$re = '/<p(?:[^>]*|\r\n|\n)>(?:.*|\r\n|\n)(<img(?:[^>]*|\r\n|\n)>)(?:.*|\r\n|\n)<\/p>/';
$str = '<p data-entity-type="" data-entity-uuid="" style="text-align: center;"><span><img alt="image.jpg" data-entity-type="" data-entity-uuid="" height="349" src="image.jpg" width="620" /><span title="Click and drag to resize">•</span></span></p>
<p><img alt="image.jpg" data-entity-type="" data-entity-uuid="" height="349" src="image.jpg" width="620" /></p>
<html>
<head></head>
<body>
some text here...
<p><img alt="image.jpg" data-entity-type="" data-entity-uuid="" height="349" src="image.jpg" width="620" />
</p>
</body>
</html>\'
// Wanted result : <html><head></head><body>some text here...<img alt="julie-bishop.jpg" data-entity-type="" data-entity-uuid="" height="349" src="/sites/default/files/inline-images/julie-bishop.jpg" width="620" /></body></html>
';
preg_match_all($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