Challenge 1: Redirecting /old-blog/anything to /blog/anything —
Solution Walkthrough

The rule:

RewriteEngine On
RewriteCond %{REQUEST_URI} ^/old-blog/(.*)$
RewriteRule ^ https://example.com/blog/%1 [L,R=301]

(An equally correct alternative captures directly in the RewriteRule
pattern itself instead of using a RewriteCond backreference:

RewriteRule ^old-blog/(.*)$ https://example.com/blog/$1 [L,R=301]

Either form is a valid solution to this exercise.)

Walking through it:

- RewriteEngine On turns on the rewrite engine for this context, the
  same as every mod_rewrite example in this chapter.
- The pattern ^old-blog/(.*)$ (or the RewriteCond form using
  %{REQUEST_URI}) captures everything after /old-blog/ into a
  backreference -- $1 when captured in the RewriteRule pattern
  itself, %1 when captured in a preceding RewriteCond.
- That captured group is placed directly into the new URL
  (https://example.com/blog/$1 or .../%1), which is what "preserving
  whatever came after /old-blog/" actually requires.
- [R=301] is what makes this an external, permanent redirect the
  client's browser actually sees and follows -- without it, this
  would silently rewrite the URI internally instead, which the
  exercise specifically asked NOT to do.
- [L] stops any further rewrite rules from also trying to process
  this already-redirected request.

WHY THIS WORKS AS AN ANSWER
------------------------------
This exercise checks that the reader can combine a capture group with
a backreference to preserve part of the original URL, and correctly
distinguishes an external redirect (needs [R]) from a silent internal
rewrite (no [R] at all) -- the exact distinction this chapter's own
mod_rewrite section draws.