gosukiwi

Start := "ab" A
       | "aba"
A     := "a"

gosukiwi

Foo := Foo "ab"
     | "ab"

gosukiwi

def my_rule
  if my_rule # infinite loop here
    do something
  else
    do something else
  end
end

gosukiwi

Assign     := Identifier EQUALS Number
Identifier := WORD
Number     := NUMBER

gosukiwi

Assign := Identifier EQUALS Number
       := WORD EQUALS NUMBER
       := foo EQUALS NUMBER # foo is a valid workd token, we can replace it
       := foo = NUMBER      # = is a valid equals token
       := foo = 1           # 1 is a valid number token

gosukiwi

               [PARAGRAPH]
                   |
                   v
      +-------[SENTENCES]-----------+
      |               |             |
      v               v             v
[TEXT="hello "] [BOLD="world"] [TEXT="."]

gosukiwi

Parser.new.parse("__Foo__ and *bar*.\n\nAnother paragraph.") 
  => #<BodyNode:0x007fc774abe008
     @consumed=14,
     @paragraphs=
      [#<ParagraphNode:0x007fc774eb25d8
        @consumed=12,
        @sentences=
         [#<Node:0x007fc774ea8150 @consumed=5, @type="BOLD", @value="Foo">,
          #<Node:0x007fc774ac5f60 @consumed=1, @type="TEXT", @value=" and ">,
          #<Node:0x007fc774ac6758 @consumed=3, @type="EMPHASIS", @value="bar">,

gosukiwi

class Parser
  def parse(tokens)
    body = body_parser.match(tokens)
    raise "Syntax error: #{tokens[body.consumed]}" unless tokens.count == body.consumed
    body
  end

  private

  def body_parser

gosukiwi

class BodyParser < BaseParser
  include MatchesStar

  def match(tokens)
    nodes, consumed = match_star tokens, with: paragraph_parser
    return Node.null if nodes.empty?
    BodyNode.new(paragraphs: nodes, consumed: consumed)
  end
end

gosukiwi

class SentencesAndNewlineParser < BaseParser
  include MatchesStar

  def match(tokens)
    nodes, consumed = match_star tokens, with: sentence_parser
    return Node.null if nodes.empty?
    return Node.null unless tokens.peek_at(consumed, 'NEWLINE', 'NEWLINE')
    consumed += 2 # consume newlines

    ParagraphNode.new(sentences: nodes, consumed: consumed)