How do I redirect from a legacy category URL to a new catgory URL using Retour
Despite having used regex's for years, I still almost never get them right from memory. A great tool for testing them out is: https://regexr.com/

What you'll need to use is a capture group. Something like this for the source:

/category/(.*$)

And then the resulting URI would be something like this:

/news/category/$1

Each set of () is a capture group; they are numbered sequentially, so $1 will output the first capture group.

If you go to https://regexr.com/ it will even explain every part of the regex, and what it does: