$el = 'li'; // Ex
$match = []; // Reserving for results

/**
 * Regex - extract HTML tag and its content
 * Array map:
 * x[0] = everything
 * x[1] = open tag
 * x[2] = attributes
 * x[3] = content & end tag
 * x[4] = content only
 *
 * Note for content: including text node + children node
 */
$reg = '/(<'.$el.'(.*?)>)((n*?.*?n*?)</'.$el.'>|)/';

if (preg_match($reg, $html_str, $match)) { echo 'Moving onward!';}

$dom = new DomDocument();
$content = mb_convert_encoding(
    get_the_content(null, true), # WordPress func, it gives input str
    'HTML-ENTITIES',
    'UTF-8'
);
$dom->loadHTML($content);

$el = $doc->getElementsByTagName('li');

<link rel="stylesheet" href="/html_5/tags/html_link_tag_example.css">