$re = '/\< *[img][^\\\\\\\\>]*[src] *= *[\\\\"\\\\\']{0,1}([^\\\\"\\\\\'\\\\ >]*)/';
$str = '<img src="//cdn2.jianshu.io/assets/web/logo-58fd04f6f0de908401aa561cda6a0688.png" alt="Logo">';
preg_match($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