regex help

I am trying to set up permanent redirects so that a link to
http://www.example.com/blog/post-title/
would be redirected to
http://www.example.com/post-title/

I have in my .htaccess the following

RewriteRule blog/([A-Za-z0-9]+)$ /$1 [R=301,L]

which doesn’t work (returns a 404).

However, if I add a manual redirect such as

RewriteRule blog/post-title/ /post-title/ [R=301,L]

this works.

I must be doing something wrong with the REGEX syntax. Can anyone help me out?