rewrite rule sub directory using .htaccess

I need help with URL redirection. For example:

• old url: http://www.example.com/sub-dir/index.php?id=1
• new url: http://www.example.com/sub-dir/1/

I need the .htaccess code file to be in /sub-dir/ folder

Here is the code that I was trying to use:

RewriteRule ^/([0-9]+)/$ index.php?id=$1 [NC,L]

But it didn't work.

You can use this rule in /sub-dir/.htaccess:

DirectoryIndex index.php
RewriteEngine On
RewriteBase /sub-dir/

RewriteRule ^([0-9]+)/?$ index.php?id=$1 [QSA,L]