I am trying to limit my site to only allow User Agents with the following to be able to hit my site:
Mozilla/5.0 (Windows NT 10.0; WOW64; Trident/7.0; rv:11.0) like Gecko
Currently in my .htaccess file (/var/www/html/) I have the following:
RewriteEngine On
RewriteCond %{HTTP_USER_AGENT} !Mozilla/5.0 (Windows NT 10.0; WOW64; Trident/7.0; rv:11.0) like Gecko [NC]
RewriteRule ^ - [F,L]
Ordinarily, the CondPattern (2nd argument to the RewriteCond directive) is a regular expression. So, you would need to convert this to a regex - notably, you would need to backslash-escape the literal dots and spaces. However, it would be easier to use the lexicographical equality operator instead, ie. = prefix. This then matches a literal string, not a regex.
The spaces in the user-agent string are a special case since the space is an argument separator in Apache config files. These either need to be backslash escaped (or use the \s shorthand character class if using a regex), or enclose the whole argument in double quotes.
Try the following instead:
RewriteCond %{HTTP_USER_AGENT} "!=Mozilla/5.0 (Windows NT 10.0; WOW64; Trident/7.0; rv:11.0) like Gecko"
RewriteRule ^ - [F]
The L flag is not required with the F flag - it is implied. If you specifically only want to allow that one user-agent then you don't need the NC flag on the condition.