_search_regex function

And here we are with #847!

This is one of those moments when I'm REALLY glad to have tests.
However, I'll look into making WARNINGs fail tests when appropriate (like, when a field thet should be there can't be found).

So, TODO:

- [ ] port those two new IEs
- [ ] check WARNINGs
- [ ] consider setting a nicer placeholder in output templates for missing fields