Rails: How to validate format only if value is present?

Question

I would like to validate the format of a form value, allowing an empty value or requiring a specific format.

To validate a value against a format, this works fine:

validates :application_bundle_identifier, :format => {
  :with => /^[a-zA-Z0-9.-]+$/,
  :message => "A bundle identifier should contain only alphanumeric (A-Z,a-z,0-9), hyphen (-), and period (.) characters."
}

Now, I would like to allow users to not enter the value at all. In this case, I could tweak the regexp to ^[a-zA-Z0-9.-]*$, but this does not seem a very good Rails-way. Depending of the format, it might be not possible, or at least less clear.

What is the best practice in this case?

Answer 1

You can use the option allow_blank: true to skip the validation when the attribute is blank/empty/nil.

https://guides.rubyonrails.org/active_record_validations.html#allow-blank