$re = '/<img.*?src="(.*?)"/';
$str = '<img size="6995" id="img570172" tabindex="0" data-outlook-trace="F:1|T:1" src="data:image/jpeg;base64,iVBORw0KGgoAAAANSUhEUgAAAQMAAADCCAMAAAB6zFdcAAACQFBMVEX///9NTU///v9CQkTzcCD/6wBKSkxFRUeEKZBAQEL/7gD+6AD///2HKJD/5QD/8QBJL5J2K5C1tbZ/KpFhLZFYLpBoLJCrKlE7Oz0Ae77r6+z6pxmsrK3+4QD/2QD/0QCoKVLi4uOdnZ4AfrukK1b9tRXzaQAAh7H"> <img size="6995" id="img570172" tabindex="0" data-outlook-trace="F:1|T:1" src="data:image/jpeg;base64,11111iVBORw0KGgoAAAANSUhEUgAAAQMAAADCCAMAAAB6zFdcAAACQFBMVEX///9NTU///v9CQkTzcCD/6wBKSkxFRUeEKZBAQEL/7gD+6AD///2HKJD/5QD/8QBJL5J2K5C1tbZ/KpFhLZFYLpBoLJCrKlE7Oz0Ae77r6+z6pxmsrK3+4QD/2QD/0QCoKVLi4uOdnZ4AfrukK1b9tRXzaQAAh7H"> <img size="6995" id="img570172" tabindex="0" data-outlook-trace="F:1|T:1" src="data:image/jpeg;base64,22222iVBORw0KGgoAAAANSUhEUgAAAQMAAADCCAMAAAB6zFdcAAACQFBMVEX///9NTU///v9CQkTzcCD/6wBKSkxFRUeEKZBAQEL/7gD+6AD///2HKJD/5QD/8QBJL5J2K5C1tbZ/KpFhLZFYLpBoLJCrKlE7Oz0Ae77r6+z6pxmsrK3+4QD/2QD/0QCoKVLi4uOdnZ4AfrukK1b9tRXzaQAAh7H">';
preg_match_all($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