How do i rewrite an easy number based Hyperlink to a sub folder?

I would like http://mysite.com/123 to redirect to http://mysite.com/en/123.

The amount might be everything from 1 - 9999. My attempt in htaccess was:

RewriteRule ^([0-9]+)$ /en/$1/ [R]

But that does not work.

your syntax is appropriate, I simply remove slash ultimately of line and delay pills work:

RewriteRule ^([0-9]+)$ /en/$1