All right, here's my attempt:
UPDATE: fixed it to account for "aba" palindromes, thanks BrowserUk:
UPDATE2: fixed again, thanks ccn:
#!/usr/bin/perl -w
use strict;
my $input = <STDIN>;
chomp($input);
my $longest_palindrome = '';
# look for two occurrences of the same character back to back or with
+another
# character in-between them to find palindromes:
while ($input =~ /((.).?\2)/g)
{
# the first character beyond the three or two character middle of
+a
# palindrome:
my $match_position = pos($input);
# get the positions of the two matching characters:
my $left_pos = $match_position - length $1;
my $right_pos = $match_position - 1;
# now go looking to the left and right of each matching character
# for more matching characters:
while (nextCharactersMatch($input, $left_pos, $right_pos))
{
last if ($left_pos <= 0 or $right_pos >= length $input);
$left_pos--;
$right_pos++;
}
# extract the palindrome:
my $offset = $left_pos;
my $length = ($right_pos - $left_pos) + 1;
my $palindrome = substr($input, $offset, $length);
$longest_palindrome = $palindrome
if ($length > length $longest_palindrome);
# backtrack, to find palindromes within this palindrome:
pos($input) -= (length($1) - 1);
}
print $longest_palindrome;
sub nextCharactersMatch
{
my ($input, $left_pos, $right_pos) = @_;
return 1 if (substr($input, $left_pos - 1, 1)
eq substr($input, $right_pos + 1, 1));
}