Another balanced parens parser

parser.rb

class Parser
  ENCLOSURES = {
    '"' => '"',
    '(' => ')',
    '{' => '}',
    '[' => ']'
  }

  def initialize
    @i = 0
  end

  def is_valid? str
    return false unless ENCLOSURES.keys.include? str[@i]

    close = ENCLOSURES[str[@i]]

    @i += 1
    valid = true
    while str[@i] != close && @i != str.length && valid
      valid = is_valid? str
      @i += 1
    end

    return valid && str[@i] == close
  end
end

parser2.rb

class Parser
  ENCLOSURES = {
    '"' => '"',
    '(' => ')',
    '{' => '}',
    '[' => ']'
  }

  def initialize
    @i = 0
  end

  def is_valid? str
    return true if @i >= str.length
    return false unless ENCLOSURES.keys.include? str[@i]

    close = ENCLOSURES[str[@i]]

    @i += 1
    while is_valid? str
      @i += 1
    end

    actual_close = str[@i]

    @i += 1
    return actual_close == close && is_valid?(str)
  end
end

parser3.rb

class Parser
  ENCLOSURES = {
    '"' => '"',
    '(' => ')',
    '{' => '}',
    '[' => ']'
  }

  def initialize
    @i = 0
  end

  def is_beginning? str
    return ENCLOSURES.key? str[@i]
  end

  def is_valid? str
    return true if @i >= str.length
    return false unless is_beginning? str

    valid = true
    while is_beginning?(str) && valid
      close = ENCLOSURES[str[@i]]

      @i += 1
      is_valid?(str)

      actual_close = str[@i]

      @i += 1
      valid &&= actual_close == close
    end

    return valid
  end
end