Tic tac toe, in Ruby!

tictactoe.rb

# A nice helper
class String
  def is_number?
    true if Float(self) rescue false
  end
end

class Board
  def initialize
    @board = [
      [nil, nil, nil],
      [nil, nil, nil],
      [nil, nil, nil],
    ]

    @moves = 0
  end

  def mark(team, x, y)
    # Some validation
    throw :err, "Invalid team specified to mark!" if ![:x, :o].include?(team)
    throw :err, "Space already taken" if @board[y][x] != nil
    throw :err, "Invalid space" if x < 0 || y < 0 || x >= @board.length || y >= @board.length

    @board[y][x] = team
    @moves += 1

    # Check for any endgame states, starting with the row
    (0...@board.length).each do |check_x|
      break if @board[y][check_x] != team
      return team if check_x == @board.length - 1
    end

    # Column
    (0...@board.length).each do |check_y|
      break if @board[check_y][x] != team
      return team if check_y == @board.length - 1
    end

    # Diagonal
    if x == y
      (0...@board.length).each do |check_n|
        break if @board[check_n][check_n] != team
        return team if check_n == @board.length - 1
      end
    end

    # Draw?
    if @moves == @board.length ** 2
      return false
    end

    return nil
  end

  def to_s
    marks = {nil => " ", :x => "X", :o => "O"}
    @board.inject("") do |row_str, row|
      str = row.each.inject("") do |col_str, col|
        "#{col_str}[#{marks[col]}] "
      end

      "#{row_str}#{str}\n"
    end
  end
end

b = Board.new

puts "Let's play a game of tic tac toe!"

active_team = :x
while true
  puts "\n#{b.to_s}"

  puts "#{active_team} is up. Enter a comma separated coordinate pair:"
  x, y = gets.split(",")
  if !x.is_number? || !y.is_number?
    puts "Invalid coordinate!"
    next
  end

  end_state = nil
  error_msg = catch :err do
    end_state = b.mark(active_team, x.to_i, y.to_i)
    nil
  end
  
  if !error_msg.nil?
    puts error_msg
    next
  elsif end_state == false
    puts b.to_s
    puts "Drat! There was a draw."
    break
  elsif !end_state.nil?
    puts b.to_s
    puts "#{end_state} wins!"
    break
  end

  # Toggle teams
  active_team = active_team == :x ? :o : :x
end