$re = '/(?<=<img)([^>]*)([0-9]+.imgdomain\.com)[^>]*?>/';
$str = '<div class="separator" style="clear: both; text-align: center;">
<img src="/demodomain.com/-13ucJuEQEUw/linktoimg.png"><a href="https://12.imgdomain.com/-13ucJuEQEUw/linktoimg.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="618" data-original-width="1062" height="372" src="https://21.imgdomain.com/-13ucJuEQEUw/WsGsjY2E2bI/-13ucJuEQEUw/linktoimg.jpg" width="640"></a></div>
<div class="separator" style="clear: both; text-align: center;"><img src="/demodomain.com/-13ucJuEQEUw/linktoimg.png"><a href="https://12.imgdomain.com/-13ucJuEQEUw/linktoimg.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="618" data-original-width="1062" height="372" src="https://21.imgdomain.com/-13ucJuEQEUw/WsGsjY2E2bI/-13ucJuEQEUw/linktoimg.jpg" width="640"></a></div>';
preg_match_all($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