activemodel-base64_validator

Add to your Gemfile:

gem 'activemodel-base64_validator'

Run:

bundle install

Then add the following to your model:

validates :my_base64_attribute, base64: true

If you need to validate a base64 outside a model, you can get the regexp:

Base64Validator::REGEXP
Base64Validator.valid?(string)  # true or false

Regular Expression based on http://stackoverflow.com/a/475217/1297336