UTF8 member names not supported

[jgoodw1n]

We are attempting to use JsonPath to look up properties from a JSON payload that utilizes German keys.

pry> @object = {'Übermorgen': 'The day after tomorrow'}
=> {:Übermorgen=>"The day after tomorrow"}
pry> JsonPath.new('$.store.Übermorgen').on(@object)
FrozenError: can't modify frozen String: "['store']" 
from (PATH)/lib/jsonpath.rb:48:in `initialize'

It seems like an issue with this regexp https://github.com/joshbuddy/jsonpath/blob/master/lib/jsonpath.rb#L33 which does not parse alphanumeric values in UTF8 outside of ASCII characters.