htaccess blocking access based on length of user agent string - .htaccess

Question

I recently came across several cloud based services image scraping my website and had to block them based on their User Agent string.
However this would be very inefficient if I had to check my logs on a regular basis to find out who they are and add them to the htaccess block list.
After investigations it transpired that these bots all have user agent strings less than about 12 characters e.g. "moe".
So I was looking for a way to block user agents based on the length of the UA string. But of course you cannot perform calculations in htaccess.
So I came up with the following:
RewriteCond %{HTTP_REFERER} ^$
RewriteCond %{HTTP_USER_AGENT} ^[a-z0-9./:;-=]{12}$ [NC,OR]
RewriteCond %{HTTP_USER_AGENT} ^[a-z0-9./:;-=]{11}$ [NC,OR]
RewriteCond %{HTTP_USER_AGENT} ^[a-z0-9./:;-=]{10}$ [NC,OR]
RewriteCond %{HTTP_USER_AGENT} ^[a-z0-9./:;-=]{9}$ [NC,OR]
RewriteCond %{HTTP_USER_AGENT} ^[a-z0-9./:;-=]{8}$ [NC,OR]
RewriteCond %{HTTP_USER_AGENT} ^[a-z0-9./:;-=]{7}$ [NC,OR]
RewriteCond %{HTTP_USER_AGENT} ^[a-z0-9./:;-=]{6}$ [NC,OR]
RewriteCond %{HTTP_USER_AGENT} ^[a-z0-9./:;-=]{5}$ [NC,OR]
RewriteCond %{HTTP_USER_AGENT} ^[a-z0-9./:;-=]{4}$ [NC,OR]
RewriteCond %{HTTP_USER_AGENT} ^[a-z0-9./:;-=]{3}$ [NC,OR]
RewriteCond %{HTTP_USER_AGENT} ^[a-z0-9./:;-=]{2}$ [NC,OR]
RewriteCond %{HTTP_USER_AGENT} ^[a-z0-9./:;-=]{1}$ [NC]
RewriteRule .*\.(js|ico|gif|jpg|png|swf|rm|mov|mpg|mpeg)$ xxxxx/hotlink.gif [F,NC,L]
I would appreciate any feedback on this if I have missed any likely characters that could be used by UA or if anyone can see any problems with this approach.
I do have all the other usual image hotlinking in place.
This is to prevent cloud services hotlinking to images from newsletters.
Thanks in advance.