$re = '/(?i)<img[^>]+? src\s*?=\s*?"(.*?\?contextId.*?)"[^\/]+?\/>/m';
$str = '<img xmlns="http://www.w3.org/1999/xhtml" src="http://10.206.38.34:8080/resource/resources/aktuelle_Ressourcen/eService/Bilder/uebersicht-router.png?contextId=eservice_1.0.0.qualifier" alt="Bild Übersicht Router" title="Übersicht Router" />
<p> sdfs </p>
<img xmlns="http://www.w3.org/1999/xhtml" src="http://localhost:8080/resource/carl/eService/rain/apple.png?contextId=eservice_aaa" alt="Communicator" title="Super Robot" />
';
preg_match_all($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