Create a Regex Redirect after changing Permalink structure Post > Category/Post

Hi all,
I had a bad SEO permalinks configuration so I’ve changed it from sitename/%post% to sitename/%category%/%post%.
Now I wold like to set a redirect from old links (sitename/%post%) to new ones.
I’ve tried a configuration like this one:
Redirection: 301
From Url: /%postname%
Regex: checked
To Url: /%category%/%postname%

But it doesn’t work.
Thank you in advanced
Edoardo