$re = '/(?<=https:\/\/strm.yandex.ru\/kal\/)(.*?)(?=\/)/';
$str = '
"content_url": "https://strm.yandex.ru/kal/probusiness_cv_supres/probusiness_cv_supres0.m3u8?from=unknown&partner_id=188262&target_ref=https%3A//yastatic.net&uuid=42b7c9a63d2c656ab5dc8c5ec0934d6f&video_category_id=1017';
preg_match($re, $str, $matches, PREG_OFFSET_CAPTURE, 0);
// Print the entire match result
var_dump($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