issue with using regex to redirect following permalink changes

hi. I have recently decided to change my url structure from year/month/post-name to /post-name. I changed it to that and added the regex expression as follows:

source URL: /[0-9]{4}\/[0-9]{2}\/(.*?)/
destination: /$1

it worked really well, HOWEVER

Now, I want to change the permalink from /post-name to /category/post-name.

I have been trying to find the proper expression to add in redirection to redirect from but it has not been successful. can anyone help me? what is the regex expression to redirect all the old urls to /category/post-name