Header("Content-type: text/plain");
$flags = array(
  "PREG_PATTERN_ORDER",
  "PREG_SET_ORDER",
  "PREG_SET_ORDER|PREG_OFFSET_CAPTURE",
);
$re   = '|<(\w+).*?>(.*?)</\1>|s';
$text = "<b>текст</b>  и еще <i>другой текст</i>";
echo "Строка: $text\n";
echo "Выражение: $re\n\n";
foreach ($flags as $name) {
  preg_match_all($re, $text, $pockets, eval("return $name;"));
  echo "Флаг $name:\n";
  print_r($pockets);
  echo "\n";
}