The RewriteCond directive defines a rule condition. Precede a RewriteRule directive with one ore more RewriteCond directives. The following rewriting rule is only used if its pattern matches the current state of the URI AND if these additional conditions apply, too.

TestString is a string which contains server-variables of the form

%{ NAME_OF_VARIABLE }

HTTP headers: HTTP_USER_AGENT HTTP_REFERER HTTP_COOKIE HTTP_FORWARDED HTTP_HOST HTTP_PROXY_CONNECTION HTTP_ACCEPT	connection & request: REMOTE_ADDR REMOTE_HOST REMOTE_USER REMOTE_IDENT REQUEST_METHOD SCRIPT_FILENAME PATH_INFO QUERY_STRING AUTH_TYPE
server internals: DOCUMENT_ROOT SERVER_ADMIN SERVER_NAME SERVER_PORT SERVER_PROTOCOL SERVER_SOFTWARE SERVER_VERSION	system stuff: TIME_YEAR TIME_MON TIME_DAY TIME_HOUR TIME_MIN TIME_SEC TIME_WDAY	specials: API_VERSION THE_REQUEST REQUEST_URI REQUEST_FILENAME

These variables all correspond to the similar named HTTP MIME-headers, C variables of the Apache server or struct tm fields of the Unix system.

Special Notes:

1. The variables SCRIPT_FILENAME and REQUEST_FILENAME contain the same value, i.e. the value of the filename field of the internal request_rec structure of the Apache server. The first name is just the commonly known CGI variable name while the second is the consistent counterpart to REQUEST_URI (which contains the value of the uri field of request_rec).

2. There is an additional format: %{ENV:variable} where variable can be any Unix environment variable. This is looked-up via getenv() from the Apache server process.

3. There is one more additional format: %{HTTP:header} where header can be any HTTP MIME-header name. This is looked-up from the HTTP request. Example: %{HTTP:Proxy-Connection} is the value of the HTTP header ``Proxy-Connection:''.

CondPattern is the condition pattern, i.e. a regular expression which gets applied to the current instance of the TestString, i.e. TestString gets evaluated and then matched against CondPattern.

Remember: CondPattern is a standard Extended Regular Expression with some additions:

1. You can precede the pattern string with a '!' character (exclamation mark) to specify a non-matching pattern.

2. There are some special variants of CondPatterns. Instead of real regular expression strings you can also use one of the following:
   • '-d' (is directory)
     Treats the TestString as a pathname and tests if it exists and is a directory.

   • '-f' (is regular file)
     Treats the TestString as a pathname and tests if it exists and is a regular file.

   • '-s' (is regular file with size)
     Treats the TestString as a pathname and tests if it exists and is a regular file with size greater then zero.

   • '-l' (is symbolic link)
     Treats the TestString as a pathname and tests if it exists and is a symbolic link.

   Notice: All of these tests can also be prefixed by a not ('!') character to negate their meaning.

Additionally you can set special flags for CondPattern by appending

[flags]

• 'nocase|NC' (no case)
  This makes the condition test case-insensitive, i.e. there is no difference between 'A-Z' and 'a-z' both in the expanded TestString and the CondPattern.

• 'ornext|OR' (or next condition)
  Use this to combine rule conditions with a local OR instead of the implicit AND. Typical example:

  RewriteCond %{REMOTE_HOST}  ^host1.*  [OR]
  RewriteCond %{REMOTE_HOST}  ^host2.*  [OR]
  RewriteCond %{REMOTE_HOST}  ^host3.*  
  RewriteRule ...some special stuff for any of these hosts...
  

  Without this flag you had to write down the cond/rule three times.

Example:

To rewrite the Home-page of a site according to the ``User-Agent:'' header of the request, you can use the following:

RewriteCond  %{HTTP_USER_AGENT}  ^Mozilla.*
RewriteRule  ^/$                 /homepage.max.html  [L]

RewriteCond  %{HTTP_USER_AGENT}  ^Lynx.*
RewriteRule  ^/$                 /homepage.min.html  [L]

RewriteRule  ^/$                 /homepage.std.html  [L]

Interpretation: If you use Netscape Navigator as your browser (which identifies itself as 'Mozilla'), then you get the max homepage, which includes Frames, etc. If you use the Lynx browser (which is Terminal-based), then you get the min homepage, which contains no images, no tables, etc. If you use any other browser you get the standard homepage.

RewriteRule

The RewriteRule directive is the real rewriting workhorse. The directive can occur more than once. Each directive then defines one single rewriting rule. The definition order of these rules is important, because this order is used when applying the rules at run-time.

Pattern can be (for Apache 1.1.x a System V8 and for Apache 1.2.x a POSIX) regular expression which gets applied to the current URL. Here ``current'' means the value of the URL when this rule gets applied. This may not be the original requested URL, because there could be any number of rules before which already matched and made alterations to it.

Some hints about the syntax of regular expressions:

^ Start of line $ End of line . Any single character [chars] One of chars [^chars] None of chars ? 0 or 1 of the preceding char * 0 or N of the preceding char + 1 or N of the preceding char \char escape that specific char (e.g. for specifying the chars ".[]()" etc.) (string) Grouping of chars (the Nth group can be used on the RHS with $N)

Additionally the NOT character ('!') is a possible pattern prefix. This gives you the ability to negate a pattern; to say, for instance: ``if the current URL does NOT match to this pattern''. This can be used for special cases where it is better to match the negative pattern or as a last default rule.

Notice! When using the NOT character to negate a pattern you cannot have grouped wildcard parts in the pattern. This is impossible because when the pattern does NOT match, there are no contents for the groups. In consequence, if negated patterns are used, you cannot use $N in the substitution string!

Substitution of a rewriting rule is the string which is substituted for (or replaces) the original URL for which Pattern matched. Beside plain text you can use

1. pattern-group back-references ($N)
2. server-variables as in rule condition test-strings (%{VARNAME})
3. mapping-function calls (${mapname:key|default})

As already mentioned above, all the rewriting rules are applied to the Substitution (in the order of definition in the config file). The URL is completely replaced by the Substitution and the rewriting process goes on until there are no more rules (unless explicitly terminated by a L flag - see below).

There is a special substitution string named '-' which means: NO substitution! Sounds silly? No, it is useful to provide rewriting rules which only match some URLs but do no substitution, e.g. in conjunction with the C (chain) flag to be able to have more than one pattern to be applied before a substitution occurs.

Notice: There is a special feature. When you prefix a substitution field with http://thishost[:thisport] then mod_rewrite automatically strips it out. This auto-reduction on implicit external redirect URLs is a useful and important feature when used in combination with a mapping-function which generates the hostname part. Have a look at the first example in the example section below to understand this. Remember: An unconditional external redirect to your own server will not work with the prefix http://thishost because of this feature. To achieve such a self-redirect, you have to use the R-flag (see below).

Additionally you can set special flags for Substitution by appending

[flags]

• 'redirect|R' (force redirect)
  Prefix Substitution with http://thishost/ (which makes the new URL a URI) to force a external redirection. Use it for rules which should canonicalize the URL and gives it back to the client, e.g. translate ``/~'' into ``/u/'' or always append a slash to /u/user, etc.
  Notice: When you use this flag, make sure that the substitution field is a valid URL! If not, you are redirecting to an invalid location!

• 'proxy|P' (force proxy)
  This flag forces the substitution part to be internally forced as a proxy request and immediately (i.e. rewriting rule processing stops here) put through the proxy module. You have to make sure that the substitution string is a valid URI (e.g. typically http://) which can be handled by the Apache proxy module. If not you get an error from the proxy module. Use this flag to achieve a more powerful implementation of the mod_proxy directive ProxyPass, to map some remote stuff into the name-space of the local server.

• 'last|L' (last rule)
  Stop the rewriting process here and don't apply any more rewriting rules. This corresponds to the Perl last command or the break command from the C language. Use this flag to prevent the currently rewritten URL from being rewritten further by following rules which may be wrong. For example, use it to rewrite the root-path URL ('/') to a real one, e.g. '/e/www/'.

• 'next|N' (next round)
  Re-run the rewriting process (starting again with the first rewriting rule). Here the URL to match is again not the original URL but the URL from the last rewriting rule. This corresponds to the Perl next command or the continue command from the C language. Use this flag to restart the rewriting process, i.e. to immediately go to the top of the loop.
  But be careful not to create a deadloop!

• 'chain|C' (chained with next rule)
  This flag chains the current rule with the next rule (which itself can also be chained with its following rule, etc.). This has the following effect: if a rule matches, then processing continues as usual, i.e. the flag has no effect. If the rule does not match, then all following chained rules are skipped. For instance, use it to remove the ``.www'' part inside a per-directory rule set when you let an external redirect happen (where the ``.www'' part should not to occur!).

• 'type|T=mime-type' (force MIME type)
  Force the MIME-type of the target file to be mime-type. For instance, this can be used to simulate the old mod_alias directive ScriptAlias which internally forces all files inside the mapped directory to have a MIME type of ``application/x-httpd-cgi''.

• 'nosubreq|NS' (used only if no internal sub-request)
  This flag forces the rewriting engine to skip a rewriting rule if the current request is an internal sub-request. For instance, sub-requests occur internally in Apache when mod_include tries to find out information about possible directory default files (index.xxx). On sub-requests it is not always useful and even sometimes causes a failure to if the complete set of rules are applied. Use this flag to exclude some rules.
  

  Use the following rule for your decision: whenever you prefix some URLs with CGI-scripts to force them to be processed by the CGI-script, the chance is high that you will run into problems (or even overhead) on sub-requests. In these cases, use this flag.

• 'passthrough|PT' (pass through to next handler)
  This flag forces the rewriting engine to set the uri field of the internal request_rec structure to the value of the filename field. This flag is just a hack to be able to post-process the output of RewriteRule directives by Alias, ScriptAlias, Redirect, etc. directives from other URI-to-filename translators. A trivial example to show the semantics: If you want to rewrite /abc to /def via the rewriting engine of mod_rewrite and then /def to /ghi with mod_alias:

      RewriteRule ^/abc(.*)  /def$1 [PT]
      Alias       /def       /ghi   
      

  If you omit the PT flag then mod_rewrite will do its job fine, i.e. it rewrites uri=/abc/... to filename=/def/... as a full API-compliant URI-to-filename translator should do. Then mod_alias comes and tries to do a URI-to-filename transition which will not work.

  Notice: You have to use this flag if you want to intermix directives of different modules which contain URL-to-filename translators. The typical example is the use of mod_alias and mod_rewrite..

  For the Apache hackers: If the current Apache API had a filename-to-filename hook additionally to the URI-to-filename hook then we wouldn't need this flag! But without such a hook this flag is the only solution. The Apache Group has discussed this problem and will add such hooks into Apache version 2.0.

• 'skip|S=num' (skip next rule(s))
  This flag forces the rewriting engine to skip the next num rules in sequence when the current rule matches. Use this to make pseudo if-then-else constructs: The last rule of the then-clause becomes a skip=N where N is the number of rules in the else-clause. (This is not the same as the 'chain|C' flag!)

Remember: Never forget that Pattern gets applied to a complete URL in per-server configuration files. But in per-directory configuration files, the per-directory prefix (which always is the same for a specific directory!) gets automatically removed for the pattern matching and automatically added after the substitution has been done. This feature is essential for many sorts of rewriting, because without this prefix stripping you have to match the parent directory which is not always possible. There is one exception: If a substitution string starts with ``http://'' then the directory prefix will be not added and a external redirect or proxy throughput (if flag P is used!) is forced!

Notice! To enable the rewriting engine for per-directory configuration files you need to set ``RewriteEngine On'' in these files and ``Option FollowSymLinks'' enabled. If your administrator has disabled override of FollowSymLinks for a user's directory, then you cannot use the rewriting engine. This restriction is needed for security reasons.

Here are all possible substitution combinations and their meanings:

Inside per-server configuration (httpd.conf)
for request ``GET /somepath/pathinfo'':

Given Rule Resulting Substitution ---------------------------------------------- ---------------------------------- ^/somepath(.*) otherpath$1 not supported, because invalid! ^/somepath(.*) otherpath$1 [R] not supported, because invalid! ^/somepath(.*) otherpath$1 [P] not supported, because invalid! ---------------------------------------------- ---------------------------------- ^/somepath(.*) /otherpath$1 /otherpath/pathinfo ^/somepath(.*) /otherpath$1 [R] http://thishost/otherpath/pathinfo via external redirection ^/somepath(.*) /otherpath$1 [P] not supported, because silly! ---------------------------------------------- ---------------------------------- ^/somepath(.*) http://thishost/otherpath$1 /otherpath/pathinfo ^/somepath(.*) http://thishost/otherpath$1 [R] http://thishost/otherpath/pathinfo via external redirection ^/somepath(.*) http://thishost/otherpath$1 [P] not supported, because silly! ---------------------------------------------- ---------------------------------- ^/somepath(.*) http://otherhost/otherpath$1 http://otherhost/otherpath/pathinfo via external redirection ^/somepath(.*) http://otherhost/otherpath$1 [R] http://otherhost/otherpath/pathinfo via external redirection (the [R] flag is redundant) ^/somepath(.*) http://otherhost/otherpath$1 [P] http://otherhost/otherpath/pathinfo via internal proxy

Inside per-directory configuration for /somepath
(i.e. file .htaccess in dir /physical/path/to/somepath containing RewriteBase /somepath)
for request ``GET /somepath/localpath/pathinfo'':

Given Rule Resulting Substitution ---------------------------------------------- ---------------------------------- ^localpath(.*) otherpath$1 /somepath/otherpath/pathinfo ^localpath(.*) otherpath$1 [R] http://thishost/somepath/otherpath/pathinfo via external redirection ^localpath(.*) otherpath$1 [P] not supported, because silly! ---------------------------------------------- ---------------------------------- ^localpath(.*) /otherpath$1 /otherpath/pathinfo ^localpath(.*) /otherpath$1 [R] http://thishost/otherpath/pathinfo via external redirection ^localpath(.*) /otherpath$1 [P] not supported, because silly! ---------------------------------------------- ---------------------------------- ^localpath(.*) http://thishost/otherpath$1 /otherpath/pathinfo ^localpath(.*) http://thishost/otherpath$1 [R] http://thishost/otherpath/pathinfo via external redirection ^localpath(.*) http://thishost/otherpath$1 [P] not supported, because silly! ---------------------------------------------- ---------------------------------- ^localpath(.*) http://otherhost/otherpath$1 http://otherhost/otherpath/pathinfo via external redirection ^localpath(.*) http://otherhost/otherpath$1 [R] http://otherhost/otherpath/pathinfo via external redirection (the [R] flag is redundant) ^localpath(.*) http://otherhost/otherpath$1 [P] http://otherhost/otherpath/pathinfo via internal proxy

Example:

We want to rewrite URLs of the form

/ Language /~ Realname /.../ File

into

/u/ Username /.../ File . Language

We take the rewrite mapfile from above and save it under /anywhere/map.real-to-user. Then we only have to add the following lines to the Apache server configuration file:

RewriteLog   /anywhere/rewrite.log
RewriteMap   real-to-user               txt:/anywhere/map.real-to-host
RewriteRule  ^/([^/]+)/~([^/]+)/(.*)$   /u/${real-to-user:$2|nobody}/$3.$1