphanumeric characters are allowed
	If regex.test(fso.GetExtensionName(fname)) = False Then
		ret = ""
	Else
		ret = "." & fso.GetExtensionName(fname)
	End If
	fileExt = ret
End Function
 
'Real Start of Program
arr_t = Array("http://example.com/download.tar.gz",	_
	      "CharacterModel.3DS",			_
	      ".desktop",				_
	      "document",				_
	      "document.txt_backup",			_
	      "/etc/pam.d/login")
 
For Each name In arr_t
	Wscript.Echo "NAME:",name
	Wscript.Echo " EXT:","<" & fileExt(name) & ">"
Next
Output:
NAME: http://example.com/download.tar.gz
 EXT: <.gz>
NAME: CharacterModel.3DS
 EXT: <.3DS>
NAME: .desktop
 EXT: <.desktop>
NAME: document
 EXT: <>
NAME: document.txt_backup
 EXT: <>
NAME: /etc/pam.d/login
 EXT: <>

Visual Basic

Works with: Visual Basic version 6
Option Explicit
'-----------------------------------------------------------------
Function ExtractFileExtension(ByVal Filename As String) As String
Dim i As Long
Dim s As String
  i = InStrRev(Filename, ".")
  If i Then
    If i < Len(Filename) Then
      s = Mid$(Filename, i)
      For i = 2 To Len(s)
        Select Case Mid$(s, i, 1)
        Case "A" To "Z", "a" To "z", "0" To "9"
            'these characters are OK in an extension; continue
        Case Else
            'this one is not OK in an extension
            Exit Function
        End Select
      Next i
      ExtractFileExtension = s
    End If
  End If
End Function
'-----------------------------------------------------------------
Sub Main()
Dim s As String
  s = "http://example.com/download.tar.gz"
  Debug.Assert ExtractFileExtension(s) = ".gz"
  s = "CharacterModel.3DS"
  Debug.Assert ExtractFileExtension(s) = ".3DS"
  s = ".desktop"
  Debug.Assert ExtractFileExtension(s) = ".desktop"
  s = "document"
  Debug.Assert ExtractFileExtension(s) = ""
  s = "document.txt_backup"
  Debug.Assert ExtractFileExtension(s) = ""
  s = "/etc/pam.d/login"
  Debug.Assert ExtractFileExtension(s) = ""
  s = "desktop."
  Debug.Assert ExtractFileExtension(s) = ""
  s = "a.~.c"
  Debug.Assert ExtractFileExtension(s) = ".c"
  s = "a.b.~"
  Debug.Assert ExtractFileExtension(s) = ""
  s = "a.b.1~2"
  Debug.Assert ExtractFileExtension(s) = ""
End Sub

Wren

Translation of: Kotlin
Library: Wren-pattern
Library: Wren-fmt
import "./pattern" for Pattern
import "./fmt" for Fmt

var p = Pattern.new("/W") // matches any non-alphanumeric character

var extractFileExtension = Fn.new { |path|
    if (path.isEmpty) return ""
    var fileName = path.split("/")[-1]
    if (path == fileName) fileName = path.split("\\")[-1]
    var splits = fileName.split(".")
    if (splits.count == 1) return ""
    var ext = splits[-1]
    return p.isMatch(ext) ? "" : "." + ext
}

var paths = [
    "http://example.com/download.tar.gz",
    "CharacterModel.3DS",
    ".desktop",
    "document",
    "document.txt_backup",
    "/etc/pam.d/login",
    "c:\\programs\\myprogs\\myprog.exe",          // using back-slash as delimiter
    "c:\\programs\\myprogs\\myprog.exe_backup"    // ditto
]
for (path in paths) {
    var ext =  extractFileExtension.call(path)
    Fmt.print("$-37s -> $s", path, ext.isEmpty ? "(empty string)" : ext)
}
Output:
http://example.com/download.tar.gz    -> .gz
CharacterModel.3DS                    -> .3DS
.desktop                              -> .desktop
document                              -> (empty string)
document.txt_backup                   -> (empty string)
/etc/pam.d/login                      -> (empty string)
c:\programs\myprogs\myprog.exe        -> .exe
c:\programs\myprogs\myprog.exe_backup -> (empty string)

XPL0

func Ext(Str);          \Return address of extension
char Str;  int I, C, End;
string 0;
[I:= 0;
while Str(I) do I:= I+1;
End:= I;
loop    [I:= I-1;
        if Str(I) = ^. then return @Str(I);
        if I = 0 then return @Str(End); \no dot found, return null
        C:= Str(I);
        if C>=^A & C<=^Z ! C>=^a & C<=^z ! C>=^0 & C<=^9 then \OK
        else return @Str(End);          \illegal char, return null
        ];
];

[Text(0, Ext("http://example.com/download.tar.gz"));  CrLf(0);
 Text(0, Ext("CharacterModel.3DS"));  CrLf(0);
 Text(0, Ext(".desktop"));  CrLf(0);
 Text(0, Ext("document"));  CrLf(0);
 Text(0, Ext("document.txt_backup"));  CrLf(0);
 Text(0, Ext("/etc/pam.d/login"));  CrLf(0);
]
Output:
.gz
.3DS
.desktop



zkl

The File object has a method splitFileName that does just that, returning a list of the parts. The method knows about the OS it was compiled on (Unix, Windows).

fcn extractFileExtension(name){
   var [const] valid=Walker.chain(".",["a".."z"],["A".."Z"],["0".."9")).pump(String);
   ext:=File.splitFileName(name)[-1];
   if(ext - valid) ext="";
   ext
}
foreach nm in (T("http://example.com/download.tar.gz","CharacterModel.3DS",
		  ".desktop","document",
		  "document.txt_backup","/etc/pam.d/login")){
   println("%35s : %s".fmt(nm,extractFileExtension(nm)));
}
Output:

Note: on Unix, .desktop is a hidden file, not an extension.

 http://example.com/download.tar.gz : .gz
                 CharacterModel.3DS : .3DS
                           .desktop : 
                           document : 
                document.txt_backup : 
                   /etc/pam.d/login :