RewriteMap & hashes: dealing with a hash miss

Sacha Varma <sacha@xxxxxxxxx> · Fri, 06 Feb 2009 16:05:15 +0000

(In the course of writing this e-mail I worked out the answer, but I 

thought I'd post it anyway, for future hapless rewriters such as myself.)

----

I have a website whose URLs often include an identifier for the 

underlying database record that powers the page.

Simplified example for illustrative purposes:

  /id/ABC123

The website has been redesigned, and is moving to a new domain, and all 

 the URLs have changed. The ids of the database records are preserved, 

 but not all the database records will exist on the new site.

I would like to rewrite URLs on the old site such that visitors to the 

old URL are redirected via an HTTP 301 (moved permanently) response to 

the new site.

I created an external DBM hash of ids to the new URLs, and configured 

Apache (2.2.0) as follows:

  RewriteMap ids dbm:/path/to/map.dbm
  RewriteRule ^/id/(.*) ${ids:$1} [R=301]

This worked fine except in the case where the id did not exist in the 

hash. I tried this:

  RewriteRule ^(/id/(.*)) ${ids:$2|$1} [R=301]

but this gives an endless redirection loop in the case of a hash miss 

(because although the URL is unchanged, the R flag causes a redirect).

I think what I need is a RewriteCond that checks whether the id is in 

the hash, but I don't know whether it's possible to express this.

----

The answer:

  RewriteCond %{REQUEST_URI} ^/id/(.*)
  RewriteCond ${ids:%1} !^$
  RewriteRule ^/id/(.*)$ ${ids:$1} [R=301]

It's a bit filthy that the first RewriteCond and the pattern in the 

RewriteRule are duplicated, but I don't know if there's a way to improve 

that.

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