This goes in whatever model you're validating. "Subject" can be any of the symbols in your model and doesn't need the : in front of it.

def validate
	if subject.split.any?{|w| w.length > 26}
		errors.add(:subject, "cannot have words more than 26 consecutive characters")
	end
end

I needed to find a way to use active record validations on non active record items so i went searching and found this blog which is useful for me..
Totally ripped off Peter Donald's blog :P
http://www.realityforge.org/articles/2005/12/02/validations-for-non-activerecord-model-objects

To get this working grab the active_form.rb file and place it in the app/models directory. You can then make your model objects extend ActiveForm and use them like regular ActiveRecord objects.

active_form.rb

# Note ".valid?" method  must occur on object for validates_associated
class ActiveForm
  
  def initialize(attributes = nil)
    if attributes
      attributes.each do |key,value|
        send(key.to_s + '=', value)
      end
    end
    yield self if block_given?
  end

  def [](key)
    instance_variable_get("@#{key}")
  end

  def method_missing( method_id, *args )
    if md = /_before_type_cast$/.match(method_id.to_s)
      attr_name = md.pre_match
      return self[attr_name] if self.respond_to?(attr_name)
    end
    super
  end

protected 
  def raise_not_implemented_error(*params)
    ValidatingModel.raise_not_implemented_error(*params)
  end

  def self.human_attribute_name(attribute_key_name)
    attribute_key_name.humanize
  end

  def new_record?
    true
  end

  # these methods must be defined before include
  alias save raise_not_implemented_error
  alias update_attribute raise_not_implemented_error

public
  include ActiveRecord::Validations

protected 

  # the following methods must be defined after include so that they overide
  # methods previously included
  alias save! raise_not_implemented_error

  class << self
    def raise_not_implemented_error(*params)
      raise NotImplementedError
    end

    alias validates_uniqueness_of raise_not_implemented_error
    alias create! raise_not_implemented_error
    alias validate_on_create raise_not_implemented_error
    alias validate_on_update raise_not_implemented_error
    alias save_with_validation raise_not_implemented_error    
  end
end

require 'dispatcher'
class Dispatcher
  class << self
    if ! method_defined?(:form_original_reset_application!) 
      alias :form_original_reset_application! :reset_application!
      def reset_application!
        form_original_reset_application!
        Dependencies.remove_subclasses_for(ActiveForm) if defined?(ActiveForm)
      end
    end
  end
end

in the model:

class User < ActiveRecord::Base
  require 'validations'

  validates_positive_or_zero :number
  
end

in /lib/validations.rb:

def validates_positive_or_zero(*attr_names)
  configuration = { :message => "Cannot be negative" }
  configuration.update(attr_names.pop) if attr_names.last.is_a?(Hash)
  validates_each attr_names do |m, a, v| m.errors.add(a, configuration[:message]) if v<0 end
end