a__naive_reopened_string.rb

class String
  def to_widget
    Widget.new self.gsub(/foo/, "bar")
  end
end

b__class_eval_reopened_string.rb

String.class_eval do
  def to_widget
    Widget.new self.gsub(/foo/, "bar")
  end
end

c__module_include_string_extension.rb

module StringExtensions
  def to_widget
    Widget.new self.gsub(/foo/, "bar")
  end
end

String.send :include, StringExtensions