validates_length_of

Validates that the specified attribute matches the length restrictions supplied. Only one option can be used at a time:

class Person < ActiveRecord::Base
  validates_length_of :first_name, :maximum => 30
  validates_length_of :last_name, :maximum => 30, :message => "less than 30 if you don't mind"
  validates_length_of :fax, :in => 7..32, :allow_nil => true
  validates_length_of :phone, :in => 7..32, :allow_blank => true
  validates_length_of :user_name, :within => 6..20, :too_long => "pick a shorter name", :too_short => "pick a longer name"
  validates_length_of :zip_code, :minimum => 5, :too_short => "please enter at least 5 characters"
  validates_length_of :smurf_leader, :is => 4, :message => "papa is spelled with 4 characters... don't play me."
  validates_length_of :essay, :minimum => 100, :too_short => "Your essay must be at least 100 words.",
                      :tokenizer => lambda { |str| str.scan(/\w+/) }
end

Configuration options:

• :minimum - The minimum size of the attribute.

• :maximum - The maximum size of the attribute.

• :is - The exact size of the attribute.

• :within - A range specifying the minimum and maximum size of the attribute.

• :in - A synonym(or alias) for :within.

• :allow_nil - Attribute may be nil; skip validation.

• :allow_blank - Attribute may be blank; skip validation.

• :too_long - The error message if the attribute goes over the maximum (default is: “is too long (maximum is %{count} characters)”).

• :too_short - The error message if the attribute goes under the minimum (default is: “is too short (min is %{count} characters)”).

• :wrong_length - The error message if using the :is method and the attribute is the wrong size (default is: “is the wrong length should be %{count} characters)”).

• :message - The error message to use for a :minimum, :maximum, or :is violation. An alias of the appropriate too_long/too_short/wrong_length message.

• :on - Specifies when this validation is active. Runs in all validation contexts by default (nil), other options are :create and :update.

• :if - Specifies a method, proc or string to call to determine if the validation should occur (e.g. :if => :allow_validation, or :if => Proc.new { |user| user.signup_step > 2 }). The method, proc or string should return or evaluate to a true or false value.

• :unless - Specifies a method, proc or string to call to determine if the validation should not occur (e.g. :unless => :skip_validation, or :unless => Proc.new { |user| user.signup_step <= 2 }). The method, proc or string should return or evaluate to a true or false value.

• :tokenizer - Specifies how to split up the attribute string. (e.g. :tokenizer => lambda {|str| str.scan(/\w+/)} to count words as in above example). Defaults to lambda{ |value| value.split(//) } which counts individual characters.

• :strict - Specifies whether validation should be strict. See ActiveModel::Validation#validates! for more information.