Is it possible to use a mod_rewrite server variable inside a RewriteCond's CondPattern?

Question

Using mod_rewrite, I'd like to use a Server Variable as part of the CondPattern of a RewriteCond, something like:

RewriteCond %{HTTP_HOST} !^%{SERVER_NAME} [NC]
                           ^^^^^^^^^^^^^^

This would be really useful. I know that the CondPattern is a Perl-compatible Regular Expression, and that that means for example the {} chars in my example would need to be escaped somehow. I've left it as-is simply to demonstrate the idea.

The aforementioned docs don't mention anything about it as far as I can find, and googling around I wasn't able to find anything definitive. My hunch is the answer is no, but I'd like to know for sure.

Answer 1

CondPattern does not expand %{VAR} variables or %N backreferences to the previous RewriteCond, so David's suggestion

RewriteCond %{SERVER_NAME} ^(.*)$
RewriteCond %{HTTP_HOST} !%1

will succeed whenever %{HTTP_HOST} does not contain the literal string %1, which is not what you want.

What you want, according to your example

RewriteCond %{HTTP_HOST} !^%{SERVER_NAME} [NC]

is to match if HTTP_HOST does not start with SERVER_NAME (case-insensitive). You can implement this with a single RewriteCond if you include SERVER_NAME in the TestString, capture it in the regex, and use \N to reference it again:

RewriteCond "%{SERVER_NAME} %{HTTP_HOST}" "!(^[^ ]*) \1" [NC]

The regex uses a space to separate SERVER_NAME and HTTP_HOST because neither variable can have spaces.

Apache expressions (version 2.4 and later)

If you actually just want to test if SERVER_NAME equals HTTP_HOST, it would be simpler to use Apache expressions:

# Case-sensitive comparison.
RewriteCond expr "%{HTTP_HOST} == %{SERVER_NAME}"

or

# Case-insensitive comparison.
RewriteCond expr "tolower(%{HTTP_HOST}) == tolower(%{SERVER_NAME})"