Redirecting all blog posts to a subfolder?

I just moved my entire blog from the root into a (blog) subfolder so I could use the root for my new main website – everything is working fine on the blog itself, but all links to actual posts from google are now broken…!

I need to redirect as in the following example:

http://www.mysite.com/2012/12/actualposts

to http://www.mysite.com/blog/2012/12/actualposts

Can anyone tell me how to do this (presumably using a 301 redirect in the htaccess file)?

Much appreciated – been tearing my hair out with different attempts that haven’t worked!