Possible Duplicate:
RegEx to parse or validate Base64 data
How to validate Base64 string in Javascript using regex?
Possible Duplicate:
RegEx to parse or validate Base64 data
How to validate Base64 string in Javascript using regex?
See if this answer fits you, from another user:
^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$