Module mod_rewrite

Rewriting URLs With Query Strings
by Dirk Brockhausen

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 % ^ov1$
RewriteRule ^$ /product1.html [L]
RewriteCond % ^ov2$
RewriteRule ^$ /product2.html [L]
RewriteCond % ^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 % ^ov1$

is made up of the following three parts:

Directive: "RewriteCond"
TestString: "%"
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 % ^ov(1)$ [or]
RewriteCond % ^ov(2)$ [or]
RewriteCond % ^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.


Dirk Brockhausen is the co-founder and principal of fantomaster.com GmbH
(Belgium), a company specializing in webmasters software development,
industrial-strength cloaking and search engine positioning services. He holds a
doctorate in physics and has worked as an SAP consultant and software developer
since 1994. He is also Technical Editor of fantomNews, a free newsletter focusing
on search engine optimization, available at:
http://fantomaster.com/fantomnews-sub.html
You can contact him at mailto:-email-
(c) copyright 2002 by fantomaster.com

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