Detect HTML tags in a string

If purpose is just to check if string contain html tag or not. No matter html tags are valid or not. Then you can try this.

function is_html($string) {
  // Check if string contains any html tags.
  return preg_match('/<\s?[^\>]*\/?\s?>/i', $string);
}

This works for all valid or invalid html tags. You can check confirm here https://regex101.com/r/2g7Fx4/3

A simple solution is:

if($string != strip_tags($string)) {
    // contains HTML
}

The benefit of this over a regex is it's easier to understand, however I could not comment on the speed of execution of either solution.

If you just want to detect/replace certain tags: This function will search for certain html tags and encapsulate them in brackets - which is pretty senseless - just modify it to whatever you want to do with the tags.

$html = preg_replace_callback(
    '|\</?([a-zA-Z]+[1-6]?)(\s[^>]*)?(\s?/)?\>|',
    function ($found) {
        if(isset($found[1]) && in_array(
            $found[1], 
            array('div','p','span','b','a','strong','center','br','h1','h2','h3','h4','h5','h6','hr'))
        ) {
            return '[' . $found[0] . ']';
        };
    },
    $html  
);

Explaination of the regex:

\< ... \>   //start and ends with tag brackets
\</?        //can start with a slash for closing tags
([a-zA-Z]+[1-6]?)    //the tag itself (for example "h1")
(\s[^>]*)? //anything such as class=... style=... etc.
(\s?/)?     //allow self-closing tags such as <br />

you need to 'delimit' the regex with some character or another. Try this:

if(!preg_match('#(?<=<)\w+(?=[^<]*?>)#', $string)){ 
    return $string;
}