.htaccess - Recursively mapping slashes to underscores - regex

Question

G'day,
As the title says, I'm trying to make the url formatted as: /this/is/mah/page/strucutre to the file this_is_mah_page_structure.php.
Now, I have that working - except that I can't know the depth of the structure. Thus I need to have some recursion going on.
The working snipit I have for one and two replacements is:
# map the slashes to underscores
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^/]+)/([^/]+)/?$ $1_$2 [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^/]+)/([^/]+)/([^/]+)/?$ $1_$2_$3 [L]
I have found this: Htaccess recursively replace underscores with hyphens which states how to do it (for different characters); and this https://www.askapache.com/htaccess/rewrite-underscores-hyphens-seo-url/ which shows similar code. But I can't make it work.
Also, I found that .htaccess recursion can cause 500 errors - somewhere in the order of 10 loops - based on LimitInternalRecursion. From that, I tried replacing multiple with the one pass, but that had the unexpected outcome of doubling up the url.
This is the example from the other StackOverflow answer to give you an idea of what I've been testing. Any ideas / thoughts / direction?
(Rewrites underscores with hyphens - so it's the wrong side, but it's a start)
RewriteEngine On
# if there is only one underscore then repalce it by - and redirect
RewriteRule ^([^_]*)_([^_]*)$ /$1-$2 [L,R=302]
# if there are more than one underscores then "repeatedly" repalce it by - and set env var USCORE
RewriteRule ^([^_]*)_([^_]*)_(.*) $1-$2-$3 [E=USCORE:1]
# if USCORE env var is set then redirect
RewriteCond %{ENV:REDIRECT_USCORE} =1
RewriteRule ^([^_]+)$ /$1 [L,R=302]
-- Edit and full solution --
So it finally dawned on me through this post, that the presence / absence of R=302 (or 301) is what shifts the idea from "Map URL to file" (ie: transparent); to "Redirect". It should have been obvious, but I thought there was more too it.
The final solution is as #anubhava suggested, but I removed the ,R=301 and added RewriteCond %{REQUEST_FILENAME} !-f and RewriteCond %{REQUEST_FILENAME} !-d to prevent collision between the rules and real files/directories of the same name. (i.e. scripts.js, /images/yomomma.jpg, etc)
# when there are more than one / then "recursively" replace it by _
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^/]+)/([^/]+/.+)$ $1_$2 [N,DPI]
# when there is only one / then replace it by _ and redirect
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^/]+)/([^/]+)$ /$1_$2 [NE,L]

Answer 1

I had posted that answer about 5 years ago though now I think it can be further improved.
You may use these rules in site root .htaccess:
RewriteEngine On
# if request is for a file [OR]
RewriteCond %{REQUEST_FILENAME} -f [OR]
# if request is for a directory
RewriteCond %{REQUEST_FILENAME} -d
# then ignore all rules below
RewriteRule ^ - [L]
# when there is only one / then replace it by _ and redirect
RewriteRule ^([^/]+)/([^/]+)$ /$1_$2 [NE,L,R=301]
# when there are more than one / then recursively replace / by _
RewriteRule ^([^/]+)/(.+)$ $1_$2 [N,DPI]
Change R=301 rule to RewriteRule ^([^/]+)/([^/]+)$ /$1_$2 [L] if you don't want an external redirect.