htaccess rewrite to a page that can or cannot have ids - .htaccess

Question

I'm trying to create a htaccess rule to rewrite the following URL's:
localhost/abc/def/1 (or with trailing slash would be fine)
to
localhost/abc/def.php?id=1
accessing just
localhost/abc/def/ is also valid, and it's working.
My code so far:
RewriteEngine On
Options +FollowSymLinks
Options All -Indexes
RewriteEngine On
RewriteCond %{REQUEST_URI} !(\.css|\.js|\.png|\.jpg|\.jpeg|robots\.txt)$ [NC]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^index$ ./src/index.php [NC,L] # it works
RewriteRule ^def/?$ ./src/def.php [NC,L] #works too!
RewriteRule ^def/(.*)?$ ./src/def.php?id=$1 [NC,L] #this doesnt
I've read some posts like this one Rewrite Rule To Detect Numbers Only but they didn't work for me.
The weird thing is, if i change the slash between def and the id part with underline, it does work, like this:
RewriteRule ^def_(.*)?$ ./src/def.php?id=$1 [NC,L]
But the URL becomes localhost/abc/def_1 and doesnt look that good.
UPDATE:
Site structure:
/home/username/public_html/abc/ (have multiple websites on public_html)
|_media/
|____js/
|____css/
|____images/
|_src/
|____def.php
|____fgh.php
|_.htaccess
When the rule to redirect def/1 does work, the media url's within the page are also being translated to:
http://localhost/abc/def/css/bootstrap.min.css
Thanks!

Answer 1

Reproducing your case scenario, I've made the follow folder structure:
/home/username/public_html/
|_ abc/
|___ .htaccess
|___ src/
|_____ def.php
In my .htaccess located inside the abc folder, I have the following:
RewriteEngine On
RewriteRule ^def/?$ src/def.php [NC,L]
RewriteRule ^def/([1-9]+)/?$ src/def.php?id=$1 [NC,L]
And all the scenarios you mentioned work as expected.
These 2 go to def.php:
domain.com/abc/def
domain.com/abc/def/
These 2 go to def.php?id=1 and def.php?id=3 respectively:
domain.com/abc/def/1
domain.com/abc/def/3/
I am afraid you might have additional .htaccess or rules taking effect or even your browser cached not giving the proper information you want.
Try using a different browser and try the above exactly as I described it.