.htaccess/mod_rewrite: West <--- Main Site ---> East - .htaccess

Question

I would really appreciate anyone's help with a mod_rewrite question. I don't know regex and am not familiar with .htaccess directives, so I can't think of how to solve the problem I am having.
Below is a description of what I am trying to accomplish...
3 websites:
http://www.example.com/ or http://example.com/
http://east.example.com/
http://west.example.com/
Have pages on main site that need to be redirected to other sites:
http://www.example.com/location1
http://www.example.com/location2
http://www.example.com/location3
http://www.example.com/location4
http://www.example.com/location5
http://www.example.com/location6
Need to redirect some pages to one website based on location, e.g http://east.example.com/:
http://www.example.com/location1 redirect to http://east.example.com/location1
http://www.example.com/location2 redirect to http://east.example.com/location2
http://www.example.com/location3 redirect to http://east.example.com/location3
(also without www) e.g.
http://example.com/location1 redirect to http://east.example.com/location1
Need to redirect some pages to another website based on location, e.g http://west.example.com/:
http://www.example.com/location4 redirect to http://west.example.com/location4
http://www.example.com/location5 redirect to http://west.example.com/location5
http://www.example.com/location6 redirect to http://west.example.com/location6
(also without www) e.g.
http://example.com/location4 redirect to http://west.example.com/location4
Want a rewrite rule that works like:
If domain name is www.example.com or domain name is example.com
And domain name is not east.example.com
And domain name is not west.example.com
And "somelocation" (in the east) is in the URL
Then Rewrite(Redirect) URL to http://east.example.com/.../somelocation/.../somepage
NOTE: the three websites actually share the same codebase, so the URLs all point to the same location/.htaccess file. So, I tried using a basic redirect, and ended up with an error that said... can't open page because of too many redirects.
If anyone knows the answer and can help with this, I would really appreciate the help!
EDIT: Example of expected results
Original URL:
"http://www.example.com/locations/eastend-web-page"
Rewritten URL:
"http://east.example.com/locations/eastend-web-page"

Answer 1

Please try to fill a rewritemap file (see here) to make a correspondance with the URLs that are part of your "east" domain. You may declare it like this:
RewriteMap mapmaintoeast \
dbm:/web/htdocs/yoursite/rewriterules/mapmaintoeast.map
Regarding your sample, your map file may be filled with things like:
eastlocation1 mainlocation1
eastlocation2 mainlocation2
eastlocation3 mainlocation3
...
I've called them 'east' and 'main' to distinguish clearly. These examples are supposed to be URLs.
Do the same for west:
RewriteMap mapmaintowest \
dbm:/web/htdocs/yoursite/rewriterules/mapmaintowest.map
In that file:
westlocation1 mainlocation4
westlocation2 mainlocation5
westlocation3 mainlocation6
...
Then here you go for the hard part:
# This cond put any non-empty string into "%1" (= parenthesis + first cond):
RewriteCond %{QUERY_STRING} (.+)
# The following rule doesn't touch the URL, but
# will try to search into the map file and
# create fill an environment variable called MAINTOEAST with the
# string found and if not found, assign MAINTOEAST to "notfound"
RewriteRule . - [QSA,E=MAINTOEAST:${mapmaintoeast:%1|notfound}]
# if MAINTOEAST not empty and different from "notfound":
RewriteCond %{ENV:MAINTOEAST} !^$
RewriteCond %{ENV:MAINTOEAST} !(notfound)
# ok found => redirect to east:
RewriteRule (.*) http://east.example.com$1 [QSA,R=301,L]
# Now do the same with west (no comments needed (see previous)):
RewriteCond %{QUERY_STRING} (.+)
RewriteRule . - [QSA,E=MAINTOWEST:${mapmaintowest:%1|notfound}]
RewriteCond %{ENV:MAINTOWEST} !^$
RewriteCond %{ENV:MAINTOWEST} !(notfound)
RewriteRule (.*) http://west.example.com$1 [QSA,R=301,L]
This should work.
I hope I've given you enough clues to finish the job ;)
If you don't have enough clues...
Two hints:
Please try to use the RewriteLog directive: it helps you to track down such problems:
# Trace:
# (!) file gets big quickly, remove in prod environments:
RewriteLog "/web/logs/mywebsite.rewrite.log"
RewriteLogLevel 9
RewriteEngine On
My favorite tool to check for regexp:
http://www.quanetic.com/Regex (don't forget to choose ereg(POSIX) instead of preg(PCRE)!)

Answer 2

RewriteEngine On
# If domain name is www.example.com or domain name is example.com
RewriteCond %{HTTP_HOST} ^(www\.)?example.com$ [NC]
# And "somelocation" (in the east) is in the URL,
# Then Rewrite(Redirect) URL to http://east.example.com/.../somelocation/.../somepage
RewriteRule ^(.*somelocation.*)$ http://east.example.com/$1 [R,L]
You'd need to do the same thing for west. Note that if the domain name is www.example.com or example.com, it cannot be east.example.com or west.example.com