$re = '/<p>DETAILS[\s\S]*|(<p>.*?<\/p>)|.*/';
$str = '<info>
<owner>
<p>Owners:</p>
<p>1. New Owner_1</p>
<p>2. New Owner_2</p>
</owner>
<addons>
<p>Name of dog: Alex</p>
<p>1. Text blah blah blah</p>
<p>2. Text blah blah blah</p>
<p>3. Text blah blah blah</p>
<p>4. Text blah blah blah</p>
<p>OR MORE Text blah blah blah</p>
</addons>
<p>DETAILS</p>
<p>1. Vicky Mears 1st dog\'s owner.</p>
<p>2. Paul Nash 2nd dog\'s owner.</p>
<p>3. Dog found last Apr. 2016</p>
</info>';
preg_match_all($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