I've been asked a few times about how mod_rewrite rules are done, so I thought that I would do a brief "primer" on how it works and some examples on how to implement it. Mod_rewrite'd URL's are a commonly used method of transforming url's into something like http://example.com/category/article-description/


Benefits:

• Search Engine Friendly - search engines such as google prefer ordinary looking URL's with no query string, you'll definitely make their job easier indexing your pages.
• You can swap technologies without breaking links, for example articles/42 could link to a page in any language (whether it be PHP, ruby, python, et all).
• Making urls "human" friendly is far easier - articles/category/article-name is far nicer to read than articles.php?c=45&id=2

How it works:

The mod_rewrite engine sits between the address typed in, and the "webserver". It intercepts and "silently" relays the "matched" URL.

Breakdown of Regular Expressions and mod_rewite symbols:

Here's a small table describing some of the symbols and their meanings in the regular expressions we're using. This not an exhaustive list, just a few commonly used symbols. 

You'll need to edit your .htaccess file for this, it is a file called ".htaccess" located in your public_html (or web root directory) . Create it if it's not there. The first line simply starts the mod_rewrite engine, it's only needed once at the top of your file.

The above rule matches any url that "starts" with "articles" and also ends with "articles". It then "silently" redirects to articles.php if a match is found. It will not match "test-articles", or "articles_january" for example.

In the next example, we'll pass along to the server, a query sting. As you can see, this will match articles/1 or articles/2 and redirect it to articles.php?id=1. The second example, adds /? which means articles/1/ with a trailing slash would also work.

Getting a bit more tricky, you could add another argument to the above examples to add a "page" argument:

You can of course, replace [0-9] with one of the symbols in the table above, to match letters, or other characters.

Tips and Tricks:

There are a few nice tips and tricks you can do using mod_rewrite. Perhaps, you want to force everyone to use your website without the www. You could add the following rule to do that: Note that ! effectively means "not", and [R] means, redirect. RewriteCond is a way that you can apply an "if this, do something" type rule.

Suppose you wanted to redirect a certain page to secure https:// you could use a rule such as:

Summary:

I hope that this article has provided a good introduction how mod_rewrite can work for you, and the very basics of regular expressions. If you have any other tricks or tips you wish to share, please feel free to comment!