$re = '/<(span).*?class="\s*(?:.*\s)?wpcf7[^\>]*>(.*)<\/\1>/is';
$str = '<span class="wpcf7-form-control wpcf7-acceptance">
<span class="wpcf7-list-item">
<input type="checkbox" name="our-policy" value="1" aria-invalid="false" class="form-check-input" id="our-policy">
</span>
</span>';
preg_match($re, $str, $matches, PREG_OFFSET_CAPTURE, 0);
// Print the entire match result
var_dump($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