Shortest code to determine if a string is a palindrome

Question

A palindrome is some string that is spelled the same way both backwards and forwards. For instance, 'Eva, can I stab bats in a cave?' is a palindrome (EVACANISTAB | BATSINACAVE)

For this code golf, using the language of your choice, determine if a given string is a palindrome or not.

Edge Cases:

• Punctuation is not counted towards palindrominess.
• Control characters are not counted towards palindrominess.
• Whitespace is not counted towards palindrominess.
• Numbers are counted towards palindrominess.
• Case in this challenge is not counted towards palindrominess.
• There is no limit to the length of strings to evaluate, except for what is imposed by your language of choice.
• For this challenge, limit yourself to the ASCII character set.

Technical Requirements:

• Only method bodies are needed; additional things like method signatures, data structure declarations, etc. do not count towards the victory requirement.
• Code must compile or interpret without compile-time errors or exceptions.
• Code must not throw any unhandled exceptions or crash. (Almost needless to say. Almost.)
• Code must return some value indicating palindrominess. The data type depends on the language you use (e.g. a C# user might use a bool, whereas a JavaScript user could use a var.)
• You may not write your own interpreter that performs this task as a 'native' capability so you can 'golf' a near-winning score. (Hopefully needless to say.)

Victory Condition:

• The shortest code in characters wins.

Current Leader: tmartin (k, 25 characters)

...Want the magic green check mark by your answer? Beat this guy's answer!

Answer 1

K, 25

{x~|x:_x@&x in,/.Q`a`A`n}

.

k){x~|x:_x@&x in,/.Q`a`A`n}"Eva, can I stab bats in a cave?"
1b

Answer 2

Perl, 26 char

s/_|\W//g;uc eq reverse uc

Evaluates to 1 when $_ is a palindrome, "" (one of Perl's false values) when it is not.

Sample usage:

sub palin {
    s/_|\W//g;uc eq reverse uc
}
while (<DATA>) {
    chomp;
    print "$_ => ",palin()?"yes":"no","\n";
}
__DATA__
Eva, can I stab bats in a cave?
A man, a plan, a canal. Panama!
Madam, I'm Adam Carolla.
757
Boeing 757
A man, a plan, a big shovel, a canal. Panama!
A man, a plan, a canoe, pasta, heros, rajahs, a coloratura, maps, snipe, percale, macaroni, a gag, a banana bag, a tan, a tag, a banana bag again (or a camel), a crepe, pins, Spam, a rut, a Rolo, cash, a jar, sore hats, a peon, a canal >> __Panama__

output:

Eva, can I stab bats in a cave? => yes
A man, a plan, a canal. Panama! => yes
Madam, I'm Adam Carolla. => no
757 => yes
Boeing 757 => no
A man, a plan, a big shovel, a canal. Panama! => no
A man, a plan, a canoe, pasta, heros, rajahs, a coloratura, maps, snipe, percale, macaroni, a gag, a banana bag, a tan, a tag, a banana bag again (or a camel), a crepe, pins, Spam, a rut, a Rolo, cash, a jar, sore hats, a peon, a canal >> __Panama__ => yes

Answer 3

C# 82 only :)

var x=s.ToLower().Where(char.IsLetterOrDigit);return x.SequenceEqual(x.Reverse());

Couldn't resist the temptation of writing a boilerplate-free program in my favorite language.

A test is available here: http://ideone.com/8bwz7z

Answer 4

GolfScript, 36 34 31 30 characters

{0"0:A[a{"@{>^}+/},{32|}%.-1%=

Similar algorithm to my previous (Javascript) solution.

0"0:A[a{"@{>^}+/ -- Optimised by Peter Taylor and Howard. My version was "/9@Z"{1$<},,2%\;. Howard donated function concatenation and Peter Taylor donated XOR for modulo-2. It's basically a generic method of comparing if the value is in a sequence of ranges.

{.96>32*-}% (11 characters) is not really an improvement over Javascript's .toUpperCase() (14 characters), especially since it mangles some weird punctuation that follows z in the ASCII table (which doesn't matter here).

as Peter Taylor's suggested, however, if we filter out alphanumerics first, we can convert to lowercase and digits just by setting one bit in each character: {32|}

.-1%= does all the palindromic heavy lifting. One part I'm not really fond of is how long it took me to find out how to reverse an array. I should have read the docs. The other two characters perform stack management and comparison.

Test: http://golfscript.apphb.com/?c=IkV2YSwgY2FuIEkgc3RhYiBiYXRzIGluIGEgY2F2ZT8iCgp7IjA6QVtheyJcez59KywsMiV9LHszMnx9JS4tMSU9

---

Further, if I can assume that none of the following control characters are present: (Data link escape, device control 1-4, negative acknowledge, synchronous idle, end of transmission block, cancel, end of medium) (we all agree these are all pretty obscure) or if I can treat them as uppercase versions of the digits 0-9, we can save another two characters:

GolfScript, 28 characters

{32|}%{0"0:a{"@{>^}+/},.-1%=

Test: http://golfscript.apphb.com/?c=IkV2YSwgY2FuIEkgc3RhYiBiYXRzIGluIGEgY2F2ZT8iCgp7MzJ8fSV7MCIwOmF7IkB7Pl59Ky99LC4tMSU9

Answer 5

Javascript, 53 characters:

(x=x.toLowerCase().match(/[a-z\d]/g))+""==x.reverse()

is a javascript expression that evaluates to true if x is a palindrome, to false if it isn't. It assumes x is a string. If that's not guaranteed, prepend x+="",

Here's a breadcrumb: Due to how reverse() works,

(x=x.toLowerCase().match(/[a-z\d]/g))==""+x.reverse()

fails. However,

""+(x=x.toLowerCase().match(/[a-z\d]/g))==x.reverse()

is perfectly fine.

Answer 6

Bash: 52 48 46 characters

s=${1,,};s=${s//[^a-z0-9]};[ $s = `rev<<<$s` ]

This takes the sting to check as first parameter and sets the exit code to 0 for palindrome and 1 for not.

Sample run:

bash-4.2$ p() { s=${1,,};s=${s//[^a-z0-9]};[ $s = `rev<<<$s` ]; }

bash-4.2$ p 'Eva, can I stab bats in a cave?'; echo $?
0

bash-4.2$ p 'A man, a plan, a canal. Panama!'; echo $?
0

bash-4.2$ p "Madam, I'm Adam Corolla."; echo $?
1

bash-4.2$ p '757'; echo $?
0

bash-4.2$ p 'Boeing 757'; echo $?
1

bash-4.2$ p 'A man, a plan, a shovel, a canal. Panama!'; echo $?
1

bash-4.2$ p 'A_man,_a_plan, a_caremer, a canal:_Panama!'; echo $?
0

Answer 7

Python 2: 49 (without counting the method signature)

def f(s):
 s=filter(str.isalnum,s.upper())
 return s==s[::-1]

A complete program, with input and output can be writte in 74 characters.

import sys
s=filter(str.isalnum,sys.stdin.read().upper())
print s==s[::-1]

Example usage:

$echo 'Eva,can I stab bats in a cave?' | python palindrome.py
True 
$ cat huge_palindrome.txt | python palindrome.py
True
$echo 'Able was I ere i SaW elBa' | python palindrome.py                                                                   
True                                         

(huge_palindrome.txt contains this 17,826 word palindrome)

This solution can be adapted to python 3 adding some characters:

Python 3: 55

def f(s):
 s=list(filter(str.isalnum,s.upper()))
 return s==s[::-1]

Answer 8

R: 66

w=grep("[a-z0-9]",strsplit(tolower(s),"")[[1]],v=T);all(w==rev(w))

Usage:

f=function(s){w=grep("[a-z0-9]",strsplit(tolower(s),"")[[1]],v=T);all(w==rev(w))}

f("Eva, can I stab bats in a cave?")
[1] TRUE

Answer 9

JAVA (or the most verbose language ever), 102 96 95 char

s=s.replaceAll("\\W|_","");return s.equalsIgnoreCase(new StringBuffer(s).reverse().toString());

Usage (with ungolfed code):

static boolean q(String s) {
    s=s.replaceAll("\\W|_","");
    return s.equalsIgnoreCase(new StringBuffer(s).reverse().toString());
}

public static void main(String[] args) {
    System.out.println(q("'A man, a plan, a canal - Panama!'"));
}

Shortened with the help of the commenter below

Answer 10

Mathematica 54 53

One byte saved thanks to CatsAreFluffy:

PalindromeQ@StringCases[ToUpperCase@#,WordCharacter]&

For those with version 10.2 or earlier:

#==Reverse@#&@StringCases[ToUpperCase@#,WordCharacter]&

Example

PalindromeQ@StringCases[ToUpperCase@#, WordCharacter]&["Eva,can I stab bats in a cave?"]

True

Answer 11

J, 30 characters

*/(=|.)tolower(#~'[^_\W]'rxE])

Usage:

   */(=|.)tolower(#~'[^_\W]'rxE])'A man, a plan, a canal - Panama!'
1
   */(=|.)tolower(#~'[^_\W]'rxE])'Doc, note: I dissent. A fast never prevents a fatness. I diet on cod'
1

Answer 12

k (50 48 45 38 chars)

Suppresses all errors, returning a default of 0b (false).

{X~|X@:&(X:_:x)in 10h$(48+!10),97+!26}

Example:

k){X~|X@:&(X:_:x)in 10h$(48+!10),97+!26} "Eva, can I stab bats in a cave?"
1b

edit: shaved three more character by avoiding intermediate variable. H/T, CS. -7: No need to suppress errors.

Answer 13

Ruby: 43 38 characters

s=s.upcase.tr'^A-Z0-9','';s==s.reverse

Sample run:

irb(main):001:0> p=->s{s=s.upcase.tr'^A-Z0-9','';s==s.reverse}
=> #<Proc:0x854592c@(irb):1 (lambda)>

irb(main):002:0> p['Eva, can I stab bats in a cave?']
=> true

irb(main):003:0> p['A man, a plan, a canal. Panama!']
=> true

irb(main):004:0> p["Madam, I'm Adam Corolla."]
=> false

irb(main):005:0> p['757']
=> true

irb(main):006:0> p['Boeing 757']
=> false

irb(main):007:0> p['A man, a plan, a shovel, a canal. Panama!']
=> false

irb(main):009:0> p['A_man,_a_plan, a_caremer, a canal:_Panama!']
=> true

Answer 14

C++, 107 (miscounted), 100 (miscounted), 81

string s;for(int c:t)if(isalnum(c))s+=c|32;return s==string(s.rbegin(),s.rend());

• Exploits ASCII bit patterns.
• Relies on an evil using namespace std;.
• Uses bitwise AND and OR instead of the logical operators.

• Uses int because it's shorter than char or auto.

  #include <string>
  using namespace std;
  
  bool IsPalindrome(const string & t) {
  string s;for(int c:t)if(isalnum(c))s+=c|32;return s==string(s.rbegin(),s.rend());
  }
  
  #include <cassert>
  
  int main() {
      assert(!IsPalindrome("gorilla"));  // simple failure
      assert( IsPalindrome("racecar"));  // simple success
      assert( IsPalindrome("Hannah"));   // case blind
      assert(!IsPalindrome("1999"));     // digit failure
      assert( IsPalindrome("2002"));     // digit success
      // Ignore spacing, punctuation, and case:
      assert( IsPalindrome(" \t09AZ/:@[`{za90"));  // boundaries
      assert( IsPalindrome("A man, a plan, a canal: Panama."));
      assert( IsPalindrome("Eva, can I stab bats in a cave?"));
      assert( IsPalindrome(""));  // empty string
      return 0;
  }
  

Answer 15

Lua, 56

a=io.read"*l":lower():gsub("%W","")print(a:reverse()==a)

Answer 16

Python 3/2 59 chars:

def pld(i):
   p=[c for c in i.lower() if c.isalnum()]
   return(p == p[::-1])

Answer 17

Haskell, 43

Using the standard libraries Control.Monad, Control.Monad.Instances, and Data.Char:

ap(==)reverse.map toLower.filter isAlphaNum

Answer 18

PHP 60 characters.

First try on codegolf.

//thank you manatwork
echo($x=preg_replace('/\W/','',strtolower($c)))==strrev($x); 

Example:

$c='Eva, can I stab bats in a cave?';
echo($x=preg_replace('/\W/','',strtolower($c)))==strrev($x);
//prints 1

Answer 19

Python 2 64 Charecters:

i =''.join(re.findall('[a-z0-9]+',i.lower()))
return i==i[::-1]

Answer 20

Haskell 48

(\x->x==reverse x).map toLower.filter isAlphaNum

used like this:

(\x->x==reverse x).map toLower.filter isAlphaNum$"Eva, can I stab bats in a cave?"

Answer 21

Smalltalk, Squeak/Pharo flavour
116 chars using traditional formatting with tabs

You add two methods to String:

selffles
    ^self = self reverse
isPalindrome
    ^(self asUppercase asDecomposedUnicode select: #isAlphaNumeric) selffles

We could of course eliminate some spaces, or use shorter method names, but let's not betray the spirit of Smalltalk.

More over, this will handle French palindromes, like in http://fr.wikipedia.org/wiki/Liste_de_palindromes_fran%C3%A7ais, not many answers in this page can.

['Léon a trop par rapport à Noël' isPalindrome] assert.

Answer 22

Python 3 (51 char)

and may be Python 2

based on abhiram solution (with more agressive golfing)

from re import findall

def palindrome(i):
 i=findall('[a-z\d]',i.lower())
 return i==i[::-1]

print(palindrome(input('Phrase: ')))

may be shortened to 46 chars, using RE '\w'

and variant with extremely shortened function body (27 chars)

import re
l=str.lower
f=re.compile('[a-z\d]').findall

def palindrome(i):i=f(l(i));return i==i[::-1]

print(palindrome(input('Phrase: ')))

Answer 23

Julia 1.0, 63 bytes

~s=((x=filter(in(['0':'9';'A':'Z']),uppercase(s)))==reverse(x))

Try it online!

Since punctuation is filtered out, "5,000 0.005" is considered a palindrome.

Answer 24

Windows PowerShell, 56 47 45 chars

Updated (see comments), and can remove the brackets around the regex:

($s=$s-replace'\W')-eq(-join$s[$s.length..0])

Original (56)

$s=$s-replace'[^\w]','';$s-eq($s[-1..-$s.length]-join'')

Original Un-golfed:

$s = "Eva, can I stab bats in a cave?"
$s = $s -replace '[^\w]', ''
$rev = $s[-1..-$s.length] -join ''
$s -eq $rev

Answer 25

C++, 74 bytes

This code is actually really elegant, and easy to understand (when formatted correctly). I don't believe it's possible to get any shorter in C++, and it doesn't use any standard library functions.

p(auto c){auto e=c;while(*e)++e;--e;while(*e==*c&e>c)--e,++c;return e<=c;}

Example usage:

p("Hello"); //Outputs 0
p(""); //Outputs 1
p("a"); //Outputs 1
p("HellolleH"); //Outputs 1

Nicely formatted version:

p(auto c)
{
    auto e=c;
    while(*e) ++e;
    --e;
    while(*e==*c & e>c)--e,++c;
    return e<=c;
}

Answer 26

PHP, 26 84 80 78 62 63 bytes

<?=strrev($s=strtolower(preg_replace("#\W#","",$argv[1])))==$s;

takes input from first command line argument; prints 1 for truthy, empty string for falsy.

---

I18n is a littly expansive, as there is no multibyte alternative for strrev (110 bytes; run with -r):

preg_match_all("#.#us",$s=strtolower(preg_replace("#\W#u","",$argv[1])),$m);echo$s==join(array_reverse($m[0]);

utf8_strrev blatantly stolen from the PHP manual. You might also want to take a look at this blog post.

Answer 27

K (ngn/k), 18 bytes

#|:\(2!"0:a{"')__:

Try it online!

Uses @scrawl's suggestion here to return 1 for palindromes and 2 for non-palindromes. If 1 and 0 are required, the solution can be adjusted by prefixing 1=.

• _: lowercase the (implicit) input
• (...)_ filter out values where the code in (...) returns a non-0 value
  • 2!"0:a{"' do a bins lookup, then mod the result by two; returns 0 if the input is within "a"-"z" or "0"-"9" and 1 otherwise
• |:\ set up a converge-scan, reversing the input until the original input is returned. on palindromes, returns a one-item list; on non-palindromes, a two-item list
• # take the count of the result from above. As noted, if a strict 1 and 0 output are required for palindrome/non-palindrome, prefix 1= for two bytes

Answer 28

Ruby, 48

p((g=gets.upcase.gsub /[^A-Z\d]/,'')==g.reverse)

Quite simple, and hastily made so not golfed too much. I shall golf it more later.

Answer 29

Pylongolf2, 24 bytes

c╨2"[^a-zA-Z]"-_╨1=~

c takes the input, ╨2 to convert to lower case.
I then push a regex onto the stack and use - to remove all non-alphabetical characters in the input.
_ duplicates the input.
╨1 reverses it and = then compares them.
~ prints the stack in the end which prints either true or false.

Answer 30

PowerShell, 194 190 bytes

A recursive implementation to show how an unnamed PowerShell scriptblock can call itself.

$P={param([string]$s)$s=($s-replace'[^a-z]').tolower();if(!$s){return $true};if($s.length-lt4){return $s[0]-eq$s[-1]};$s[0]-eq$s[-1]-and(&$MyInvocation.MyCommand.ScriptBlock $s.trim($s[0]))}

ungolfed:

$P={
param([string]$s)
$s=($s-replace'[^a-z]').tolower();
if(!$s){return $true};
if($s.length-lt4){return $s[0]-eq$s[-1]};
$s[0]-eq$s[-1]-and(&$MyInvocation.MyCommand.ScriptBlock $s.trim($s[0]))
}

tests:

&$P "Eva, can I stab bats in a cave?"
&$P "Eva, can I stab cats in a cave?"
&$P "A man, a plan, a canal. Panama!"
&$P "A man, a plan, a big shovel, a canal. Panama!"
&$P "Madam, I'm Adam."
&$P "Madam, I'm Adam Corolla."
&$P "757"
&$P "Boeing 757"