You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: utils.rb
    path of file: ./repos/stackneveroverflow/vendor/bundle/ruby/2.3.0/gems/mail-2.6.5/lib/mail/multibyte
    
    the code of the file until where you have to start completion: # encoding: utf-8
# frozen_string_literal: true

module Mail #:nodoc:
  module Multibyte #:nodoc:
    if RUBY_VERSION >= "1.9"
      # Returns a regular expressio
    
      def self.clean(string)
        if expression = valid_character
          # Splits the string on character boundaries, which are determined based on $KCODE.
          string.split(//).grep(expression).join
        else
          string
        end