brackets.rb

class BracketsChecker

  OPENING = ['[', '{', '(']
  CLOSING_MAP = {'[' => ']', '{' => '}', '(' => ')'}

  def valid?(input)
    return false if input.empty?
    open_brackets = []
    input.each_char do |char|
      open_brackets << char if OPENING.include? char
      last_open = open_brackets[-1]
      open_brackets.pop if CLOSING_MAP[last_open] == char
    end
    open_brackets.empty?
  end
end

brackets_spec.rb

require 'rspec'
require './brackets'

RSpec.describe BracketsChecker do
  subject(:checker) { BracketsChecker.new }
  it {expect(checker.valid?("(a[0]+b[2c[6]]) {24 + 53}")).to be true }
  it {expect(checker.valid?("f(e(d))")).to be true }
  it {expect(checker.valid?("[()]{}([])")).to be true }
  it {expect(checker.valid?("((b)")).to be false }
  it {expect(checker.valid?("(c]")).to be false }
  it {expect(checker.valid?("{(a[])")).to be false }
  it {expect(checker.valid?("([)]")).to be false }
  it {expect(checker.valid?(")(")).to be false }
  it {expect(checker.valid?("")).to be false }
end