Allow me to celebrate the JRuby team (and nearby efforts) in that the next patch release of it will include \K as a regular expression special character.
This GitHub Issue was just closed with a resolution in the Joni library: https://github.com/jruby/jruby/issues/4871
Its support is in a little select company, randomly listed at StackOverflow, which has such google-juice that it felt worth it to shout about JRuby on there.
I’ll repeat the example from the GitHub Issue:
$ irb 2.3.1 :001 > 'street'.match( /s\Kt/) => #<MatchData "t">