Wordpress custom link to wrap inserted images from media library - php

Question

I didn't expect this to be difficult but so far have failed at implementing it.
Simply I want to add a class name to the link tag that wraps the img tag when you insert an image into a post using the media library.
I want to turn this
<img src="http://..." alt="..." width="780" height="490" class="alignnone size-full wp-image-12" />
Into this
<img src="http://..." alt="..." width="780" height="490" class="alignnone size-full wp-image-12" />
I can do it simply enough using jQuery, but I'd much prefer to use a wordpress filter or custom function. SO the output in the post includes the class name.

Answer 1

Thanks msbodetti, the solution is as follows.
function add_colorbox_class_to_image_links($html, $attachment_id, $attachment) {
$linkptrn = "/<a[^>]*>/";
$found = preg_match($linkptrn, $html, $a_elem);
// If no link, do nothing
if($found <= 0) return $html;
$a_elem = $a_elem[0];
// Check to see if the link is to an uploaded image
$is_attachment_link = strstr($a_elem, "wp-content/uploads/");
// If link is to external resource, do nothing
if($is_attachment_link === FALSE) return $html;
if(strstr($a_elem, "class=\"") !== FALSE){
// If link already has class defined inject it to attribute
$a_elem_new = str_replace("class=\"", "class=\"fancylink ", $a_elem);
$html = str_replace($a_elem, $a_elem_new, $html);
}else{ // If no class defined, just add class attribute
$html = str_replace("<a ", "<a class=\"fancylink \" data-fancybox-group=\"gallery\" ", $html);
}
return $html;
}
add_filter('image_send_to_editor', 'add_colorbox_class_to_image_links', 10, 3);