$re = '/(?<=<w-block-content[\s\S]*?<span[\s\S]*?<div[\s\S]*?>)[\s\S]*?(?=<\/w-block-content>)/';
$str = '
<w-block-content id="A">
CONTENT OF #A
<span id="B">
CONTENT OF #B
<div id="C">
<p>CONTENT OF #C</p>
</div>
CONTENT OF #B
</span>
CONTENT OF #A
</w-block-content>
';
preg_match_all($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