Rewriting with modrewrite if file exists for clean urls - .htaccess

Question

In order to have nice clean urls with some kind of site archetecture I am using modrewrite in my .htaccess file. I don't want file extensions (.php or .html).
I have the following:
<IfModule mod_rewrite.c>
RewriteEngine On
</IfModule>
RewriteRule ^([a-zA-Z0-9-]+)/$ $1.html [L]
RewriteRule ^([a-zA-Z0-9-]+)/([a-zA-Z0-9-]+)/$ $1/$2.html [L]
RewriteRule ^([a-zA-Z0-9-]+)/([a-zA-Z0-9-]+)/([a-zA-Z0-9-]+)/$ $1/$2/$3.html [L]
But I want it to check if an index.html file exists in the sub-directory first.
For example, I want
http://example.com/first/
to rewrite to
http://example.com/first.html
but if that does not exist then rewrite to
http://example.com/first/index.html
The same for sub-directies, for example I want
http://example.com/first/second/
to rewrite to
http://example.com/first/second.html
but if that does not exist then rewrite to
http://example.com/first/second/index.html
Is this possible, and how do I achieve this?

Answer 1

This should work for you (works for me):
DirectoryIndex index.html
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
# Do not do anything for already existing files and folders
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule .+ - [L]
# add .html file extension (if such file does exist)
RewriteCond %{DOCUMENT_ROOT}/$1\.html -f
RewriteRule ^(.+[^/])/?$ $1.html [L,QSA]
</IfModule>
If you are using <IfModule mod_rewrite.c> then place ALL rewrite rule into between the tags, not just RewriteEngine On.
The rule checks if such file with .html extension does exist before rewriting. For http://example.com/first/second/ it will check if WEBSITE_ROOT/first/second.html does exist. If not -- the will leave as is, and Apache will automatically pick up http://example.com/first/second/index.html because of DirectoryIndex index.html directive (see the very first line).
Please note: these 2 URLs are treated as the same from rewrite engine point of view: http://example.com/first/second/ and http://example.com/first/second (notice that there is no trailing slash). If you want this to be 2 different URLs (only rewrite one with trailing slash) then remove ? and [^/] from last rewrite rule (e.g. RewriteRule ^(.+)/$ $1.html [L,QSA]).
For URL http://example.com/first/second/ -- if you have both folder /first/second/ and file /first/second.html present, then request will go to a folder (and /first/second/index.html will be the final URL -- see #2).

Answer 2

You can use a RewriteCond something -f to check if something is actually a file. For directories, you can use the -d flag instead.