Remove double slash at the begining of URL path - apache

Question

Due to moving of a site, the old hoster created a redirect to the new location. However, there is a leading slash / in the redirection and the former hoster is not able/willing to fix it. So I end up with all the redirects coming in like this:
http://sub.domain.com//path/to/file.html
So I tried to remove the leading slash:
using mod_alias
RedirectMatch 301 ^//(.*)$ http://sub.domain.com/$1
using mod_rewrite
RewriteEngine on
RewriteCond %{REQUEST_METHOD} !=POST
RewriteCond %{REQUEST_URI} ^(.*?)(/{2,})(.*)$
RewriteRule . %1/%3 [R=301,L]
Neither works. The latter removes multiple slashes inside the path, but not at the beginning.
There are already questions regarding slash removing, but they don't solve this problem:
Issue In Removing Double Or More Slashes From URL By .htaccess
.htaccess - how to remove repeated characters from url?
Does Apache somehow treat this case differently?
How do I get rid of one of the leading slashes?

Answer 1

The Problem
The point is that Apache 2 do not include leading slashes in the Requested URI, so you can't do this:
RewriteRule ^/+(.*)$ /$1 [R=301,L]
or that:
RewriteCond %{REQUEST_URI} ^/+(.*)$
RewriteRule ^.*$ /%1 [R=301,L]
what workes grat for any other kind of redirects or rewrites.
The Solution
But what you can do is to use the the Apache request variable that usually looks something like this:
GET /some-page HTTP/1.1
With this in mind we can do the following:
RewriteCond %{THE_REQUEST} ^[A-Z]{3,7}\s//+(.*)\sHTTP.*$
RewriteRule ^.*$ /%1 [R=301,L]
So all leading slashes will be reduced to the one that that we need.
I tested this positive on apache 2.4

Answer 2

This code work for me
RewriteEngine On
RewriteCond %{THE_REQUEST} \s/+(.*?)/{2,}([^\s]*)
RewriteRule ^ %1/%2 [R=301,L,NE]

Answer 3

The First / is Implied
Try this:
RewriteEngine on
RewriteRule ^/(.+) $1 [R=301,L,B,NE]
Why?
To match two slashes at the beginning of the path, in htaccess, we don't need to look for two slashes—just one—because the first slash is assumed. For instance, if you wanted to match example.com/pasta, your match rule would be ^pasta$, not ^/pasta (which would only match on example.com//pasta).
This is different from httpd.conf, where you would need to specify both slashes.
Confusing, but that's how it works.