/usr/lib/ruby/2.3.0/rexml/ is in libruby2.3 2.3.0-5ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/ruby/2.3.0/rexml/attlistdecl.rb
/usr/lib/ruby/2.3.0/rexml/attribute.rb
/usr/lib/ruby/2.3.0/rexml/cdata.rb
/usr/lib/ruby/2.3.0/rexml/child.rb
/usr/lib/ruby/2.3.0/rexml/comment.rb
/usr/lib/ruby/2.3.0/rexml/doctype.rb
/usr/lib/ruby/2.3.0/rexml/document.rb
/usr/lib/ruby/2.3.0/rexml/dtd/
/usr/lib/ruby/2.3.0/rexml/dtd/attlistdecl.rb
/usr/lib/ruby/2.3.0/rexml/dtd/dtd.rb
/usr/lib/ruby/2.3.0/rexml/dtd/elementdecl.rb
/usr/lib/ruby/2.3.0/rexml/dtd/entitydecl.rb
/usr/lib/ruby/2.3.0/rexml/dtd/notationdecl.rb
/usr/lib/ruby/2.3.0/rexml/element.rb
/usr/lib/ruby/2.3.0/rexml/encoding.rb
/usr/lib/ruby/2.3.0/rexml/entity.rb
/usr/lib/ruby/2.3.0/rexml/formatters/
/usr/lib/ruby/2.3.0/rexml/formatters/default.rb
/usr/lib/ruby/2.3.0/rexml/formatters/pretty.rb
/usr/lib/ruby/2.3.0/rexml/formatters/transitive.rb
/usr/lib/ruby/2.3.0/rexml/functions.rb
/usr/lib/ruby/2.3.0/rexml/instruction.rb
/usr/lib/ruby/2.3.0/rexml/light/
/usr/lib/ruby/2.3.0/rexml/light/node.rb
/usr/lib/ruby/2.3.0/rexml/namespace.rb
/usr/lib/ruby/2.3.0/rexml/node.rb
/usr/lib/ruby/2.3.0/rexml/output.rb
/usr/lib/ruby/2.3.0/rexml/parent.rb
/usr/lib/ruby/2.3.0/rexml/parseexception.rb
/usr/lib/ruby/2.3.0/rexml/parsers/
/usr/lib/ruby/2.3.0/rexml/parsers/baseparser.rb
/usr/lib/ruby/2.3.0/rexml/parsers/lightparser.rb
/usr/lib/ruby/2.3.0/rexml/parsers/pullparser.rb
/usr/lib/ruby/2.3.0/rexml/parsers/sax2parser.rb
/usr/lib/ruby/2.3.0/rexml/parsers/streamparser.rb
/usr/lib/ruby/2.3.0/rexml/parsers/treeparser.rb
/usr/lib/ruby/2.3.0/rexml/parsers/ultralightparser.rb
/usr/lib/ruby/2.3.0/rexml/parsers/xpathparser.rb
/usr/lib/ruby/2.3.0/rexml/quickpath.rb
/usr/lib/ruby/2.3.0/rexml/rexml.rb
/usr/lib/ruby/2.3.0/rexml/sax2listener.rb
/usr/lib/ruby/2.3.0/rexml/security.rb
/usr/lib/ruby/2.3.0/rexml/source.rb
/usr/lib/ruby/2.3.0/rexml/streamlistener.rb
/usr/lib/ruby/2.3.0/rexml/syncenumerator.rb
/usr/lib/ruby/2.3.0/rexml/text.rb
/usr/lib/ruby/2.3.0/rexml/undefinednamespaceexception.rb
/usr/lib/ruby/2.3.0/rexml/validation/
/usr/lib/ruby/2.3.0/rexml/validation/relaxng.rb
/usr/lib/ruby/2.3.0/rexml/validation/validation.rb
/usr/lib/ruby/2.3.0/rexml/validation/validationexception.rb
/usr/lib/ruby/2.3.0/rexml/xmldecl.rb
/usr/lib/ruby/2.3.0/rexml/xmltokens.rb
/usr/lib/ruby/2.3.0/rexml/xpath.rb
/usr/lib/ruby/2.3.0/rexml/xpath_parser.rb