如何获取字符串中嵌套标签的完整内容？

$str = "
{if 'a'}
    111111
    {if 'c'}33333{/if}
    {if 'd'}44444{/if}
    ......
{/if}
{if 'b'}22222{/if}
";

[
"{if 'a'}
    111111
    {if 'c'}33333{/if}
    {if 'd'}44444{/if}
    ......
{/if}",
"{if 'b'}22222{/if}"
]

$stack = [];
$top = null;
$result = [];
preg_match_all('!({/?if)!', $str, $matches, PREG_OFFSET_CAPTURE);
foreach ($matches[0] as [$match, $offset]) {
    if ($match === '{if') {
        $stack[] = $offset;
        if ($top === null) { $top = $offset; }
    } else {
        $pop = array_pop($stack);
        if ($pop === null) { throw new Exception('语法错误，存在多余的 {/if} 标签'); }
        if (empty($stack)) {
            $newOffset = $offset + strlen($match);
            $result[] = substr($str, $top, $newOffset);
            $top = $newOffset + 1;
        }
    }
}
if (!empty($stack)) { throw new Exception('语法错误，存在未闭合的 {if} 标签'); }
var_dump($result);