class Source < ActiveRecord::Base
   belongs_to :source_type
   def url=(value)
       SourceType.each do |type|
          # here i match type's regexp to input value and if match,
          # assign it to the new source object
       end
   end
end

class Person
  include ActiveModel::Validations
  attr_accessor :title

  validates :title, presence: true
end

class Source < ActiveRecord::Base
   belongs_to :source_type, inverse_of: :sources
   attr_accessor :url
end

class SourceType < ActiveRecord::Base
   has_many :sources, inverse_of: :source_type
   validates :source_type_attr, presence: { if: :url_match? }

   def url_match?
      self.sources.url == [your_regex]
   end
end