module Jekyll
  module Converters
    class CoffeeScript < Converter
      safe true
      priority :low

      def matches(ext)
        ext =~ /\A\.coffee\z/
      end

      def output_ext(ext)
        ".js"
      end

      def convert(content)
        ::CoffeeScript.compile(content)
      end
    end
  end
end

module Jekyll
  module Converters
    class CoffeeScript < Converter
      safe true
      priority :low

      def matches(ext)
        ext =~ /^\.coffee$/i
      end

      def output_ext(ext)
        ".js"
      end

      def convert(content)
        ::CoffeeScript.compile(content)
      end
    end
  end
end

Change Matching Regex to use standard symbols
Change Matching Regex to use standard symbols

For beginning and end of line, ^ and $.
And added i flag for case insensitivity.

This also more closely matches the sass/scss converters