RewriteCond file exists - regex

Question

Such a simple problem but htaccess and I have never got along.
Serve the file if it exists.
If the URI is just / and if index.html exists then serve index.html
Otherwise serve app.php
Here is .htaccess:
# Disable the directory processing, Apache 2.4
DirectoryIndex disabled xxx
RewriteEngine On
# Serve existing files
RewriteCond %{REQUEST_FILENAME} -f
RewriteRule .? - [L]
# For / serve index.html if it exists
RewriteCond %{REQUEST_URI} ^/$
RewriteCond index.html -f
RewriteRule .? index.html [L]
# Otherwise use front controller
RewriteRule .? app.php [L]
It works if I comment out the RewriteCond index.html -f line as long as index.html does in fact exists. But I also want to check for index.php so I need the file exists check. And if nothing else I'd like to understand why the posted lines do not work.

Answer 1

This should work:
# Disable the directory processing, Apache 2.4
DirectoryIndex disabled xxx
RewriteEngine On
# Serve existing files
RewriteCond %{REQUEST_FILENAME} -f
RewriteRule ^ - [L]
# For / serve index.html if it exists
RewriteCond %{DOCUMENT_ROOT}/index.html -f [NC]
RewriteRule ^/?$ index.html [L]
# Otherwise use front controller
RewriteRule ^ app.php [L]
Remember that RewriteCond with -f or -d needs full path to the file or directory that's why you need to prefix the filename with %{DOCUMENT_ROOT}/. No need to escape dots here.