1)RewriteEngine on
2)RewriteBase /ci/
3)RewriteCond $1 !^(index\.php|images|captcha|css|js|robots\.txt)
4)RewriteRule ^(.*)$ /ci/index.php/$1 [L]
------------------------------------------------------------------
thats my htaccess file , as i understand this should remove the index.php file ok , but i dont understand how it do it exactly .

Q1) what the command in line 3 do exactly ?

Q2) line number 3 capture any url that do not start with index.php
like for example www.claudia.com/welcome/about_us
now $1 = welcome/about_us
line number 4 redirect this $1 to its real path :
www.claudia.com/index.php/welcome/about_us
do i understand right ?

Q3) as i understand this htaccess will prevent any url that start with index.php or images or captcha or css or js or robots.txt ,
my question here what will happen if our script need to download somthing from /images/ by an ajax call ? the htaccess will prevent this call from targeting this directory?

Q4)the same question 3 but regarding how search engine robots will use robots.txt if this htaccess dont redirect to this file ?

thanks.