validations_user.rb

class User < ActiveRecord::Base
  validates :username, :password, :email, :phone, :age, presence: true

  validates :username,
    length: { within: 5..20 },
    uniqueness: true,
    format: { with: /\A\w*\z/ }

  validates :password, length: { within: 5..20 }
  validates :email, format: { with: /.+@.+\..+/i }
  validates :phone, format: { with: /\A\+48/ }
  validates :age, numericality: true

  # lots of other stuff that usually is here
end