validates_format_of

Validates whether the value of the specified attribute is of the correct form, going by the regular expression provided. You can require that the attribute matches the regular expression:

class Person < ActiveRecord::Base
  validates_format_of :email, with: /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\z/i, on: :create
end

Alternatively, you can require that the specified attribute does not match the regular expression:

class Person < ActiveRecord::Base
  validates_format_of :email, without: /NOSPAM/
end

You can also provide a proc or lambda which will determine the regular expression that will be used to validate the attribute.

class Person < ActiveRecord::Base
  # Admin can have number as a first letter in their screen name
  validates_format_of :screen_name,
                      with: ->(person) { person.admin? ? /\A[a-z0-9][a-z0-9_\-]*\z/i : /\A[a-z][a-z0-9_\-]*\z/i }
end

Note: use \A and \Z to match the start and end of the string, ^ and $ match the start/end of a line.

Due to frequent misuse of ^ and $, you need to pass the multiline: true option in case you use any of these two anchors in the provided regular expression. In most cases, you should be using \A and \z.

You must pass either :with or :without as an option. In addition, both must be a regular expression or a proc or lambda, or else an exception will be raised.

Configuration options:

• :message - A custom error message (default is: “is invalid”).

• :with - Regular expression that if the attribute matches will result in a successful validation. This can be provided as a proc or lambda returning regular expression which will be called at runtime.

• :without - Regular expression that if the attribute does not match will result in a successful validation. This can be provided as a proc or lambda returning regular expression which will be called at runtime.

• :multiline - Set to true if your regular expression contains anchors that match the beginning or end of lines as opposed to the beginning or end of the string. These anchors are ^ and $.

There is also a list of default options supported by every validator: :if, :unless, :on, :allow_nil, :allow_blank, and :strict. See ActiveModel::Validation#validates for more information

# File activemodel/lib/active_model/validations/format.rb, line 108
      def validates_format_of(*attr_names)
        validates_with FormatValidator, _merge_attributes(attr_names)
      end