Normalizer to add prefix on root-relative paths

normalizr.rb

Normalizr.configure do |config|
  # Match root-relative paths that begin with a leading slash
  #
  #     (/path/to/image.jpg)    in CSS url(/...)
  #     "/path/to/image.jpg"    in HTML src="/..."
  #     '/path/to/image.jpg'    in HTML src='/...'
  #
  # The contents of the parentheses or quotes are captured as matches that can
  # be prefixed with a hostname for absolute URLs.
  #
  ROOT_RELATIVE_PATHS = %r{
    (?<=            # Look behind before match start to find:
      \( |          # - an opening parenthesis, or
      (['"])        # - an opening quote captured as \1 for closing pair
    )               #
    (?=             # Then, match the string that follows
      /             # if it starts with a leading slash,
      (?:\S+?)      # has no whitespace and at least one character long,
      (?:           # and ends with either:
        \1          # - the paired opening quote we captured
        | \)        # - or a closing parenthesis
      )             #
    )               #
  }x

  add :qualify_root_relative do |t, prefix:|
    next unless t.present? && t.is_a?(String)
    t.gsub(ROOT_RELATIVE_PATHS, prefix)
  end
end