redirecting subdomain to main domain with htaccess - php

Question

I am building a website with two main sections,
so I have created two subdomains:
web1.example.com
web2.example.com
since i'm building websites with codeigniter, I want to redirect to the index.php page. here's what I did:
RewriteEngine On
RewriteCond %{HTTP_HOST} ^web1.example.com$ [NC]
RewriteRule ^(.*)$ http://example.com/web1 [R=301,NC,L]
it works as expected, but it changes the url to example.com/web1..
is there any way to keep the url web1.example.com, but still redirecting to example.com/web1?
I already am using .htaccess file:
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_URI} ^system.*
RewriteRule ^(.*)$ /index.php?/$1 [L]
RewriteCond %{REQUEST_URI} ^application.*
RewriteRule ^(.*)$ /index.php?/$1 [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ index.php?/$1 [L]
</IfModule>
<IfModule !mod_rewrite.c>
ErrorDocument 404 /index.php
</IfModule>
when I enter web1.example.com, the actual url to be executed should be:
example.com/web1, and file to be executed is index.php ..

Answer 1

Didnt tested, Modify and try this
RewriteRule ^([a-zA-Z0-9-/]+).example.com$ http://example.com/$1 [L]
RewriteRule ^([a-zA-Z0-9-/]+).example.com/([a-zA-Z0-9-/]+) http://example.com/$1&$2 [L]
Edited 1
RewriteEngine on
RewriteCond %{HTTP_HOST} ^(.*)\.example\.com
RewriteRule ^(.*)$ http://example.com/index.php/%1&$1 [L,NC,QSA]