$re = '/(?(DEFINE)
(?P<list>(?:apple|banana|orange);)
)
^(?!(?P>list)*?((?P>list))(?P>list)*?\2)
(?P>list)+$/mx';
$str = 'banana;apple;
banana;apple;orange;
banana;apple;apple;
banana;apple;orange;banana;apple;orange;';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php