solution

user_spect.rb

require 'rails_helper'

RSpec.describe User, type: :model do
  let(:user) {FactoryGirl.create :user}
  subject { user }

  context "attributes" do
  	it { is_expected.to respond_to(:username) }
  	it { is_expected.to respond_to(:full_name) }
  	it { is_expected.to respond_to(:bio) }
	end

	context "validations" do
		it { is_expected.to validate_presence_of(:username) }
		it { is_expected.to validate_presence_of(:full_name) }

		it { is_expected.to validate_length_of(:bio).is_at_most(300) }

		it { is_expected.to allow_value('Jesuslerma3').for(:username) }
		it { is_expected.to_not allow_value('jesuslerma').for(:username) }
		it { is_expected.to validate_uniqueness_of(:username)}
	end
end

validations_cookbook.rb

class User < ActiveRecord::Base
  # Include default devise modules. Others available are:
  # :confirmable, :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable
  
  validates :username, :full_name,  presence: true
  validates :bio, length: { maximum: 300 }
  validates :username, format: { 
  	with: /^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).+$/,
  	multiline: true,
    message: "Only allow username with at least 1 number and 1 upcase letter"
  }
  validates :username,  uniqueness: true
end