maxab.rb

def resolve(string)
  string.gsub(/\d\[(.*?)\]/){|a|
    formated = a.split("[")[1..-1].join("[")
    if formated.include?('[')
      resolve(formated) * a.split("[").first.to_i
    else
      a.split("[").last * a.split("[").first.to_i
    end
  }.gsub("]", "")
end

input1 = "4[ab]"
input2 = "2[b3[a]]"
input3 = "z1[y]zzz2[abc]"

raise "oh oh missing this input #{input1} with the following result #{resolve(input1)}" unless resolve(input1) == 'abababab'
raise "oh oh missing this input #{input2} with the following result #{resolve(input2)}" unless resolve(input2) == 'baaabaaa'
raise "oh oh missing this input #{input3} with the following result #{resolve(input3)}" unless resolve(input3) == 'zyzzzabcabc'

p "all tests pass Yah :D "