$re = '/<div\sclass=\"sg-text\">(.*?)<\/div>/ims';
$str = '<div class="brn-answer__content sg-content-box__content">
<div class="sg-content-box sg-content-box--spaced-bottom">
<div class="brn-answer__text">
<div class="sg-text">
Тут немного текста
</div>
</div>
<div class="brn-attachments brn-attachments--spaced-top">
<a href="https://site.ru/files/de1/b96e8076bba5b1c3bbd39a53d301c8a5.jpg" class="brn-attachments__attachment" target="_blank">
<div class="sg-box sg-box--image-wrapper ">
<img class="sg-box__image" src="https://site.ru/files/de1/b96e8076bba5b1c3bbd39a53d301c8a5.jpg" title="Photo" alt="">
</div>';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php