palindrome using regular expressions

szabgab has asked for the wisdom of the Perl Monks concerning the following question:

I am trying to understand how Regexp::Common finds palindromes. I took out the code from the module that looks like this:

use re 'eval';
my $ch = '[a-zA-Z]';
my $palindrome;
my $r   = "(??{\$palindrome})";
$palindrome = qr/$ch|($ch)($r)?\1/;

while (<>) {
    print if /^$palindrome$/;
}
[download]

Would someone care to exmplain how it works? Are there other solutions not using (??{ code }) ?

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