“Replace Method With Method Object” to the rescue.

1_old.rb

module Partners::DailymotionHelper
  def dailymotion_sanitize_film_summary(summary)
    summary.gsub!(/\r\n/, "\n")
    summary.gsub!(%r{(\n\s*)?<br[^>]*>(\s*\n)?}i, "\n")
    summary.gsub!(%r{</p[^>*]>}i, "</p>\n\n")
    summary = strip_tags(summary)
    summary.gsub!(/\u0003/, '')
    summary.gsub!(/\n\n+/, "\n\n")
    summary.strip!
    summary
  end
end

2_new.rb

module Partners::DailymotionHelper
  def dailymotion_sanitize_film_summary(summary)
    DailymotionSanitizer.sanitize!(summary)
  end
  
  class DailymotionSanitizer
    NEWLINE = "\n".freeze
    DOUBLE_NEWLINE = "\n\n".freeze
    EMPTY_STRING = ''.freeze
    
    include ActionView::Helpers::SanitizeHelper
    
    def self.sanitize!(string)
      new(string).sanitize!
    end
    
    def initialize(string)
      @string = string
    end
    
    def sanitize!
      normalize_newlines
      replace_br_tags_by_newlines
      insert_double_newlines_after_p_tags
      remove_control_characters
      strip_html_tags
      limit_to_two_following_newlines
      @string.strip!
      @string
    end
    
    protected
      def normalize_newlines
        # The following reads better but is approx 10 times slower...
        # @string.encode!(Encoding::UTF_8, universal_newline: true)
        @string.gsub!(/\r\n?/, NEWLINE)
      end
      
      def replace_br_tags_with_newlines
        @string.gsub!(%r{
          \s*
          <br[^>]*>
          \s*
        }ix, NEWLINE)
      end
      
      def insert_double_newlines_after_p_tags
        @string.gsub!(%r{</p[^>*]>}i, "</p>#{DOUBLE_NEWLINE}")
      end
      
      def remove_control_characters
        # All control characters except \n
        @string.gsub!(/[[:cntrl:]&&[^\n]]/, EMPTY_STRING)
      end
      
      def strip_html_tags
        @string = strip_tags(@string)
      end
      
      def limit_to_two_following_newlines
        @string.gsub!(/\n\n+/, DOUBLE_NEWLINE)
      end
  end
end