Treetop Nested Non-Unique Delimiters

README.md

Trying to show a problem with the Treetop Documentation at http://cjheath.github.io/treetop/pitfalls_and_advanced_techniques.html

Matching Nested Structures With Non-Unique Delimeters

Say I want to parse a diabolical wiki syntax in which the following interpretations apply.

** *hello* ** --> <strong><em>hello</em></strong>
* **hello** * --> <em><strong>hello</strong></em>

rule strong
  '**' (em / !'*' . / '\*')+ '**'
end

rule em
  '**' (strong / !'*' . / '\*')+ '**'    
end

Emphasized text is allowed within strong text by virtue of em being the first alternative. Since em will only successfully parse if a matching * is found, it is permitted, but other than that, no * characters are allowed unless they are escaped.

It gives me the following error.

$ rake
Expected one of **, \* at line 1, column 4 (byte 4) after ** 
1
4
rake aborted!
Exception: Parse error at offset: 0
/Users/spundun/Documents/code_samples/treetop_nested_non_unique_delimiters/parser.rb:24:in `parse'
/Users/spundun/Documents/code_samples/treetop_nested_non_unique_delimiters/Rakefile:6:in `block in <top (required)>'
/Users/spundun/.rvm/gems/ruby-2.1.2/bin/ruby_executable_hooks:15:in `eval'
/Users/spundun/.rvm/gems/ruby-2.1.2/bin/ruby_executable_hooks:15:in `<main>'
Tasks: TOP => default => test
(See full trace by running task with --trace)
$ 

Gemfile

gem 'rake'
gem 'pry'
gem 'treetop'

Gemfile.lock

GEM
  specs:
    coderay (1.1.0)
    method_source (0.8.2)
    polyglot (0.3.5)
    pry (0.10.1)
      coderay (~> 1.1.0)
      method_source (~> 0.8.1)
      slop (~> 3.4)
    rake (10.3.2)
    slop (3.6.0)
    treetop (1.5.3)
      polyglot (~> 0.3)

PLATFORMS
  ruby

DEPENDENCIES
  pry
  rake
  treetop

nested_nonunique.treetop

grammar NestedNonunique
  rule body
    strong / em
  end
  rule strong
    '**' (em / !'*' . / '\*')+ '**'
  end
  rule em
    '**' (strong / !'*' . / '\*')+ '**'
  end
end

parser.rb

require 'treetop'

class Parser
  
  # Load the Treetop grammar from the 'sexp_parser' file, and 
  # create a new instance of that parser as a class variable 
  # so we don't have to re-create it every time we need to 
  # parse a string

  base_path = File.expand_path(File.dirname(__FILE__))

  Treetop.load(File.join(base_path, 'nested_nonunique.treetop'))
  @@parser = NestedNonuniqueParser.new

  def self.parse(data)
    tree = @@parser.parse(data)
    
    # If the AST is nil then there was an error during parsing
    # we need to report a simple error message to help the user
    if(tree.nil?)
      puts @@parser.failure_reason
      puts @@parser.failure_line
      puts @@parser.failure_column
      raise Exception, "Parse error at offset: #{@@parser.index}"
    end
    
    return tree
  end
end

Rakefile

require './parser.rb'

task default: [:test]

task :test do |variable|
	p Parser.parse("** *hello* **")
	p Parser.parse("* **hello** *")
end

Thanks, applied. Sorry it took a while to get to it.