balanced_parens.rb

require "./tinyparse"

parser = Tinyparse.build do
  paren "(", :main, ")", :main
  
  main [:paren, ""]
end

["((()))", "(((", "()()((()()))", "))(("].each do |str|
  puts str
  if parser =~ str
    puts "  \e[32;1mis valid\e[0m"
  else
    puts "  \e[31;1mis invalid\e[0m"
  end
end

tinyparse.rb

class Tinyparse
  def self.build(&bk)
    new.tap { |t| t.instance_eval(&bk) }.to_regexp
  end
  
  attr_reader :rules
  
  def initialize
    @rules = {}
  end
  
  def to_regexp
    Regexp.new("\\A" << @rules.map { |name,re| "(?<#{name}>#{re}){0}" }.join << "\\g<main>\\z")
  end
  
  def method_missing(rule, *args)
    compile = ->x {
      case x
      when Regexp
        Regexp.union(//, x).source["(?-mix:)|".length..-1]
      when String
        Regexp.escape(x)
      when Symbol
        "\\g<#{x}>"
      when Array
        "(#{x.map(&compile).join("|")})"
      end
    }
    @rules[rule] = args.map(&compile).join
  end
end