Support for the Ruby 2.4 series has ended. See here for reference.
# File rexml/dtd/notationdecl.rb, line 10 def initialize src super() if src.match( PUBLIC ) md = src.match( PUBLIC, true ) elsif src.match( SYSTEM ) md = src.match( SYSTEM, true ) else raise ParseException.new( "error parsing notation: no matching pattern", src ) end @name = md[1] @middle = md[2] @rest = md[3] end