(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.)
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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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