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:

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

Answer 2

Add unless nil? || blank?, which should skip the validation if it's blank (empty) or nil, I believe.

See the last code example on this page.