what's the use of back references in Rewriterule in mod_rewrite?
See this for excruciating details:
Backreferences are used to address specific match-groups in
(2008|2009) is a reference, and $1 is the backreference. Same for (.*) and $2. As you can see, they're used for URL rewriting. See the "RewriteRule matching syntax" section of the answer I linked to.