How to check that a string is a palindrome using regular expressions?

(.)(.)21

([a-z])([a-z])[a-z]21

$a = "teststring";
while(length $a > 1)
{
   $a =~ /(.)(.*)(.)/;
   die "Not a palindrome: $a" unless $1 eq $3;
   $a = $2;
}
print "Palindrome";

/^((.)(?1)2|.?)$/

(.?)(.?)(.?)(.?)(.?).?54321

bool isPalindrome(String s1)

{

    String s2 = s1.reverse;

    return s2 == s1;
}

$re = qr/
  .                 # single letter is a palindrome
  |
  (.)               # first letter
  (??{ $re })??     # apply recursivly (not interpolated yet)
  1                # last letter
/x;

while(<>) {
    chomp;
    say if /^$re$/; # print palindromes
}

(?<N>.)+.?(?<-N>k<N>)+(?(N)(?!))

WHILE string.length > 1
    IF /(.)(.*)1/ matches string
        string = 2
    ELSE
        REJECT
ACCEPT

/(.?)(.?)(.?)(.?)(.?)(.?)(.?)(.?)(.?).?987654321/

str == str.reverse ? true : false