How and where can I learn regular expressions and .htaccess redirect settings?

Question

PHP

Now the task has become to set up a redirect in a tricky way. Minimum knowledge of both regular expressions and redirects. Therefore, I have 2 questions:
1. What is the most effective way to figure this out?
2. Where and what would you recommend to read?
PS You need to set up a redirect from site.com/.../ to site.com/ru/.../ while there are still 2 languages ​​that the client selects. In this case, the path will already be site.com/en/.../

Answer 1

www.egoroff.spb.ru/portfolio
https://beget.ru/articles/htaccess