# Module mod_rewrite

This module is contained in the `mod_rewrite.c` file, with Apache 1.2 and later. It provides a rule-based rewriting engine to rewrite requested URLs on the fly. `mod_rewrite` is not compiled into the server by default. To use `mod_rewrite` you have to enable the following line in the server build Configuration file:

        Module  rewrite_module   mod_rewrite.o

## Summary

This module uses a rule-based rewriting engine (based on a regular-expression parser) to rewrite requested URLs on the fly.

It supports an unlimited number of additional rule conditions (which can operate on a lot of variables, including HTTP headers) for granular matching and external database lookups (either via plain text tables, DBM hash files or external processes) for advanced URL substitution.

It operates on the full URLs (including the PATH_INFO part) both in per-server context (httpd.conf) and per-dir context (.htaccess) and even can generate QUERY_STRING parts on result. The rewritten result can lead to internal sub-processing, external request redirection or to internal proxy throughput.

This module was originally written in April 1996 and gifted exclusively to the The Apache Group in July 1997 by

> Ralf S. Engelschall
> `rse@engelschall.com`
> `www.engelschall.com`

## Directives

- RewriteEngine
- RewriteOptions
- RewriteLog
- RewriteLogLevel
- RewriteMap
- RewriteBase
- RewriteCond
- RewriteRule

# Configuration Directives

### RewriteEngine

Syntax: `RewriteEngine` {`on,off`}
Default: `RewriteEngine off`
Context: server config, virtual host, per-directory config

The `RewriteEngine` directive enables or disables the runtime rewriting engine. If it is set to `off` this module does no runtime processing at all. It does not even update the `SCRIPT_URx` environment variables.

Use this directive to disable the module instead of commenting out all `RewriteRule` directives!

### RewriteOptions

Syntax: `RewriteOptions` Option ...
Default: -None-
Context: server config, virtual host, per-directory config

The `RewriteOption` directive sets some special options for the current per-server or per-directory configuration. The Option strings can be one of the following:

- '`inherit`'
  This forces the current configuration to inherit the configuration of the parent. In per-virtual-server context this means that the maps, conditions and rules of the main server gets inherited. In per-directory context this means that conditions and rules of the parent directory's `.htaccess` configuration gets inherited.

### RewriteLog

Syntax: `RewriteLog` Filename
Default: -None-
Context: server config, virtual host

The `RewriteLog` directive sets the name of the file to which the server logs any rewriting actions it performs. If the name does not begin with a slash ('`/`') then it is assumed to be relative to the Server Root. The directive should occur only once per server config.

To disable the logging of rewriting actions it is not recommended to set Filename to `/dev/null`, because although the rewriting engine does not create output to a logfile it still creates the logfile output internally. This will slow down the server with no advantage to the administrator! To disable logging either remove or comment out the `RewriteLog` directive or use `RewriteLogLevel 0`!

SECURITY: See the Apache Security Tips document for details on why your security could be compromised if the directory where logfiles are stored is writable by anyone other than the user that starts the server.

Example:

>     RewriteLog "/usr/local/var/apache/logs/rewrite.log"

### RewriteLogLevel

Syntax: `RewriteLogLevel` Level
Default: `RewriteLogLevel 0`
Context: server config, virtual host

The `RewriteLogLevel` directive set the verbosity level of the rewriting logfile. The default level 0 means no logging, while 9 or more means that practically all actions are logged.

To disable the logging of rewriting actions simply set Level to 0. This disables all rewrite action logs.

Notice: Using a high value for Level will slow down your Apache server dramatically! Use the rewriting logfile only for debugging or at least at Level not greater than 2!

Example:

>     RewriteLogLevel 3

### RewriteMap

Syntax: `RewriteMap` Mapname `{txt,dbm,prg}:`Filename
Default: not used per default
Context: server config, virtual host

The `RewriteMap` directive defines an external Rewriting Map which can be used inside rule substitution strings by the mapping-functions to insert/substitute fields through a key lookup.

The Mapname is the name of the map and will be used to specify a mapping-function for the substitution strings of a rewriting rule via

> `${` Mapname `:` LookupKey `|` DefaultValue `}`

When such a directive occurs the map Mapname is consulted and the key LookupKey is looked-up. If the key is found, the map-function directive is substituted by SubstValue. If the key is not found then it is substituted by DefaultValue.

The Filename must be a valid Unix filepath, containing one of the following formats:

1. Plain Text Format

   This is a ASCII file which contains either blank lines, comment lines (starting with a '#' character) or

   > MatchingKey SubstValue

   pairs - one per line. You can create such files either manually, using your favorite editor, or by using the programs `mapcollect` and `mapmerge` from the `support` directory of the mod_rewrite distribution.

   To declare such a map prefix, Filename with a `txt:` string as in the following example:

       #
       #   map.real-to-user -- maps realnames to usernames
       #

       Ralf.S.Engelschall    rse   # Bastard Operator From Hell
       Dr.Fred.Klabuster     fred  # Mr. DAU

       RewriteMap real-to-host txt:/path/to/file/map.real-to-user

2. DBM Hashfile Format

   This is a binary NDBM format file containing the same contents as the Plain Text Format files. You can create such a file with any NDBM tool or with the `dbmmanage` program from the `support` directory of the Apache distribution.

   To declare such a map prefix Filename with a `dbm:` string.

3. Program Format

   This is a Unix executable, not a lookup file. To create it you can use the language of your choice, but the result has to be a run-able Unix binary (i.e. either object-code or a script with the magic cookie trick '`#!/path/to/interpreter`' as the first line).

   This program gets started once at startup of the Apache servers and then communicates with the rewriting engine over its `stdin` and `stdout` file-handles. For each map-function lookup it will receive the key to lookup as a newline-terminated string on `stdin`. It then has to give back the looked-up value as a newline-terminated string on `stdout` or the four-character string ```NULL`'' if it fails (i.e. there is no corresponding value for the given key). A trivial program which will implement a 1:1 map (i.e. key == value) could be:

       #!/usr/bin/perl
       $| = 1;
       while (<STDIN>) {
           # ...here any transformations
           # or lookups should occur...
           print $_;
       }

   But be very careful:

   1. ``Keep the program simple, stupid'' (KISS), because if this program hangs it will lead to a hang of the Apache server when the rule occurs.
   2. Avoid one common mistake: never do buffered I/O on `stdout`! This will cause a deadloop! Hence the ```$|=1`'' in the above example...

   To declare such a map prefix Filename with a `prg:` string.

The `RewriteMap` directive can occur more than once. For each mapping-function use one `RewriteMap` directive to declare its rewriting mapfile. While you cannot declare a map in per-directory context it is of course possible to use this map in per-directory context.

For plain text and DBM format files the looked-up keys are cached in-core until the `mtime` of the mapfile changes or the server does a restart. This way you can have map-functions in rules which are used for every request. This is no problem, because the external lookup only happens once!

### RewriteBase

Syntax: `RewriteBase` BaseURL
Default: default is the physical directory path
Context: per-directory config

The `RewriteBase` directive explicitly sets the base URL for per-directory rewrites. As you will see below, `RewriteRule` can be used in per-directory config files (`.htaccess`). There it will act locally, i.e. the local directory prefix is stripped at this stage of processing and your rewriting rules act only on the remainder. At the end it is automatically added.

When a substitution occurs for a new URL, this module has to re-inject the URL into the server processing. To be able to do this it needs to know what the corresponding URL-prefix or URL-base is. By default this prefix is the corresponding filepath itself. But at most websites URLs are NOT directly related to physical filename paths, so this assumption will be usually be wrong! There you have to use the `RewriteBase` directive to specify the correct URL-prefix.

So, if your webserver's URLs are not directly related to physical file paths, you have to use `RewriteBase` in every `.htaccess` files where you want to use `RewriteRule` directives.

Example:

> Assume the following per-directory config file:

>     #
>     #  /abc/def/.htaccess -- per-dir config file for directory /abc/def
>     #  Remember: /abc/def is the physical path of /xyz, i.e. the server
>     #            has a 'Alias /xyz /abc/def' directive e.g.
>     #

>     RewriteEngine On

>     #  let the server know that we are reached via /xyz and not
>     #  via the physical path prefix /abc/def
>     RewriteBase   /xyz

>     #  now the rewriting rules
>     RewriteRule   ^oldstuff\.html$  newstuff.html

> In the above example, a request to `/xyz/oldstuff.html` gets correctly rewritten to the physical file `/abc/def/newstuff.html`.

> For the Apache hackers: The following list gives detailed information about the internal processing steps:
>     Request:
>       /xyz/oldstuff.html

>     Internal Processing:
>       /xyz/oldstuff.html     -> /abc/def/oldstuff.html    (per-server Alias)
>       /abc/def/oldstuff.html -> /abc/def/newstuff.html    (per-dir    RewriteRule)
>       /abc/def/newstuff.html -> /xyz/newstuff.html        (per-dir    RewriteBase)
>       /xyz/newstuff.html     -> /abc/def/newstuff.html    (per-server Alias)

>     Result:
>       /abc/def/newstuff.html
> This seems very complicated but is the correct Apache internal processing, because the per-directory rewriting comes too late in the process. So, when it occurs the (rewritten) request has to be re-injected into the Apache kernel! BUT: While this seems like a serious overhead, it really isn't, because this re-injection happens fully internal to the Apache server and the same procedure is used by many other operations inside Apache. So, you can be sure the design and implementation is correct.

### RewriteCond

Syntax: `RewriteCond` TestString CondPattern
Default: -None-
Context: server config, virtual host, per-directory config

The `RewriteCond` directive defines a rule condition. Precede a `RewriteRule` directive with one or 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 `}`

where NAME_OF_VARIABLE can be a string of the following list:

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 TIME | specials: API_VERSION THE_REQUEST REQUEST_URI REQUEST_FILENAME IS_SUBREQ

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 the special format: `%{ENV:variable}` where variable can be any environment variable. This is looked-up via internal Apache structures and (if not found there) via `getenv()` from the Apache server process.

3. There is the special 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:`''.

4. There is the special format: `%{LA-U:url}` for look-aheads like `-U`. This performs a internal sub-request to look-ahead for the final value of url.

5. There is the special format: `%{LA-F:file}` for look-aheads like `-F`. This performs a internal sub-request to look-ahead for the final value of file.

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:

   - '<CondPattern' (is lexicographically lower)
     Treats the CondPattern as a plain string and compares it lexicographically to TestString and results in a true expression if TestString is lexicographically lower then CondPattern.

   - '>CondPattern' (is lexicographically greater)
     Treats the CondPattern as a plain string and compares it lexicographically to TestString and results in a true expression if TestString is lexicographically greater then CondPattern.

   - '=CondPattern' (is lexicographically equal)
     Treats the CondPattern as a plain string and compares it lexicographically to TestString and results in a true expression if TestString is lexicographically equal to CondPattern, i.e the two strings are exactly equal (character by character).

   - '-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.

   - '-F' (is existing file via subrequest)
     Checks if TestString is a valid file and accessible via all the server's currently-configured access controls for that path. This uses an internal subrequest to determine the check, so use it with care because it decreases your servers performance!

   - '-U' (is existing URL via subrequest)
     Checks if TestString is a valid URL and accessible via all the server's currently-configured access controls for that path. This uses an internal subrequest to determine the check, so use it with care because it decreases your servers performance!

   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`]`

as the third argument to the `RewriteCond` directive. Flags is a comma-separated list of the following 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 Homepage 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

Syntax: `RewriteRule` Pattern Substitution
Default: -None-
Context: server config, virtual host, per-directory config

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}`)

Back-references are `$`N (N=1..9) identifiers which will be replaced by the contents of the Nth group of the matched Pattern. The server-variables are the same as for the TestString of a `RewriteCond` directive. The mapping-functions come from the `RewriteMap` directive and are explained there. These three types of variables are expanded in the order of the above list.

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.

One more note: You can even create URLs in the substitution string containing a query string part. Just use a question mark inside the substitution string to indicate that the following stuff should be re-injected into the QUERY_STRING. When you want to erase an existing query string, end the substitution string with just the question mark.

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`]`

as the third argument to the `RewriteRule` directive. Flags is a comma-separated list of the following flags:

- '`redirect|R`[=code]' (force redirect)
  Prefix Substitution with `http://thishost[:thisport]/` (which makes the new URL a URI) to force a external redirection. If no code is given a HTTP response of 302 (MOVED TEMPORARILY) is used. If you want to use other response codes in the range 300-400 just specify them as a number or use one of the following symbolic names: `temp` (default), `permanent`, `seeother`. 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! And remember that this flag itself only prefixes the URL with `http://thishost[:thisport]/`, but rewriting goes on. Usually you also want to stop and do the redirection immediately. To stop the rewriting you also have to provide the 'L' flag.

- '`forbidden|F`' (force URL to be forbidden)
  This forces the current URL to be forbidden, i.e. it immediately sends back a HTTP response of 403 (FORBIDDEN). Use this flag in conjunction with appropriate RewriteConds to conditionally block some URLs.

- '`gone|G`' (force URL to be gone)
  This forces the current URL to be gone, i.e. it immediately sends back a HTTP response of 410 (GONE). Use this flag to mark no longer existing pages as gone.

- '`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 namespace of the local server.

  Notice: You really have to put `ProxyRequests On` into your server configuration to prevent proxy requests from leading to core-dumps inside the Apache kernel. If you have not compiled in the proxy module, then there is no core-dump problem, because mod_rewrite checks for existence of the proxy module and if lost forbids proxy URLs.

- '`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.

- '`qsappend|QSA`' (query string append)
  This flag forces the rewriting engine to append a query string part in the substitution string to the existing one instead of replacing it. Use this when you want to add more data to the query string via a rewrite rule.

- '`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!)

- '`env|E=`VAR:VAL' (set environment variable)
  This forces an environment variable named VAR to be set to the value VAL, where VAL can contain regexp backreferences `$N` which will be expanded. You can use this flag more than once to set more than one variable. The variables can be later dereferenced at a lot of situations, but the usual location will be from within XSSI (via `<!--#echo var="VAR"-->`) or CGI (e.g. `$ENV{'VAR'}`). But additionally you can also dereference it in a following RewriteCond pattern via `%{ENV:VAR}`. Use this to strip but remember information from URLs.

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

# Additional Features

## Environment Variables

This module keeps track of two additional (non-standard) CGI/SSI environment variables named `SCRIPT_URL` and `SCRIPT_URI`. These contain the logical Web-view to the current resource, while the standard CGI/SSI variables `SCRIPT_NAME` and `SCRIPT_FILENAME` contain the physical System-view.

Notice: These variables hold the URI/URL as they were initially requested, i.e. in a state before any rewriting. This is important because the rewriting process is primarily used to rewrite logical URLs to physical pathnames.

Example:

>     SCRIPT_NAME=/v/sw/free/lib/apache/global/u/rse/.www/index.html
>     SCRIPT_FILENAME=/u/rse/.www/index.html
>     SCRIPT_URL=/u/rse/
>     SCRIPT_URI=http://en2.en.sdm.de/u/rse/