.htaccess - How to disable caching for only internet explorer 8 on Wordpress? -

i'm loading different html , css explorer 6-8 following code:

<?php  $ie6 = (ereg('msie 6',$_server['http_user_agent'])) ? true : false; $ie7 = (ereg('msie 7',$_server['http_user_agent'])) ? true : false; $ie8 = (ereg('msie 8',$_server['http_user_agent'])) ? true : false; if (($ie6 == 1) || ($ie7 == 1) || ($ie8 == 1)) {     // code }  ?>  

and works fine until turn on caching plugin wp-rocket. plugin adds code in .htaccess file.

i added code: rewritecond %{http_user_agent} !(msie\ [6-8]) [nc] in .htaccess:

<ifmodule mod_rewrite.c> rewriteengine on rewritebase / rewritecond %{https} on rewriterule .* - [e=wpr_ssl:-https] rewritecond %{server_port} ^443$ rewriterule .* - [e=wpr_ssl:-https] rewritecond %{http:accept-encoding} gzip rewriterule .* - [e=wpr_enc:_gzip] rewritecond %{request_method} rewritecond %{query_string} ="" rewritecond %{http:cookie} !(wordpress_logged_in_|wp-postpass_|wptouch_switch_toggle|comment_author_|comment_author_email_) [nc] rewritecond %{request_uri} !^(.*/feed/)$ [nc]  #my code rewritecond %{http_user_agent} !(msie\ [6-8]) [nc]  rewritecond %{http_user_agent} !^(facebookexternalhit).* [nc] rewritecond "%{document_root}/wp-content/cache/wp-rocket/%{http_host}%{request_uri}/index%{env:wpr_ssl}.html%{env:wpr_enc}" -f rewriterule .* "/wp-content/cache/wp-rocket/%{http_host}%{request_uri}/index%{env:wpr_ssl}.html%{env:wpr_enc}" [l] </ifmodule> 

but doesn't work.

so, there way turn off caching explorer 6-8?