Try this in RM:
/01\.(?<year>20\d\d) \- 31\.\k<year>/
and it will be highlighted by inspection as illegal escape syntax. Why?
This is perfectly correct regexp expression in Ruby. Here's the proof: 'Named groups can be backreferenced with \k<name>, where name is the group name.' (see Ruby docs for Regexp class, 'Capturing' section)