Semantic HTML

<?php
$html = 'html=<!doctype html>
<html>
<body>
<header>
<div style="color:red" id="nav" >
Nav
<div class="aside"  style="color:blue">
Aside
<div align = "left" class = "section" style = "color:blue">
    <p class="header">
        <span class="main">
            Hi, I have class="main".
        </span>
    </p>
</div> <!--  section-->
</div> <!-- aside -->
</div>  <!--nav -->
Hello
</header>
</body>
</html>';
;
//$html = $_GET['html'];
$openingRegex = '/\s*<\s*div\s+(.*?)\s*((?:id|class)\s*=\s*\"(\s*header\s*|\s*aside\s*|\s*footer\s*|\s*main\s*|\s*article\s*|\s*section\s*|\s*nav\s*)\")\s*[^\n]+/';
$closingRegex = '/\s*<\s*\/\s*div\s*>\s*<\s*!\s*-\s*-\s*(header|nav|main|aside|footer|section|article)\s*-\s*-\s*>\s*/';

$changed = array();

$lineByLine = explode(PHP_EOL, $html);
$lineByLine = array_filter($lineByLine, function($val) {
    if($val == "") {
        return false;
    }
    return true;
});

foreach ($lineByLine as &$line) {
    preg_match($openingRegex, $line, $opening);
    preg_match($closingRegex, $line, $closing);
    //var_dump($opening);
    if($opening) {
        $line = str_replace($opening[2], '', $line);
        $line = preg_replace("/\s*<\s*div/", "<" . $opening[3], $line);
        $line = preg_replace("/\s+/", ' ', $line);
        preg_match("/\s*<\s*" . $opening[3] ."\s*>/", $line, $res);
        $changed[] = trim($opening[3]);
        preg_match("/(\s+)>/", $line, $res2);
        if($res) {
            $line = preg_replace("/([\s]+)/", '', $line);
        }
        if($res2) {
            $line = preg_replace("/([\s]+)(?=>)/", '', $line);
        }
    }
    if($closing) {
        if(count($changed) != 0) {
            if($changed[count($changed) - 1] == $closing[1]) {
                preg_match("/\s+/", $line, $result);
                $line = str_replace($line, "</" . $closing[1] . ">", $line);
                $line = $result[0] . preg_replace("/([\s]+)/", '', $line);
                unset($changed[count($changed) - 1]);
            } else {
                preg_match("/\s+/", $line, $result);
                $line = $result[0] . "</div>";
            }
        } else {
            preg_match("/\s+/", $line, $result);
            $line = $result[0] . "</div>";
        }
    }
}

echo implode(PHP_EOL, $lineByLine);