Paco is a parser combinator library inspired by Haskell's Parsec and Parsimmon.

"But I don't need to write another JSON parser or a new language, why do I need your library then?"

Well, most probably you don't. But I can think of rare cases when you do. Say, you need to write a validation for git branch names.

You can go with easy-peasy regex:

branch_name_regex = /^(?!\/|.*(?:[\/.]\.|\/\/|@{|\\|\.lock$|[\/.]$))[^\040\177 ~^:?*\[]+$/
branch_name_regex.match?("feature/branch-validation")

With Paco, you can go with a little more verbose version of that rule:

module BranchNameParser
  extend Paco
  class << self
    def parse(input)
      parser.parse(input)
    end
    def parser
      lookahead(none_of("/")).next(valid_chars.join)
    end
    def valid_chars
      any_char.not_followed_by(invalid_sequences).at_least(1)
    end
    
    def invalid_sequences
      alt(invalid_chars, invalid_endings)
    end
    def invalid_chars
      alt(
        string("/."),
        string(".."),
        string("//"),
        string("@{"),
        string("\\\\"),
        one_of("\040\177 ~^:?*\\[")
      )
    end
    def invalid_endings
      seq(
        alt(string(".lock"), one_of("/.")),
        eof
      )
    end
  end
end
BranchNameParser.parse("feature/branch-validation")

Easy? Not really, but there is a chance you can read it. 😅

See API documentation, examples and specs for more info on usage.

Add to your Gemfile:

gem "paco"

And then run bundle install.

After checking out the repo, run bin/setup to install dependencies. Then, run bundle exec rspec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install.

Bug reports and pull requests are welcome on GitHub at https://github.com/skryukov/paco.

• parslet - A small (but featureful) PEG based parser library.
• parsby — Parser combinator library for Ruby inspired by Haskell's Parsec.

The gem is available as open source under the terms of the MIT License.