You didn't specify which regex standard you are looking to use. It's going to be different in PHP, Java, grep, sed etc.
If you accept my bid I will provide you with a bash script that will recursively remove the entire base64 decode line from any file in any sub-directory beneath where ever you start it at.
My underlaying assumption is that this is static HTML. However there is the strong possibility that these pages are being generated and if that's the case then you're going to need to clean out whatever is generating them. That would likely mean you've suffered a SQL injection attack. Anyways, pick me and I'll help you clean it up, regardless of what the source is for the same price.