$re = '/<h2\s*>\s*([\S\s]*?)\s*</h2\s*>\s*([\S\s]*?)(?=\s*<h2\s*>|\s*$)/';
$str = '<h2>heading one</h2>
<p>different content in here <a>test</a> <b>test</b></p>
<p>different content in here <a>test</a> <b>test</b></p>
<h2>heading two</h2>
<p>different content in here <a>test</a> <b>test</b></p>
<h2>heading three</h2>
<p>different content in here <a>test</a> <b>test</b></p>
<p>different content in here <a>test</a> <b>test</b></p>
<p>different content in here <a>test</a> <b>test</b></p>
';
preg_match_all($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