ReWrite rule to add .html extension

[S_Combs]

I need a rule that will add an .html extension whenever there is 'not' a trailing slash if this is possible.

I have a new client that has changed ecommerce scripts and, the new version handles SEO differently and changed all of their 16,000+ product links. This was not caught prior to the site being re-indexed so it's now a big problem and I need a little help with this.

All of their products used to have links like this
domain.com/category/productname
but are now
domain.com/category/productname.html

Category links did not change and all are like this
domain.com/category/ (with trailing slash)

I tried a variety of things but couldn't figure out how to limit the redirect only to links without a trailing slash. I then found an old thread here and the bottom replies seemed to be exactly what I'm needing but are not working.
[webmasterworld.com...]

These are the most promising methods I have tried

RewriteCond %{REQUEST_URI} !(\.|/$)
RewriteRule (.*) /$1.html [L]

RewriteCond %{REQUEST_URI} !(\.|/$)
RewriteRule ^([^.]*[^./])$ $1.html [L]

RewriteCond %{REQUEST_URI} !(\.|/$)
RewriteRule ^([^./]+)$ $1.html [L]

Existing rules from cart script

RewriteCond %{REQUEST_FILENAME} !\.(png|gif|ico|swf|jpe?g|js|css)$
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . index.php?sef_rewrite=1 [L,QSA]

Thanks for helping if you can

[S_Combs]

I was able to resolve this problem using this rule

RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !(\.|/$)
RewriteRule ^(.*)$ /$1.html [R=301,L]

[g1smd]

You would have been far better off changing the new site to use extensionless URLs and then redirecting recently indexed new .html URLs to the extensionless version.

[jdMorgan]

More efficiently-coded version of current solution (see important comment in post above):

RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^.]*[^./])$ /$1.html [R=301,L]

Jim