$file = '{include file="{$COMMON_TPL_PATH}common/header_admin.tpl"}
{include file="{$mainPage}"}
{include file="{$COMMON_TPL_PATH}common/footer_admin.tpl"}';
preg_match('/^(\{\})|^(\{\*([\S\s]*?)\*\})|^(<\?(?:php\w+|=|[a-zA-Z]+)?)|^([     ]*[
]+[  ]*)|^(\{strip\})|^(\{\/strip\})|^(\{literal\})|^(\{\s{1,}\/)|^(\{\s{1,})|^(\{\/)|^(\{)|^(([\S\s]*?)(?=([    ]*[
]+[  ]*|\{|<\?)))|^([\S\s]+)/', $file, $matches);
var_dump($matches);

Neden çıktı:

array(13) {
  [0]=>
  string(1) "{"
  [1]=>
  string(0) ""
  [2]=>
  string(0) ""
  [3]=>
  string(0) ""
  [4]=>
  string(0) ""
  [5]=>
  string(0) ""
  [6]=>
  string(0) ""
  [7]=>
  string(0) ""
  [8]=>
  string(0) ""
  [9]=>
  string(0) ""
  [10]=>
  string(0) ""
  [11]=>
  string(0) ""
  [12]=>
  string(1) "{"
}

Bu ^([\S\s]+) bütün dize maç gibi geliyor bana ..