```ruby
# frozen_string_literal: true

# A simple library to prepend magic comments to all Ruby files in a given folder

module AddMagicComment
  MAGIC_COMMENT_PREFIX  = "frozen_string_literal"
  MAGIC_COMMENT_PATTERN = /^(-|(<%))?#\s*#{MAGIC_COMMENT_PREFIX}\s*(%>)?/m
  MAGIC_COMMENT         = "#{MAGIC_COMMENT_PREFIX}: true"

  EXTENSION_COMMENTS = {
    "rb"   => "# #{MAGIC_COMMENT}",
    "rake" => "# #{MAGIC_COMMENT}",
    "haml" => "-# #{MAGIC_COMMENT}",
    "erb"  => "<%# #{MAGIC_COMMENT} %>"
  }

  def self.process(argv)
    directory = argv.first || Dir.pwd

    count = 0
    EXTENSION_COMMENTS.each do |ext, comment|
      filename_pattern = File.join(directory, "**", "*.#{ext}")
      Dir.glob(filename_pattern).each do |filename|
        next unless File.size(filename) > 0
        File.open(filename, "r+") do |file|
          lines = file.readlines

          # remove current encoding comment(s)
          while lines.first && lines.first.match(MAGIC_COMMENT_PATTERN)
            lines.shift
          end

          # set current encoding
          lines.insert(0, comment + "\n")
          count += 1

          file.pos = 0
          file.puts(lines.join)
          file.truncate(file.pos)
        end
      end
    end

    puts "Magic comments added to #{count} source file(s)"
  end
end
```

Remove /m since it is no longer needed
user
```ruby
# frozen_string_literal: true

# A simple library to prepend magic comments to all Ruby files in a given folder

module AddMagicComment
  MAGIC_COMMENT_PREFIX  = "frozen_string_literal"
  MAGIC_COMMENT_PATTERN = /^(-|(<%))?#\s*#{MAGIC_COMMENT_PREFIX}\s*(%>)?/
  MAGIC_COMMENT         = "#{MAGIC_COMMENT_PREFIX}: true"

  EXTENSION_COMMENTS = {
    "rb"   => "# #{MAGIC_COMMENT}",
    "rake" => "# #{MAGIC_COMMENT}",
    "haml" => "-# #{MAGIC_COMMENT}",
    "erb"  => "<%# #{MAGIC_COMMENT} %>"
  }

  def self.process(argv)
    directory = argv.first || Dir.pwd

    count = 0
    EXTENSION_COMMENTS.each do |ext, comment|
      filename_pattern = File.join(directory, "**", "*.#{ext}")
      Dir.glob(filename_pattern).each do |filename|
        next unless File.size(filename) > 0
        File.open(filename, "r+") do |file|
          lines = file.readlines

          # remove current encoding comment(s)
          while lines.first && lines.first.match(MAGIC_COMMENT_PATTERN)
            lines.shift
          end

          # set current encoding
          lines.insert(0, comment + "\n")
          count += 1

          file.pos = 0
          file.puts(lines.join)
          file.truncate(file.pos)
        end
      end
    end

    puts "Magic comments added to #{count} source file(s)"
  end
end
```
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