Wordpress `the_content` filter, for adding class to img element in the page content

functions.php

<?php
// other stuff ...

// register our filter to the wordpress action 'the_content', 
// it will then be called when we call the_content() from anywhere in our template
add_filter( 'the_content', 'add_classlist_to_img' );

function add_classlist_to_img( $content ) {
        // used to detect img tag element
    $locateElement = '/\<img.*?\/\>/';

    $content = preg_replace_callback($locateElement, function($imgElement) {
        // add this class list to the element
        $additionalClass = 'any additional classlist here';

        // used to locate class attribute
        $locateClass = '/class=(["\'])(.*?)\1/';

        // the complete match
        // see preg_replace_callback doc at php.net
        $imgElement = $imgElement[0];

        if (preg_match($locateClass, $imgElement, $class)) {
            // the element already has class attribute, so we simply append our $additionalClass in it
            $replacement = 'class=$1$2 '.$additionalClass.'$1';
        } else {
            // aim at the end of the element to insert class attribute
            $locateClass = '/(\/\>)/';

            // add class attribute to the element
            $replacement = 'class="'.$additionalClass.'" $1';
        }

        return  preg_replace($locateClass, $replacement, $imgElement); 
        
    }, $content);

    // this will be content that will be printed on the page
    return $content;
}

// other stuff ...