You can actually do this in Ruby it just means you need to be a tad more fancy
utf_pattern = Regexp.new("\\p{Lower}".force_encoding("UTF-8"))
a = "Go234"
a.match(utf_pattern) # => #<MatchData "o">
b = "GO234"
b.match(utf_pattern) # => nil
b = "ÜÖ234"
b.match(utf_pattern) # => nil
b = "Über234"
b.match(utf_pattern) # => #<MatchData "b">