language: PHP (php 5.4.4)
date: 978 days 13 hours ago
link:
visibility: private 
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<?php
 
$palindrome = '/(?x)
^
  (?:
      (.) (?=
              .*
              (
                \1
                (?(2) \2 | )
              )
              $
          )
  )*
  .?
  \2?
$
 
/';
 
$tests = array(
  '',
  'a',
  'aa',
  'aaa',
  'aba',
  'aaaa',
  'abba',
  'aaaaa',
  'abcba',
  'ababa',
  'aab',
  'abab',
  'xyz',
);
 
foreach ($tests as $test) {
  echo sprintf("%s '%s'\n", preg_match($palindrome, $test), $test);  
}
 
# ------------ extra snippet for the recursive pattern in man page
 
$man_palindrome = '/^(?:((.)(?1)\2|)|((.)(?3)\4|.))$/';
 
echo "man page recursive palindrome test result:\n";
foreach ($tests as $test) {
  echo sprintf("%s '%s'\n", preg_match($man_palindrome, $test), $test);  
}
 
?>