Rewriting URL with selected query string parameters in .htaccess

Question

I recently changed my CMS, and want to rewrite some of my URLs to match the new URL/query string parameter format.

The old URL was:

http://www.mysite         


        

Answer 1

Here is another version to get specifically what the OP meant. @smhmic's answer is close to this. I just prefer explicit beginning-or-ampersand to find the parameter, rather than the word-boundary approach. (The accepted answer is overly complicated, and has some issues.)

RewriteCond %{QUERY_STRING} (?:^|&)tag=([^&]*)
RewriteRule ^search\.cgi$ /?s=%1 [NC,R=302]

This is saying:
(?: ) don't capture this as a % number
^|& at beginning of querystring or after an ampersand (this acts like the word boundary, but specifically for urls).
tag= the param we're looking for.
( ) capture this, into %1.
[^&]* zero or more characters not an ampersand (and stopping at ampersand or running to the end).

Answer 2

You can try to use regular expression and grouping, if your server supports that. If I am not mistaken you have to rewrite only one parameter, I guess you could try something like (if you are using apache with mod_rewrite):

RewriteCond %{QUERY_STRING} ^.*(\btag\b=(\w+|&\w+;)+)
RewriteRule ^(.+) /$1?%1 [L]

Edit: I improved the regular expression a little bit to match "tag" regardless of its position in the query string and to preserve special characters sequences such as & In addition it should avoid matches with similar parameters (i.e.: it wouldn't match a parameter called "alttag").

Edit #2: An alternative (especially if you have to filter several parameters) is to use an external program to do the rewrite. This page: http://httpd.apache.org/docs/2.2/mod/mod_rewrite.html#rewritemap (in particular the section "External rewriting program") contains useful information.

Answer 3

On an Apache webserver, placing the following rule into .htaccess in your document root will rewrite URLs as you described:

RewriteCond %{QUERY_STRING} .*\btag=([^&]*).*
RewriteRule ^search\.cgi /?s=%1 [R=302]

The rule above uses a 302 redirect, so your browser doesn't "memorize" the redirect rule while you're testing and tweaking the rule. Once you're done testing, change the [R] to [R=301]. A 301 is a permanent redirect, and is better for SEO.