Check for matching brackets in a string

check_balanced_brackets.rb

def check_string(str)
  str.empty? || is_balanced(0, str)
end

def is_balanced(num_left_brackets, str)
  if str.head == "["
    is_balanced(num_left_brackets + 1, str.tail)
  elsif str.head == "]" 
    return false if num_left_brackets == 0
    is_balanced(num_left_brackets - 1, str.tail)
  else
    num_left_brackets == 0
  end
end

# utility methods

class String
  def head
    self[0]
  end

  def tail
    self[1..-1]
  end
end

p check_string("[]")                    # true 
p check_string("")                      # true
p check_string("[[]")                   # false
p check_string("[][]")                  # true
p check_string("]]]")                   # false
p check_string("[[[]]")                 # false
p check_string("][][][")                # false
p check_string("[[[[]]][][][[]]]")      # true
p check_string("[[[]][]]]][][][[]]]")   # false