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!

• John Dvorak: Is I/O a part of the challenge, or a function body will do?
• Peter Taylor: The "breadcrumbs" for showing how the work is refined are available site-wide in all answers via revision history. There's no need to have a full history visible in the current version of the answer.
• Gareth: @WernerCD I'm sure the OP will change who gets the green tick when he comes back to check on the new responses.
• Gusdor: Not specifying a language really trivialises this challenge for me. As seen below, interpreted languages with a focus on high order text manipulation functions always get the shortest results. What is to stop me throwing together my own interpreter with a single function, ip(). My compete algorithm is now 'ip:i'. 4 characters. done.
• AakashM: @Gusdor see J and GolfScript suck all the enjoyment out of Code Golf and Language Handicap and related questions on this site's meta.

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

• skeevey: Looks like a mixture of Q and k :P
• tmartin: Well, I wouldn't exactly call .Q.A etc q. They're just string wrappers, there's no real k code underneath them. Now, if I said .q.inter I'd be in trouble.
• skeevey: I guess it's a case of quibbling over semantics but they are defined in q.k
• tmartin: Could just say Q for 31: {x~(|)x:(_)x inter(,/).Q`a`A`n}
• scrawl: i know this is very old but... you can save 1 byte with {#|:\_x@&x in,/.Q`a`A`n} , where 1 is true and 2 is false

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

• Gareth: Damn, you beat me to it. Though this won't work properly if there's a _ in the string will it? And it doesn't take any input so won't you need to use the -p option?
• John Dvorak: \W in regexes excludes underscore. I'm afraid you'll need [^a-z\d] in your regex. I'm afraid I'm beaten anyways.
• Howard: Save one with _|\W instead of [_\W].
• Kobi: You can shorten _|\W to \Pl: ideone.com/0ufdaQ . Should be just the Unicode letters.
• Kobi: Actually, thinking again - \Pl would not match digits, so I guess _|\W is the best you could do.

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

• Andrew Gray: Simple...but elegant!
• Cristian Lupascu: @AndrewGray thank you for the comment and for the permissive Technical Requirements. C# boilerplate adds seriously to the char count, making it impractical for golf otherwise.
• Andrew Gray: No prob. I'm a fellow C# dev, and I like writing concise, efficient code. It's a shame there's so much boilerplate in the language...

Answer 4

GolfScript, (削除) 36 34 31 (削除ここまで) 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

• Howard: You can get rid of the clean up and save two chars: {"0:A["\{>}+,,2%},
• John Dvorak: @Howard Thanks. I think I misunderstood + on blocks. That's a cool trick.
• Peter Taylor: You can save three chars by reversing the filter and the case standardisation. You have to add a{ to the filter's list of characters to include the lower-case letters, but you can then upper-case with just {32|}%. The end result is {"0:A[a{"\{>}+,,2%},{32|}%.-1%=
• Peter Taylor: There also another saving to be made on the filter. Counting modulo 2 is xor, so 0"0:A[a{"@{>^}+/ does the job.
• John Dvorak: @PeterTaylor If I don't come up with another optimisation myself soon, I probably should convert to community wiki. Thanks again :-)

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.

• Andrew Gray: Nice tidbit on concatenating a null string! Points for good infoz!
• John Dvorak: Hints: +"" casts to a string, + casts to a number, |0 and ~~ cast to an integer, !! casts to a boolean.
• SuperPrograman: Is a char shorter with regex /[^\W_]/g

Answer 6

Bash: (削除) 52 (削除ここまで) (削除) 48 (削除ここまで) 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]

• CalculatorFeline: sys.stdin.read is basically the same as raw_input
• MilkyWay90: In Python 3.8 you can shorten this to 58 bytes
• MilkyWay90: Also, your Python 2 & 3 solution is 61 bytes and 67 bytes, respectively.
• Bakuriu: @MilkyWay90 You probably missed the without counting the method signature. In 2013 it was customary to not include it. In other solution in this question in other languages they simply omitted it and assumed you have the input in an s variable. I decided to provide the complete definition but, as customary in 2013, do not include the method signature in the length
• MilkyWay90: @Bakuriu Ah, I see. I've only been here a few months, so I don't know customs from 2013

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), (削除) 102 (削除ここまで) (削除) 96 (削除ここまで) 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

• Andrew Gray: While I don't know Java (I'm a C# guy, funny enough), couldn't you save 1 character by taking out the curly braces on the if? E.G.: if(s==null) return 1==0;? Or does Java enforce curlies on if statements?
• jsedano: You are absolutely right, I took that out thank you!!... I haven't notice they were there, I guess I'm a Java developer that doesn't C#?
• manatwork: 1) Checking whether the parameter is null is a good habit, but not practiced in CodeGolf. As I can tell, nobody else did it in this question. Skip it. 2) The space between arguments helps readability, but not golfing. Remove it; 3) Instead of explicitly toLowerCase() the string use equalsIgnoreCase() later instead of equals(). This way you have to adjust the regular expression, but still 1 character shorter. pastebin.com/s7H84faj
• assylias: replaceAll("[^A-Za-z0-9]","") => replaceAll("\\W","")
• Kevin Cruijssen: I know this has been answered more than three years ago, but you can golf two more things: .toString() can be +"", and you can make it a one-lined to save another byte like this: return(s=s.replaceAll("\\W|_","")).equalsIgnoreCase(new StringBuffer(s).reverse()+""); (86 bytes)

Answer 10

Mathematica (削除) 54 (削除ここまで) 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

• CalculatorFeline: PalindromeQ is 1 byte shorter than #==Reverse@#&
• DavidC: I wondered why I had never seen it. Version 10.3!
• CalculatorFeline: PalindromeQ= #==Reverse@#&
• DavidC: Yes, I downloaded v. 10.4 to try it out.
• CalculatorFeline: That's out? OHNO

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 ((削除) 50 48 45 (削除ここまで) 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.

• John Dvorak: interesting that at just 3 characters more, my code is much more readable :-)
• skeevey: An extra char slipped in, I have removed it.

Answer 13

Ruby: (削除) 43 (削除ここまで) 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

• Howard: You can even write s.tr!('^A-Za-z0-9','').upcase!.reverse==s
• manatwork: Man, how long I combined around the use of the ! methods! You keep amazing me @Howard with your tricks.
• manatwork: Small problem though, @Howard. tr and tr! seems to behave different when they have nothing to transliterate: pastebin.com/4YThW2qN That makes the p['757'] test to crash with "NoMethodError: undefined method `upcase!' for nil:NilClass" error.
• Howard: Oops, sorry. Didn't think about that case.
• Cristian Lupascu: can be shortened further: s=s.upcase.tr('^A-Z0-9','');s==s.reverse

Answer 14

C++, (削除) 107 (削除ここまで) (miscounted), (削除) 100 (削除ここまで) (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)

• John Dvorak: If %W is anything like \W in regex, it also excludes characters like underscore. This is unwanted.
• mniip: i checked, %w doesn't include _ (%W obviously includes it, then)
• Egor Skriptunoff: Only function body is needed. s=s:lower():gsub('%W','')return s:reverse()==s 46 chars
• mniip: but what about function(s)

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

• SlimJim: I can't make this to work in ghci. import Data.Char import Control.Monad ap(==)reverse.map toLower.filter isAlphaNum neither does it with adding spaces or such, sorry I'm not too keen with Haskell :)
• Jon Purdy: @SlimJim: Oops, you need Control.Monad.Instances as well. (I’m using the Reader monad instance with ap, but that instance isn’t exported by Control.Monad.)

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

• manatwork: The parenthesis around the ternary operator's first operand are not needed. Actually neither the ternary operator is needed – just output true or false in your language's representation. And by avoiding the separate statement for the assignment to $x 1 more character can be shortened: echo($x=preg_replace('/\W/','',strtolower($c)))==strrev($x);.

Answer 19

Python 2 64 Charecters:

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

• Bakuriu: @manatwork If you replace all the identifiers with one character identifiers you obtain a 77 characters version. No idea why abhiram posted the ungolfed version though.
• manatwork: @Bakuriu, never mind, I missed one statement in the question: "Only method bodies are needed". Though lines 2 and 3 gives me only 73 characters. Regarding further length reduction, no need for raw string and the capture group, making the re.findall('[a-z0-9]+',input.lower()) part 3 characters shorter.
• abhiram: @manatwork, I edited the post and changed the variables a little. That brought it down to 66. Yes, the raw string attribute is of no use here.
• manatwork: Ok, but why you insist on the capturing group in the regular expression? Works correctly without: pastebin.com/JzpNRRZU
• abhiram: I think it caught on with me after my recent python project involving re's :) Thanks for pointing it out.

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.

• Andrew Gray: Clever and useful answer!

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, (削除) 56 (削除ここまで) (削除) 47 (削除ここまで) 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

• SpellingD: You can golf this down to 47 chars like this: ($s=$s-replace'[\W]')-eq(-join$s[$s.length..0])
• goric: @SpellingD: Nice! I'm new at code golfing, thanks for the improvement
• mazzy: you can remove the right parenthesis also: ($s=$s-replace'\W')-eq-join$s[$s.length..0].

Answer 25

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.

• MickyT: Does this ignore the punctuation, whitespace and control characters?
• user47018: My bad, this version does not. I will fix it.
• user47018: Fixed the problem.

Answer 26

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;
}

• Karl Napf: error: ISO C++ forbids declaration of ‘p’ with no type your function should have a return type.
• Titus: I bet this does not ignore case, whitespace and control characters.
• Nick Matteo: You can save a character with while(*++e); instead of while(*e)++e;. But as Titus mentioned this answer is invalid.

Answer 27

PHP, (削除) 26 84 80 78 62 (削除ここまで) 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.

• Christoph: Come on Titus... This fails the spec.
• Titus: @Christoph Yeah I sometimes should fully read that. Fixed.
• Christoph: Hm -R would cause problems with line breaks in the string to test wouldn't it? From the spec " -R <code> Run PHP <code> for every input line". Moreover <?=strrev($s=strtolower(preg_replace("#\W#","",$argn)))==$s;would be shorter.
• Titus: @Christoph Nice idea, but there is no $argn without -R.
• Titus: $argv[1]="O Genie, der Herr ehre dein Ego!"; # :D

Answer 28

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 29

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 30

PowerShell, (削除) 194 (削除ここまで) 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"

• Titus: Shouldn´t that be [^a-z0-9]?