I’m Neio

from: Apache2.2_chinese_manual

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.

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. 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.

Query String

Pattern will not match against the query string. Instead, you must use a RewriteCond with the %{QUERY_STRING} variable. You can, however, 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. To combine a new query string with an old one, use the [QSA] flag (see below).

Substitution of Absolute URLs

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:

• ‘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]

注意

Never forget that Pattern is 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!) is 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 not be added and an external redirect or proxy throughput (if flag P is used!) is forced!

注意

To enable the rewriting engine for per-directory configuration files you need to set “RewriteEngine On” in these files 和 “Options FollowSymLinks” must be 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 /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