preg_replace_callback question

I read this php code in site php.net

my question How can I know deep level ?

PHP Code:

---

plain 
[indent]  >> DEB LEVEL=1

deep 
               [indent] >> >> DEB LEVEL=2

             deeper 

[/indent]deep 

[/indent]plain 


---

PHP Code:

---

<?php
$input = "plain [indent] deep [indent] deeper[/indent]deep[/indent]plain";

function parseTagsRecursive($input)
{

    $regex = '#\[indent]((?:[^[]|\[(?!/?indent])|(?R))+)\[/indent]#';

    if (is_array($input)) {
        $input = '<div style="margin-left: 10px">'.$input[1].'</div>';
    }

    return preg_replace_callback($regex, 'parseTagsRecursive', $input);
}

$output = parseTagsRecursive($input);

echo $output;
?>

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