Why does a $1 backreference work in a rewrite with no subgroups?

Question

Someone I'm working with committed a RewriteRule such as the following to SVN:

RewriteRule ^admin/ebay.*$ /yii.php/$1 [L]

I warned him that it may not work because there is no subgroup in the match that would correspond to the $1 backreference. It does work, and I'm perplexed. I'm pretty sure what he intended was either of the following:

RewriteRule ^admin/ebay.*$ /yii.php/$0 [L] # $0 is whole match

...or...

RewriteRule ^admin/ebay(.*)$ /yii.php/$1 [L] # $1 subgroup

Does Apache make an assumption about backreferences that I never knew about? Why does his RewriteRule (the top one) work?

Answer 1

As it turns out, the $1 really doesn't do anything here. The following works just as well:

RewriteRule ^admin/ebay.*$ /yii.php [L]

(Notice no backreference at all in the rewrite part.)

This works because Yii is looking at $_SERVER['REQUEST_URI'] to figure out what the user intended. In fact Apache was just passing empty for the $1 backreference as expected.

So no server fault here. :)

Answer 2

Try setting RewriteLog and RewriteLogLevel. The log might give you a hint about where that value is coming from.