prepare_trim_mode

# File lib/erb.rb, line 661
    def prepare_trim_mode(mode) # :nodoc:
      case mode
      when 1
        return [false, '>']
      when 2
        return [false, '<>']
      when 0, nil
        return [false, nil]
      when String
        unless mode.match?(/\A(%|-|>|<>){1,2}\z/)
          warn_invalid_trim_mode(mode, uplevel: 5)
        end

        perc = mode.include?('%')
        if mode.include?('-')
          return [perc, '-']
        elsif mode.include?('<>')
          return [perc, '<>']
        elsif mode.include?('>')
          return [perc, '>']
        else
          [perc, nil]
        end
      else
        warn_invalid_trim_mode(mode, uplevel: 5)
        return [false, nil]
      end
    end