Add Fancybox attribute to image links

fancybox-image-links.php

<?php
/**
 * Fancybox image links
 * @link http://fancyapps.com/fancybox/3/docs/#usage
 * 
 */
add_filter( 'the_content', 'ac_the_content_fancybox_image_links', 10 ); // Adjust priorities if necessary
add_filter( 'widget_text', 'ac_the_content_fancybox_image_links', 10 );

function ac_the_content_fancybox_image_links( $content ) {
	
	$attribute = 'data-fancybox';
	$group     = '="group"'; // Omit if grouping undesired

	$search = '/<a href="([^"]+)\.(jpe?g|png|gif)"/';
	$replace = '<a href="$1.$2" ' . esc_attr( $attribute . $group );
	$content = preg_replace( $search, $replace, $content );

	return $content;

}