Redirection Permlink- help needed with regex

I recently changed the permlink of our website and all previous external links, especially from search engines, gives “Error” page not found.

I reasearched deeply and found out that, when I removed the /month/ and .html from the Not found_ url and replaced it with a trailing slash(/), I worked fine, and I have over a 1000 post with this issue.

How can I probably use any redirection plugin/regex to sort this.

See example links here:

Old url: https://nigerianuniversitynews.com/2012/08/university-of-nigeria-unn-admission-list-for-20122013-to-be-out-soon.html
.
.
Redirected to:

Expected New url: https://nigerianuniversitynews.com/2012/university-of-nigeria-unn-admission-list-for-20122013-to-be-out-soon/

How can I do this for over a 1000 post in one redirection?

Thanks as I expect your reply.