Mod Rewrites
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=$4
This rule rewrites URL: < http://www.yourdomain.com/cat/cars/ product/bmw >
into: < http://www.yourdomain.com/shop.cgi? cat=cars&product=bmw >
This way, you can submit static URLs (the first one in our example above) to the search engines, with visitors still being directed to a cgi script with dynamic parameters.
The two examples above are part and parcel of mod_rewrite's standard features.
Matters tend to get more sophisticated and involved, however, if you want to redirect URLs with different tags to various different web pages. This procedure will now be explained in detail.
For a practical example, let's consider three PPC campaigns with Overture. In Overture's admin center your will assign your domain's URL, followed by unique tags to track the traffic generated, e.g.:
http://www.yourdomain.com/?ov1
http://www.yourdomain.com/?ov2
http://www.yourdomain.com/?ov3
Overture actually recommends tracking URLs. However, we don't want to restrict ourselves to mere traffic analysis, we also want to guide visitors to different pages.
To this effect, the following ".htaccess" file entries are generated:
RewriteEngine on Options +FollowSymlinks RewriteBase / RewriteCond %{QUERY_STRING} ^ov1$ 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.
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:fntecheditor@fantomaster.com (c) copyright 2002 by fantomaster.com
You can reprint this article (if not stated otherwise above) on your website or publication with notice "Reprinted from Zongoo! Computers".
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