mod_rewrite regex (too many redirects) - .htaccess

Question

I am using mod_rewrite, to convert subdomains into directory urls. (solution from here). When I explicity write a rule for one subdomain, it works perfectly:
RewriteCond %{HTTP_HOST} ^[www\.]*sub-domain-name.domain-name.com [NC]
RewriteCond %{REQUEST_URI} !^/sub-domain-directory/.*
RewriteRule ^(.*) /sub-domain-directory/$1 [L]
However, if I try to match all subdomains, it results in 500 internal error (log says too many redirects). The code is:
RewriteCond %{HTTP_HOST} ^[www\.]*([a-z0-9-]+).domain-name.com [NC]
RewriteCond %{REQUEST_URI} !^/%1/.*
RewriteRule ^(.*) /%1/$1 [L]
Can anyone suggest what went wrong and how to fix it?

Answer 1

Your second RewriteCond will never return false, because you can't use backreferences within your test clauses (they're compiled during parsing, making this impossible since no variable expansion will take place). You're actually testing for paths beginning with the literal text /%1/, which isn't what you wanted. Given that you're operating in a per-directory context, the rule set will end up being applied again, resulting in a transformation like the following:
path -> sub/path
sub/path -> sub/sub/path
sub/sub/path -> sub/sub/sub/path
...
This goes on for about ten iterations before the server gets upset and throws a 500 error. There are a few different ways to fix this, but I'm going to chose one that most closely resembles the approach you were trying to take. I'd also modify that first RewriteCond, since the regular expression is a bit flawed:
RewriteCond %{HTTP_HOST} ^([^.]+)\.example\.com$ [NC]
RewriteCond %1 !=www
RewriteCond %1#%{REQUEST_URI} !^([^#]+)#/\1/
RewriteRule .* /%1/$0 [L]
First, it checks the HTTP_HOST value and captures the subdomain, whatever it might be. Then, assuming you don't want this transformation to take place in the case of www, it makes sure that the capture does not match that. After that, it uses the regular expression's own internal backreferences to see if the REQUEST_URI begins with the subdomain value. If it doesn't, it prepends the subdomain as a directory, like you have now.
The potential problem with this approach is that it won't work correctly if you access a path beginning with the same name as the subdomain the request is sent to, like sub.example.com/sub/. An alternative is to check the REDIRECT_STATUS environment variable to see if an internal redirect has already been performed (that is, this prepending step has already occurred):
RewriteCond %{HTTP_HOST} ^([^.]+)\.example\.com$ [NC]
RewriteCond %1 !=www
RewriteCond %{ENV:REDIRECT_STATUS} =""
RewriteRule .* /%1/$0 [L]