RewriteBase 指令

RewriteBase指令顯式地設(shè)置了目錄級重寫的基準(zhǔn)URL。在下文中，你可以看見RewriteRule可以用于目錄級的配置文件中(.htaccess)并在局部范圍內(nèi)起作用，即規(guī)則實(shí)際處理的只是剝離了本地路徑前綴的一部分。處理結(jié)束后，這個路徑會被自動地附著回去。默認(rèn)值是"RewriteBase physical-directory-path"。

在對一個新的URL進(jìn)行替換時，此模塊必須把這個URL重新注入到服務(wù)器處理中。為此，它必須知道其對應(yīng)的URL前綴或者說URL基準(zhǔn)。通常，此前綴就是對應(yīng)的文件路徑。但是，大多數(shù)網(wǎng)站URL不是直接對應(yīng)于其物理文件路徑的，因而一般不能做這樣的假定! 所以在這種情況下，就必須用RewriteBase指令來指定正確的URL前綴。

例如，目錄級配置文件內(nèi)容如下：

#
#  /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 例如，
#

RewriteEngine On

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

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

上述例子中，對/xyz/oldstuff.html的請求被正確地重寫為物理的文件/abc/def/newstuff.html 。

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

RewriteCond 指令

RewriteCond指令定義了一個規(guī)則的條件，即在一個RewriteRule指令之前有一個或多個RewriteCond指令。條件之后的重寫規(guī)則僅在當(dāng)前URI與pattern匹配并且符合這些條件的時候才會起作用。

TestString是一個純文本的字符串，但是還可以包含下列可擴(kuò)展的成分：

• RewriteRule反向引用 ，引用方法是：

  $N

  (0 <= N <= 9)引用當(dāng)前(帶有若干RewriteRule指令的)RewriteCond中的與pattern匹配的分組成分(圓括號!)。
• RewriteCond反向引用 ，引用方法是：

  %N

  (1 <= N <= 9)引用當(dāng)前若干RewriteCond條件中最后符合的條件中的分組成分(圓括號!)。
• RewriteMap擴(kuò)展 ，引用方法是：

  ${mapname:key|default}

  細(xì)節(jié)請參見RewriteMap 指令。
• 服務(wù)器變量 ，引用方法是：

  %{ NAME_OF_VARIABLE }

  NAME_OF_VARIABLE可以是下表列出的字符串之一：

  HTTP headers:	connection & request:
  HTTP_USER_AGENT HTTP_REFERER HTTP_COOKIE HTTP_FORWARDED HTTP_HOST HTTP_PROXY_CONNECTION HTTP_ACCEPT	REMOTE_ADDR REMOTE_HOST REMOTE_PORT REMOTE_USER REMOTE_IDENT REQUEST_METHOD SCRIPT_FILENAME PATH_INFO QUERY_STRING AUTH_TYPE
  server internals:	date and time:	specials:
  DOCUMENT_ROOT SERVER_ADMIN SERVER_NAME SERVER_ADDR SERVER_PORT SERVER_PROTOCOL SERVER_SOFTWARE	TIME_YEAR TIME_MON TIME_DAY TIME_HOUR TIME_MIN TIME_SEC TIME_WDAY TIME	API_VERSION THE_REQUEST REQUEST_URI REQUEST_FILENAME IS_SUBREQ HTTPS

  These variables all correspond to the similarly named HTTP MIME-headers, C variables of the Apache server or struct tm fields of the Unix system. Most are documented elsewhere in the Manual or in the CGI specification. Those that are special to mod_rewrite include:

  IS_SUBREQ

  Will contain the text "true" if the request currently being processed is a sub-request, "false" otherwise. Sub-requests may be generated by modules that need to resolve additional files or URIs in order to complete their tasks.

  API_VERSION

  This is the version of the Apache module API (the internal interface between server and module) in the current httpd build, as defined in include/ap_mmn.h. The module API version corresponds to the version of Apache in use (in the release version of Apache 1.3.14, for instance, it is 19990320:10), but is mainly of interest to module authors.

  THE_REQUEST

  The full HTTP request line sent by the browser to the server (e.g., "GET /index.html HTTP/1.1"). This does not include any additional headers sent by the browser.

  REQUEST_URI

  The resource requested in the HTTP request line. (In the example above, this would be "/index.html".)

  REQUEST_FILENAME

  The full local filesystem path to the file or script matching the request.

  HTTPS

  Will contain the text "on" if the connection is using SSL/TLS, or "off" otherwise. (This variable can be safely used regardless of whether mod_ssl is loaded).

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: %{SSL:variable} where variable is the name of an SSL environment variable; this can be used whether or not mod_ssl is loaded, but will always expand to the empty string if it is not. Example: %{SSL:SSL_CIPHER_USEKEYSIZE} may expand to 128.
4. 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:".
5. There is the special format %{LA-U:variable} for look-aheads which perform an internal (URL-based) sub-request to determine the final value of variable. Use this when you want to use a variable for rewriting which is actually set later in an API phase and thus is not available at the current stage. For instance when you want to rewrite according to the REMOTE_USER variable from within the per-server context (httpd.conf file) you have to use %{LA-U:REMOTE_USER} because this variable is set by the authorization phases which come after the URL translation phase where mod_rewrite operates. On the other hand, because mod_rewrite implements its per-directory context (.htaccess file) via the Fixup phase of the API and because the authorization phases come before this phase, you just can use %{REMOTE_USER} there.
6. There is the special format: %{LA-F:variable} which performs an internal (filename-based) sub-request to determine the final value of variable. Most of the time this is the same as LA-U above.

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

Remember: CondPattern is a perl compatible regular expression with some additions:

1. You can prefix 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 lexically lower)
     Treats the CondPattern as a plain string and compares it lexically to TestString. True if TestString is lexically lower than CondPattern.
   • '>CondPattern' (is lexically greater)
     Treats the CondPattern as a plain string and compares it lexically to TestString. True if TestString is lexically greater than CondPattern.
   • '=CondPattern' (is lexically equal)
     Treats the CondPattern as a plain string and compares it lexically to TestString. True if TestString is lexically equal to CondPattern, i.e the two strings are exactly equal (character by character). If CondPattern is just "" (two quotation marks) this compares TestString to the empty string.
   • '-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 than zero.
   • '-l' (is symbolic link)
     Treats the TestString as a pathname and tests if it exists and is a symbolic link.
   • '-x' (has executable permissions)
     Treats the TestString as a pathname and tests if it exists and has execution permissions. These permissions are determined depending on the underlying OS.
   • '-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 server's performance!

   Notice

   All of these tests can also be prefixed by an exclamation mark ('!') 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 test case-insensitive, i.e., there is no difference between 'A-Z' and 'a-z' both in the expanded TestString and the CondPattern. This flag is effective only for comparisons between TestString和CondPattern. It has no effect on filesystem and subrequest checks.
• 'ornext|OR' (或 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 would have to write 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.

RewriteEngine 指令

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 the RewriteRule directives!

Note that, by default, rewrite configurations are not inherited. This means that you need to have a RewriteEngine on directive for each virtual host in which you wish to use it.

RewriteLock 指令

This directive sets the filename for a synchronization lockfile which mod_rewrite needs to communicate with RewriteMap programs. Set this lockfile to a local path (not on a NFS-mounted device) when you want to use a rewriting map-program. It is not required for other types of rewriting maps.

RewriteLog 指令

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.

RewriteLogLevel 指令

RewriteLogLevel directive sets 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.

RewriteMap 指令

RewriteMap directive defines a Rewriting Map which can be used inside rule substitution strings by the mapping-functions to insert/substitute fields through a key lookup. The source of this lookup can be of various types.

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 one of the following constructs:

${ MapName : LookupKey }
${ MapName : LookupKey | DefaultValue }

When such a construct occurs the map MapName is consulted and the key LookupKey is looked-up. If the key is found, the map-function construct is substituted by SubstValue. If the key is not found then it is substituted by DefaultValue or by the empty string if no DefaultValue was specified.

For example, you might define a RewriteMap as:

You would then be able to use this map in a RewriteRule as follows:

The following combinations for MapType和MapSource can be used:

• Standard Plain Text
  MapType: txt, MapSource: Unix filesystem path to valid regular file

  This is the standard rewriting map feature where the MapSource is a plain ASCII file containing either blank lines, comment lines (starting with a '#' character) or pairs like the following - one per line.

  MatchingKey SubstValue

  示例

  ##
  ##  map.txt -- rewriting map
  ##
  
  Ralf.S.Engelschall    rse   # Bastard Operator From Hell
  Mr.Joe.Average        joe   # Mr. Average
  

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

• Randomized Plain Text
  MapType: rnd, MapSource: Unix filesystem path to valid regular file

  This is identical to the Standard Plain Text variant above but with a special post-processing feature: After looking up a value it is parsed according to contained "|" characters which have the meaning of "or". In other words they indicate a set of alternatives from which the actual returned value is chosen randomly. For example, you might use the following map file and directives to provide a random load balancing between several back-end server, via a reverse-proxy. Images are sent to one of the servers in the 'static' pool, while everything else is sent to one of the 'dynamic' pool.

  Example:

  Rewrite map file

  ##
  ##  map.txt -- rewriting map
  ##
  
  static   www1|www2|www3|www4
  dynamic  www5|www6
  

  Configuration directives

  RewriteMap servers rnd:/path/to/file/map.txt

RewriteRule ^/(.*\.(png|gif|jpg)) http://${servers:static}/$1 [NC,P,L]
RewriteRule ^/(.*) http://${servers:dynamic}/$1 [P,L]

• Hash File
  MapType: dbm[=type], MapSource: Unix filesystem path to valid regular file

  Here the source is a binary format DBM file containing the same contents as a Plain Text format file, but in a special representation which is optimized for really fast lookups. The type can be sdbm, gdbm, ndbm, or db depending on compile-time settings. If the type is omitted, the compile-time default will be chosen. You can create such a file with any DBM tool or with the following Perl script. Be sure to adjust it to create the appropriate type of DBM. The example creates an NDBM file.

  #!/path/to/bin/perl
  ##
  ##  txt2dbm -- convert txt map to dbm format
  ##
  
  use NDBM_File;
  use Fcntl;
  
  ($txtmap, $dbmmap) = @ARGV;
  
  open(TXT, "<$txtmap") or die "Couldn't open $txtmap!\n";
  tie (%DB, 'NDBM_File', $dbmmap,O_RDWR|O_TRUNC|O_CREAT, 0644)
    or die "Couldn't create $dbmmap!\n";
  
  while (<TXT>) {
    next if (/^\s*#/ or /^\s*$/);
    $DB{$1} = $2 if (/^\s*(\S+)\s+(\S+)/);
  }
  
  untie %DB;
  close(TXT);
  

  $ txt2dbm map.txt map.db

• Internal Function
  MapType: int, MapSource: Internal Apache function

  Here the source is an internal Apache function. Currently you cannot create your own, but the following functions already exists:

  • toupper:
    Converts the looked up key to all upper case.
  • tolower:
    Converts the looked up key to all lower case.
  • escape:
    Translates special characters in the looked up key to hex-encodings.
  • unescape:
    Translates hex-encodings in the looked up key back to special characters.
• External Rewriting Program
  MapType: prg, MapSource: Unix filesystem path to valid regular file

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

  This program is started once at startup of the Apache servers and then communicates with the rewriting engine over its stdin和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>) {
      # ...put here any transformations or lookups...
      print $_;
  }
  

  But be very careful:

  1. "Keep it simple, stupid" (KISS), because if this program hangs it will hang 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...
  3. Use the RewriteLock directive to define a lockfile mod_rewrite can use to synchronize the communication to the program. By default no such synchronization takes place.

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.

RewriteOptions 指令

RewriteOptions directive sets some special options for the current per-server or per-directory configuration. The Option string can be currently only one:

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 are inherited. In per-directory context this means that conditions and rules of the parent directory's .htaccess configuration are inherited.

RewriteRule 指令

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 is a perl compatible 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 originally requested URL, because any number of rules may already have matched and made alterations to it.

Some hints about the syntax of regular expressions:

Text:
  .           Any single character
  [chars]     Character class: One  of chars
  [^chars]    Character class: None of chars
  text1|text2 Alternative: text1 or text2

Quantifiers:
  ?           0 or 1 of the preceding text
  *           0 or N of the preceding text (N > 0)
  +           1 or N of the preceding text (N > 1)

Grouping:
  (text)      Grouping of text
              (either to set the borders of an alternative or
              for making backreferences where the Nth group can 
              be used on the RHS of a RewriteRule with $N)

Anchors:
  ^           Start of line anchor
  $           End   of line anchor

Escaping:
  \char       escape that particular char
              (for instance to specify the chars ".[]()" etc.)

For more information about regular expressions have a look at the perl regular expression manpage ("perldoc perlre"). If you are interested in more detailed information about regular expressions and their variants (POSIX regex etc.) have a look at the following dedicated book on this topic:

Mastering Regular Expressions, 2nd Edition
Jeffrey E.F. Friedl
O'Reilly & Associates, Inc. 2002
ISBN 0-596-00289-0

Additionally in mod_rewrite 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 this pattern". This can be used for exceptional cases, where it is easier to match the negative pattern, or as a last default rule.

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. back-references $N to the RewriteRule pattern
2. back-references %N to the last matched RewriteCond pattern
3. server-variables as in rule condition test-strings (%{VARNAME})
4. mapping-function calls (${mapname:key|default})

Back-references are $N (N=0..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, 例如，in conjunction with the C (chain) flag to be able to have more than one pattern to be applied before a substitution occurs.

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:

• 'chain|C' (chained with next rule)
  This flag chains the current rule with the next rule (which itself can be chained with the 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!).
• 'cookie|CO=NAME:VAL:domain[:lifetime[:path]]' (set cookie)
  This sets a cookie on the client's browser. The cookie's name is specified by NAME and the value is VAL. The domain field is the domain of the cookie, such as '.apache.org',the optional lifetime is the lifetime of the cookie in minutes, and the optional path is the path of the cookie
• '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和%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 in many situations, but usually from within XSSI (via <!--#echo var="VAR"-->) or CGI (例如， $ENV{'VAR'}). Additionally you can dereference it in a following RewriteCond pattern via %{ENV:VAR}. Use this to strip but remember information from URLs.
• '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 pages which no longer exist as gone.
• 'handler|H=Content-handler' (force Content handler)
  Force the Content-handler of the target file to be Content-handler. For instance, this can be used to simulate the mod_alias directive ScriptAlias which internally forces all files inside the mapped directory to have a handler of "cgi-script".
• '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. For example, use it to rewrite the root-path URL ('/') to a real one, 例如，'/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 an infinite loop!
• 'nocase|NC' (no case)
  This makes the Pattern case-insensitive, i.e., there is no difference between 'A-Z' and 'a-z' when Pattern is matched against the current URL.
• 'noescape|NE' (no URI escaping of output)
  This flag keeps mod_rewrite from applying the usual URI escaping rules to the result of a rewrite. Ordinarily, special characters (such as '%', '$', ';', and so on) will be escaped into their hexcode equivalents ('%25', '%24', and '%3B', respectively); this flag prevents this from being done. This allows percent symbols to appear in the output, as in

  RewriteRule /foo/(.*) /bar?arg=P1\%3d$1 [R,NE]

  which would turn '/foo/zed' into a safe request for '/bar?arg=P1=zed'.
• '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.

• '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 (例如，typically starting with http://hostname) 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 ProxyPass directive, to map some remote stuff into the namespace of the local server.

  注意：mod_proxy must be enabled in order to 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.

  Note: 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和mod_rewrite..

• '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.
• '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 give it back to the client, 例如，translate "/~" into "/u/" or always append a slash to /u/user, etc.
  

  Note: 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]/, rewriting continues. Usually you also want to stop and do the redirection immediately. To stop the rewriting you also have to provide the 'L' flag.

• '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 skip=N where N is the number of rules in the else-clause. (This is not the same as the 'chain|C' flag!)
• '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 setup the content-type based on some conditions. For example, the following snippet allows .php files to be displayed by mod_php if they are called with the .phps extension:

  RewriteRule ^(.+\.php)s$ $1 [T=application/x-httpd-php-source]

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 /path/to/file/map.txt. Then we only have to add the following lines to the Apache server configuration file:

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