Add a class to the P tag wrapping aligncenter and alignnone images in WordPress

add-class-to-ptags-on-images.php

<?php

/**
* Adds the img-parent class to the <p> tags WordPress automatically adds around aligncenter images.
* Adds <p> tags with the img-parent class to alignnone images
* I used https://css-tricks.com/snippets/wordpress/remove-paragraph-tags-from-around-images/ as a base
*/

function filter_ptags_on_images($content){
   return preg_replace('/(<p>)?\s*(<a .*>)?\s*(<img.*class=".*(alignnone|aligncenter).*".*>)\s*(<\/a>)??\s*(<\/p>)??/iU', '<p class="img-parent">\2\3\5</p>', $content);
}

add_filter('the_content', 'filter_ptags_on_images');