How does this PCRE pattern detect palindromes?

_{This question is an educational demonstration of the usage of lookahead, nested reference, and conditionals in a PCRE pattern to match ALL palindromes, including the ones that can't be matched by the recursive pattern given in the PCRE man page.}

Examine this PCRE pattern in PHP snippet:

$palindrome = '/(?x)
^
  (?:
      (.) (?=
              .*
              (
                \1
                (?(2) \2 | )
              )
              $
          )
  )*
  .?
  \2?
$


/';

This pattern seems to detect palindromes, as seen in this test cases (see also on ideone.com):

$tests = array(
  # palindromes
  '',
  'a',
  'aa',
  'aaa',
  'aba',
  'aaaa',
  'abba',
  'aaaaa',
  'abcba',
  'ababa',

  # non-palindromes
  'aab',
  'abab',
  'xyz',
);

foreach ($tests as $test) {
  echo sprintf("%s '%s'\n", preg_match($palindrome, $test), $test);  
}

So how does this pattern work?

Notes

This pattern uses a nested reference, which is a similar technique used in How does this Java regex detect palindromes?, but unlike that Java pattern, there's no lookbehind (but it does use a conditional).

Also, note that the PCRE man page presents a recursive pattern to match some palindromes:

# the recursive pattern to detect some palindromes from PCRE man page
^(?:((.)(?1)\2|)|((.)(?3)\4|.))$

The man page warns that this recursive pattern can NOT detect all palindromes (see: Why will this recursive regex only match when a character repeats 2ⁿ - 1 times? and also on ideone.com), but the nested reference/positive lookahead pattern presented in this question can.