Should I be using Regex to uppercase uncommon characters?
One-to-one substitution is precisely what tr is for.

def norwegian_upcase(string)
  string.upcase.tr('æ-ý','Æ-Ý')
end


That being said, the unicode_utils gem provides a method for this:

def norwegian_upcase(string)
  UnicodeUtils.upcase(string, :no)
end


I don't know Norwegian, but you can supply language subtags :nb (Norwegian Bokmal) or :nn (Norwegian Nynorsk) if they would behave differently than general :no (Norwegian).