Add Custom Class To Post Images In WordPress Code Snippet

Here is how you can apply conditionally a CSS class to link elements which contain images embedded into posts. Just add the code bellow into your wordpress functions.php file then check your image links, they should have post-image-link CSS class.

/**
 * Attach a custom CSS class to linked images' parent anchors <a>..</a>
 * Works for existing content too
 */
function linked_images_custom_class($content) {
    $classes = 'post-image-link'; // space separate classes

    //if there is already a class attribute assigned to the anchor tag
    if ( preg_match('/<a.*? class=".*?"><img/', $content) ) {
        $content = preg_replace('/(<a.*? class=".*?)(".*?><img)/', '$1 ' . $classes . '$2', $content);
    } else {
        // If there is not an existing class attribute, create a class attribute and assign it
        $content = preg_replace('/(<a.*?)><img/', '$1 class="' . $classes . '" ><img', $content);
    }
    return $content;
}

add_filter('the_content','linked_images_custom_class');

Thanks for your time!
Best regards,
Lucas