SimpleMachines / SMF2.1

	// Quickly remove any tags which are back to back.

	$backToBackPattern = '~\\[(' . implode('|', array_diff(array_keys($valid_tags), array('td', 'anchor'))) . ')[^<>\\[\\]]*\\]\s*\\[/\\1\\]~';

	$lastlen = 0;

	while (strlen($text) !== $lastlen)

		$lastlen = strlen($text = preg_replace($backToBackPattern, '', $text));

	// Need to sort the tags by name length.

	uksort($valid_tags, function ($a, $b) {

	// Final clean up of back to back tags.

	$lastlen = 0;

	while (strlen($text) !== $lastlen)

		$lastlen = strlen($text = preg_replace($backToBackPattern, '', $text));

	return $text;

}