$re = '/\!\[.+?\]\((.+?)\)/m';
$str = '![Screenshot_20210618-090636](/uploads/7afe1f092f836d61a5fa008cf9a0c991/Screenshot_20210618-090636.jpg)
![screenshot](uploads/9d461c7a3fd8cffb4c7a84c1948f345f/bootloader.jpg)';
$subst = "<img src=\"$1\" width=\"300\">";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
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