Codeigniter remove index.php and prefix the URL with www - php

Question

I want to remove the default index.php that auto comes with Codeigniter.
I've been able to remove that with this code
RewriteRule ^(.*)$ index.php/$1 [L]
example.com/index.php/blog can now be accessed by example.com/blog
I later wanted to prefix the URL with a www i.e example.com/blog should redirect to www.example.com/blog with these rewrite rules
RewriteCond %{HTTP_HOST} ^(?!www\.)(.+) [NC]
RewriteRule ^(.*) http://www.%1/$1 [R=301,NE,L]
After adding this code above to the end of my .htaccess file, it began to misbehave.
If I enter www.example.com/blog into a URL bar it works fine but if I enter example.com/blog it redirects to www.example.com/index.php/blog
What I'm I doing wrong?
I want example.com/blog to redirect to www.example.com/blog
Note: I am using the Codeigniter framework.
Added: This code is just on top of the previous ones I have up. Maybe this is it's problem please HELP!!!
RewriteCond $1 !^{index\.php|[assests/images/themes/fonts/style/scripts/js/install]|robot\.txt|favicon\.ico}

Answer 1

Try this:
RewriteCond %{HTTP_HOST} ^example.com [NC]
RewriteRule ^(.*)$ example.com/$1 [L,R=301]

Answer 2

After much tricks and following the post given by #Tpojka I was able to come up with this
RewriteEngine On
RewriteBase /
# Removes trailing slashes (prevents SEO duplicate content issues)
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.+)/$ $1 [L,R=301]
# no www -> www
RewriteCond %{HTTP_HOST} ^(?!www\.)(.+) [NC]
RewriteRule ^(.*) http://www.%1/$1 [R=301,NE,L]
# Checks to see if the user is attempting to access a valid file,
# such as an image or css document, if this isn't true it sends the
# request to index.php
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ index.php/$1 [L]
This guy was my problem
RewriteCond $1 !^{index\.php|[assests/images/themes/fonts/style/scripts/js/install]|robot\.txt|favicon\.ico}
I would need help on how to exclude folders and some files like robot.txt file like I tried in the malfunctioning line.

Answer 3

This is the complete .htaccess file which I use personally:
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{HTTP_HOST} !^$
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteCond %{HTTPS}s ^on(s)|
RewriteRule ^ http%1://www.%{HTTP_HOST}%{REQUEST_URI} [R=301,L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ index.php?/$1 [L]
</IfModule>