Apache's Alias directive is not working as expected - php

Question

I am trying to integrate my portal with my website.
My website :
http://example.com
and My portal :
http://portal.com
Now I want to see my portal from :
http://example.com/portal
Part of my core apache config file (sites-enabled/website):
<VirtualHost *:80>
ServerAdmin webmaster#localhost
ServerName example.com
DocumentRoot /home/example/WebSite2.0/WebContent
DirectoryIndex index.php index.html
<Directory /home/example/WebSite2.0/WebContent>
Options +IncludesNOEXEC
AllowOverride None
Order allow,deny
allow from all
XBitHack On
AddType text/html .html
AddHandler server-parsed .html
</Directory>
Alias /portal /home/example/portal/CodeIgniter_2.1.0
<Directory /home/example/portal/CodeIgniter_2.1.0>
DirectoryIndex "index.php"
allow from all
Options +Indexes
#Options FollowSymLinks MultiViews
Order allow,deny
RewriteEngine On
RewriteBase /portal
#RewriteRule ^test\.html$ test.php
RewriteCond $1 !^(index\.php|css|images|robots\.txt)
RewriteRule ^(.*)$ index.php/$1 [L]
RewriteCond $1 ^(css|images|js)
RewriteRule ^(.*)$ $1
</Directory>
</VirtualHost>
As you see my portal functions on top of CodeIgniter; Hence -
RewriteCond $1 !^(index\.php|css|images|robots\.txt)
RewriteRule ^(.*)$ index.php/$1 [L]
Part of my core apache config file (sites-enabled/portal) :
<VirtualHost *:443>
ServerAdmin webmaster#localhost
ServerName portal.com
ServerAlias www.portal.com
DocumentRoot /home/example/portal/CodeIgniter_2.1.0
DirectoryIndex "index.php"
SSLEngine On
SSLCertificateFile "ssl/portal.com.crt"
SSLCertificateKeyFile "ssl/portal.com.key"
<Directory /home/example/portal/CodeIgniter_2.1.0>
Options FollowSymLinks MultiViews
AllowOverride All
Order allow,deny
allow from all
Header unset Server
ServerSignature Off
</Directory>
</VirtualHost>
Now the real problem is when I open http://example.com/portal the browser is looking for the images in the DocumentRoot and not in Alias.
e.g. for image from portal,
<img src="/images/example.png" style="margin-left:30px;height:50px;">
apache error log says -
File does not exist: /home/example/WebSite2.0/WebContent/images/example.png
I would hate to make changes to my code. I just want to get this thing working from the apache config file itself. Please help me do this.

Answer 1

RewriteBase /portal require that the url should begin with /portal. So:
RewriteCond $1 !^(index\.php|css|images|robots\.txt)
will not be hit.
<img src="/images/example.png" style="margin-left:30px;height:50px;">
will try to search file from DocumentRoot.
update1
For there is RewriteBase /portal, example.com/portal/images will hit the Rewrite rule, but example.com/images will not, so:
<img src="/images/example.png" style="margin-left:30px;height:50px;">
should be:
<img src="/portal/images/example.png" style="margin-left:30px;height:50px;">
update2
It is the answer given by #Hussain Tamboli himself, with:
RewriteRule /(images|js|css)/(.+)\.(.+)$ /portal/$1/$2.$3 [PT].
/images/Invoice.png will rewrite to /portal/images/Invoice.png