String#normalize_encoding! for working with differently encoded strings in Ruby 1.9.3

string.rb

# -*- encoding : utf-8 -*-
class String
  # method always returns string with valid encoding which is equal to Encoding#default_internal
  # handy for working with strings which encoding may differ (all kinds of imports and free text inputs)
  def normalize_encoding!
    return self unless !!defined?(Encoding) # apply for Ruby 1.9.3 only

    encoding_equal_to_default_internal = (self.encoding == Encoding.default_internal)

    # return unchanged if encoding is valid and equal to default_internal
    return self if self.valid_encoding? && encoding_equal_to_default_internal
    # try to force to default_internal and return if encoding is valid in result
    return self.force_encoding(Encoding.default_internal) if self.dup.force_encoding(Encoding.default_internal).valid_encoding?

    if encoding_equal_to_default_internal
      # there might be a cases when encoding is the same as default_internal, but it's not valid
      # so we need to force it to something else in order to make String#encode! method work.
      non_default_encoding = Encoding.list.detect{|enc| enc != Encoding.default_internal}
      self.force_encoding(non_default_encoding).encode!
    else
      self.encode!(Encoding.default_internal, self.encoding, { :undef => :replace, :invalid => :replace})
    end
  end
end