Validate string is base64 format using RegEx?

I have been looking how to validate a base64 string and came across this.

 ^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$

I need a little help to make it allow "==" aswell as "=".

Thanks