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Module mod_rewrite:
Rewriting URLs With Query Strings
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by
Dirk Brockhausen
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The Apache server's module mod_rewrite is typically
used to rewrite one URL to turn it into another one.

Example:
--------
RewriteRule ^index\.html$ homepage.html

"^index\.html$" is a regular expression.
"^" represents the beginning and "$" the end of a
string.
The dot "." in a regular expression is a meta symbol
(wildcard) and signifies any random character.
If you want to use an actual, real period/dot "."
character, you will have to mask it with a preceding
backslash.

The next example is slightly more complicated:

RewriteRule ^(.*)/(.*)/(.*)/(.*)$ shop.cgi?$1=$2&$3=$4

This rule rewrites URL:
< http://www.yourdomain.com/cat/cars/product/bmw >

into:
< http://www.yourdomain.com/shop.cgi?cat=cars&product=bmw >

This way, you can submit static URLs (the first one
in our example above) to the search engines, with
visitors still being directed to a cgi script with
dynamic parameters.

The two examples above are part and parcel of
mod_rewrite's standard features.

Matters tend to get more sophisticated and involved,
however, if you want to redirect URLs with different
tags to various different web pages. This procedure
will now be explained in detail.

For a practical example, let's consider three PPC
campaigns with Overture. In Overture's admin center
your will assign your domain's URL, followed by unique
tags to track the traffic generated, e.g.:

http://www.yourdomain.com/?ov1
http://www.yourdomain.com/?ov2
http://www.yourdomain.com/?ov3

Overture actually recommends tracking URLs. However, we
don't want to restrict ourselves to mere traffic
analysis, we also want to guide visitors to different
pages.

To this effect, the following ".htaccess" file entries
are generated:

RewriteEngine on
Options +FollowSymlinks
RewriteBase /
RewriteCond %{QUERY_STRING} ^ov1$
RewriteRule ^$ /product1.html [L]
RewriteCond %{QUERY_STRING} ^ov2$
RewriteRule ^$ /product2.html [L]
RewriteCond %{QUERY_STRING} ^ov3$
RewriteRule ^$ /product3.html [L]

This code example makes use of rewrite conditions by
analyzing the query string, i.e, everything following
the question mark "?" character.

Our line:

RewriteCond %{QUERY_STRING} ^ov1$

is made up of the following three parts:

Directive: "RewriteCond"
TestString: "%{QUERY_STRING}"
CondPattern: "^ov1$"

TestString is a server variable. CondPattern is a
regular expression. "^" represents the beginning and
"$" the end of a string.

It follows that the query string must exactly be "ov1"
and nothing else. For example, "ov11", would not
work: in this case, the condition would not be met.

The next line:

RewriteRule ^$ /product1.html [L]

will rewrite the URL "^$" into the new URL
"/product1.html".

The regular expression "^$" matches, if there is no
file name included in the URL. This is exactly the case
for these URLs:

http://www.yourdomain.com/?ov1
http://www.yourdomain.com/?ov2
http://www.yourdomain.com/?ov3

Here, we only have the domain name, followed by
"/?ov1", etc. Thus, there is no file name included.

A URL like the following:

http://www.yourdomain.com/page1.html?ov1

will not match the condition.

The [L] flag stops the rewriting process
if a condition matches.

You can achieve an even shorter form by making use of
backreferences, e.g.:

RewriteEngine on
Options +FollowSymlinks
RewriteBase /
RewriteCond %{QUERY_STRING} ^ov(1)$ [or]
RewriteCond %{QUERY_STRING} ^ov(2)$ [or]
RewriteCond %{QUERY_STRING} ^ov(3)$
RewriteRule ^$ /product%1.html? [L]

By adding the brackets in "ov(1)", etc. the value
included within brackets can be stored in a variable.
This variable can then be used for further processing.

In our second example further above:

RewriteRule ^(.*)/(.*)/(.*)/(.*)$ shop.cgi?$1=$2&$3=$4

we worked with backreferences already. Here, the
variables were "$1", "$2", etc.

If you want to make use of variables from a RewriteCond
entry in the following RewriteRule, the variables have
to adhere to the syntax "%1", "%2", etc.

There's a further detail to be found in the expression
"/product%1.html?": the trailing question mark "?"
character will inhibit transfer of the the query string
from the old to the new URL. Without this trailing "?"
character, we would get the following results:

old URL - http://www.yourdomain.com/?ov1
new URL - http://www.yourdomain.com/product1.html?ov1

As you can see, the query string "ov1" would be added
to the new URL, which is not desirable.

The above goes to illustrate how you can make use of
very sparse mod_rewrite syntax to efficiently
manipulate URLs for your SEO campaigns.

This text may freely be republished or distributed in unmodified form provided the following resource box is included intact either at the beginning or the end of the article and a complimentary copy or notice (link) is sent to the author at the address specified below:

Ralph Tegtmeier is the co-founder and principal of fantomaster.com GmbH (Belgium), < http://fantomaster.com/ >, a company specializing in webmasters software development, industrial-strength cloaking and search engine positioning services.

He has been a web marketer since 1994 and is editor-in-chief of fantomNews, a free newsletter focusing on search engine optimization, available at: < http://fantomaster.com/fantomnews-sub.html > You can contact him at mailto:fneditor@fantomaster.com
(c) copyright 2002 by fantomaster.com
All rights reserved.
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