Why doesn't the regular expression for mod_rewrite work in .htaccess?

Question

Apache HTTP Server

There are some pages with addresses like /dir1/dir2/page_name_123.html, they should be redirected using mod_rewrite to
/dir1/dir2/page_name.html having the extension "html".
The code turned out offhand like this (cut out irrelevant):

# ...
RewriteEngine On
Options +FollowSymlinks
RewriteBase /
# ...

RewriteCond %{REQUEST_URI} ^/dir1/dir2/(.+)_[0-9]{1,3}\.html$
RewriteRule /dir1/dir2/(.+)_[0-9]{1,3}\.html$ http://sitename.ru/dir1/dir2/$1.html [L,R=301]

Help me figure out what's wrong? It does not complain about the syntax, mod_rewrite is enabled and successfully fulfills the rest of the requests, but this particular one does not give anything.
I apologize in advance if the mistake is amateurish, but I'm stupid.
PS In the load - should (.+) be replaced with something like (^/)+ to cut off subdirectories? In fact, they are not in the structure, but you never know, they suddenly appear.

Answer 1

It worked for me without the 1st slash in the rewriterule:
RewriteRule dir1/dir2/(.+)_[0-9]{1,3}\.html$

Answer 2

Thanks, it worked!
However, what is the reason for this, I did not understand. I will dig the documentation further, most importantly, the urgent task is solved.