Fallback integration method breaks some image tags (attributes before the src attribute are lost)

Hmmm just playing around real quick, this could be handled with a preg_replace:

$pattern = "#((<img\s+|<img\s+[^>]*\s+)src\s*=\s*[\"|\'](($root)([^\"|^\']*)(\?.*)?)[\"|\'](.*?)>)#i";
$new = preg_replace($pattern,'$2 src="'.$cdn.'/$5" $7>', $text);

Set $cdn to the CDN URL. In my testing I also added '/' in the root (plus added the trailing slash to $base_url). That then grabbed non absolute images (ie: /sites/default/files) and changed them to the right CDN url. It also captures the image source if it's wrapped in single or double quotes.

Here's the patch. I modified it from my original idea some. I went ahead and did it with a preg_match_callback so that we can still run hook_file_url on each image. I've tested it in a few posts and it seems to be working fine on root-relative and absolute URLs, plus it keeps all the other attributes in there.

It wasn't grabbing root-relative images and it also wasn't grabbing src attributes enclosed with a single quote. It took some playing with the regex pattern, but this patch seems to be getting it all now.

It's not a module that's generating single quotes around attributes, it's from people entering them that way. Single quotes are legal around attributes (src), so IMHO it would be best to catch those as well.

I reworked the pattern some, dropping/combining groups. It cut it from 9 groups to 5, so that should be a nice improvement. I don't see much way to make it any smaller and still be able to catch everything.

As far as root-relative paths, it's catching them for me. I did have one problem, but that is unrelated and should probably be a new issue. I uploaded one image that had an upper case extension on it. Perhaps we should to a strtolower() on the extension/key in _cdn_basic_parse_raw_mapping() and then do another on $file_extension in cdn_basic_get_servers()?

I checked the logs on a live site I put the patch on last night and the only images coming off the main server were those with weird casing in the extensions.

Just gave it a good testing and all seems to be working very well!