Niall's Weblog

I was converting some mod_rewrite rules from the Lighttpd webserver to Apache today.

While Lighttpd and Apache both have request rewriting modules with pretty equivalent functionality, there are some significant differences nonetheless. Specifically, I was trying to rewrite a URL of the form:

/script?key=123abcxyz

to a file on the local disk:

/abc/123/123abcxyz

In Lighttpd, I had a single rewrite rule handling both the URL and its query string:

"^/script?key=(([0-9a-f]{3})([0-9a-f]{3}).*)" => "/$2/$3/$1"

The regular expression might be a little confusing at first, but its reasonably straight forward.

My first thought was that I could do pretty much exactly the same thing with an Apache RewriteRule, like so:

RewriteRule ^/script?key=(([0-9a-f]{3})([0-9a-f]{3}).*) /$2/$3/$1

Unfortunately this won’t work – the RewriteRule will only be passed the URL – that is, /script without the query string (?key=foo).

So how do you make the RewriteRule aware of the value of the query string to rewrite to the local on-disk file correctly? You must have a RewriteCond directive preceeding the RewriteRule. RewriteCond can run a grouping regular expression over the query string, and RewriteRule can pull those groups out of the previous RewriteCond with a special syntax:

RewriteCond %{QUERY_STRING} key=(([0-9a-f]{3})([0-9a-f]{3}).*)
RewriteRule ^/script /%2/%3/%1

So there you are – how to have an Apache RewriteRule operate on parts of the query string as well as on the URL. The solution ends up being a little more convoluted under Apache than under Lighttpd, but is still manageable.