Rewriter: ability to replace substrings

[Volker Mische <vo...@gmail.com>]

Hi devs,

I wanted to rewrite URL beyond that what's currently possible with the 
CouchDB rewrite. I especially had in mind URLs where that need to 
conform to a certain standard and where you need a proxy in front of it 
to serve them up in that specific way. I have two use cases in mind.

First one with for wikis with subpages. You might want to have URLs like:
http://example.com/wiki/apache-projects/couchdb/info
as opposed to
http://example.com/wiki/apache-projects%2fcouchd%2f/info

While I tried this with JChris' Pages wiki, I found out that if a _show 
function is used, you can also have such URLs with the current rewriter 
(though directly accessing the documents with non URL encoded slashes 
won't be possible).

My second one is a TMS tilecache [1]. Those who don't know what it is, 
it's for serving up map tiles like Google or OpenStreetmap does. I would 
create a document per tile and attach the file to it. My URL would look 
like this (where "tile" is the attachement):
http://example.com/tilecache/3-5-2/tile
But I'd need an URL like
http://example.com/tilecache/tiles/3/5/2.png
(resp. 
http://example.com/tilecache/_design/tilecache/_rewrite/tiles/6/9/9.png)

My modification to the rewriter allows you to replace substrings with 
other ones. Here's the rewriter rule for this example:
{
     "from" : "/tiles/<x>/<y>/<z>.png",
     "to" : "..\\/..\\/:x/:y/:z/tile",
     "replace": {
         "old": "/",
         "new": "-",
         "limit": -1
     }
}

It replaces all occurences of "/" with "-" on the "to"-path. Two slashes 
("\\") escape a character, so it won't be replaced. The "limit" 
parameter specifies how many occurences should be replaced. It could be 
either positive, then it will just count how often a string was already 
replaced and stop at the limit. If "limit" is negative, it will count 
from the back. This is especially useful if you want to replace slashes 
and know how many slashes the attachment has, but not how many the 
document has.

The changes are available here: 
https://github.com/vmx/couchdb/commit/889024df75892fb46a1e513e54466fa5bbe69e49

Comments please :)

[1] http://wiki.osgeo.org/wiki/Tile_Map_Service_Specification

Cheers,
   Volker