Use regular expression to detect if tag is closed

function check_html($html) {
 preg_match_all("/<([a-zA-Z0-9]+)\s*[^\/>]*>/",$html,$ start_tags);
 preg_match_all("/<\/([a-zA-Z0-9]+)>/", $html, $end_tags);
 if(count($start_tags[1]) != count ($end_tags[1])) return false;
 for($i = 0; $i < count($start_tags[1]); $i++) {
 if(!in_array($start_tags[1][$i ], $end_tags[1])) return false;
 }
 return true;
}

Explanation:

/]*>/ This pattern is used to match HTML tags (such as:

,

,

, etc., but except
), and holds the name of the tag (such as: head, div) in $start_tags wait). The // pattern is used to match closed HTML tags (such as: ,

, etc.). And keep the closed tag name in $end_tags. Then we use the conditional statement count($start_tags[1]) != count($end_tags[1]) to determine whether the starting tag and the closing tag are equal. If they are not equal, it means they are not closed. Finally, use in_array($start_tags[1][$i], $end_tags[1]) to determine whether the start tag and the closing tag are equal. At this point, we have completed the HTML matching!

Regular expression