$re = '/<div\s+class="preview item"[^>]*>\s*<img\s+[\s\S]*?src=\s?".*?([^\/]+?)"/i';
$str = '<div class="preview item"><img alt=""
sizes="(max-width: 440px) 320px"
src= "https://m.testlink.com/test/zx320y230c_4130512.jpg"
srcset= "https://m.testlink.com/test/zx320y230c_4130512.jpg 320w, https://m.testlink.com/test/zx640y460c_4130512.jpg 640w"></div>';
preg_match_all($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