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 => lambda{ |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.

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”).

• :allow_nil - If set to true, skips this validation if the attribute is nil (default is false).

• :allow_blank - If set to true, skips this validation if the attribute is blank (default is false).

• :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.

• :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.