Email Validation with Regex for Ruby on Rails [duplicate]

This question already has an answer here:

• Rails ActiveRecord regex validation of non-mandatory fields 2 answers

I want to validate the input email as right format. This is my current code in the model:

EMAIL_REGEX = /\A[^@]+@([^@\.]+\.)+[^@\.]+\z/
validates :email, :format => {:with => EMAIL_REGEX}, :uniqueness => {case_sensitive:false}

This works fine. However, since the email address is not required. User don't necessarily need to input anything. How can I allow the situation that user let the field blank?