Interesting, thanks for the heads up. I've been using a similar regex to parse the link relation and haven't come across an issue with boulder or pebble.
Your regex change makes sense, but I guess this is just an example of where we should be properly parsing the url instead of using a regex to extract what we need. That is, use go's url parser and extract the relations from the url fragment. EDIT: Thinking about this, it looks like go won't parse these urls properly anyway, so short of rolling your own or using an external url parser which supports these relations, the modified regex might be the easiest solution.
It would be interesting to see what acme server implementation is being used here and how.