$re = '/<img.*src="([^"]+)/';
$str = '<div dir="ltr" style="text-align: left;" trbidi="on">
<div style="text-align: center;">
<img border="0" height="355" src="http://i.imgur.com/8OplH.png" width="728">
</div>';
preg_match_all($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