Regex pattern for permalink change

If there is a permalink structure change from
/%year%/%monthnum%/%day%/%postname%/
To
/%category%/%postname%

or any such change such as from
/%postname%
to
/%category%/%postname%

What is the correct Regex pattern from
Source:
to
Target:

To create a site-wide 301 redirect for all the articles that will have this change