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<div id="preamble"><h1>Apache Module mod_rewrite</h1>
<table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td>Provides a rule-based rewriting engine to rewrite requested
URLs on the fly</td></tr>
<tr><th><a href="module-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="module-dict.html#ModuleIdentifier">Module Identifier:</a></th><td>rewrite_module</td></tr>
<tr><th><a href="module-dict.html#SourceFile">Source File:</a></th><td>mod_rewrite.c</td></tr>
<tr><th><a href="module-dict.html#Compatibility">Compatibility:</a></th><td>Available in Apache 1.3 and later</td></tr></table>
<h3>Summary</h3>

      <blockquote>
            <p>``The great thing about mod_rewrite is it gives you
            all the configurability and flexibility of Sendmail.
            The downside to mod_rewrite is that it gives you all
            the configurability and flexibility of Sendmail.''</p>

            <p class="cite">-- <cite>Brian Behlendorf</cite><br />
            Apache Group</p>

      </blockquote>

      <blockquote>
            <p>`` Despite the tons of examples and docs,
            mod_rewrite is voodoo. Damned cool voodoo, but still
            voodoo. ''</p>

            <p class="cite">-- <cite>Brian Moore</cite><br />
            bem@news.cmc.net</p>

      </blockquote>


      <p>Welcome to mod_rewrite, the Swiss Army Knife of URL
      manipulation!</p>

      <p>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 rules and an
      unlimited number of attached rule conditions for each rule to
      provide a really flexible and powerful URL manipulation
      mechanism. The URL manipulations can depend on various tests,
      for instance server variables, environment variables, HTTP
      headers, time stamps and even external database lookups in
      various formats can be used to achieve a really granular URL
      matching.</p>

      <p>This module operates on the full URLs (including the
      path-info part) both in per-server context
      (<code>httpd.conf</code>) and per-directory context
      (<code>.htaccess</code>) and can even generate query-string
      parts on result. The rewritten result can lead to internal
      sub-processing, external request redirection or even to an
      internal proxy throughput.</p>

      <p>But all this functionality and flexibility has its
      drawback: complexity. So don't expect to understand this
      entire module in just one day.</p>

      <p>This module was invented and originally written in April
      1996 and gifted exclusively to the The Apache Group in July 1997
      by</p>

      <p class="indent">
        <a href="http://www.engelschall.com/"><code>Ralf S.
        Engelschall</code></a><br />
         <a href="mailto:rse@engelschall.com"><code>rse@engelschall.com</code></a><br />
         <a href="http://www.engelschall.com/"><code>www.engelschall.com</code></a>
      </p>
</div>
<div id="quickview"><h3 class="directives">Directives</h3>
<ul id="toc">
<li><img alt="" src="../images/down.gif" /> <a href="#rewritebase">RewriteBase</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#rewritecond">RewriteCond</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#rewriteengine">RewriteEngine</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#rewritelock">RewriteLock</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#rewritelog">RewriteLog</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#rewriteloglevel">RewriteLogLevel</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#rewritemap">RewriteMap</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#rewriteoptions">RewriteOptions</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#rewriterule">RewriteRule</a></li>
</ul>
<h3>Topics</h3>
<ul id="topics">
<li><img alt="" src="../images/down.gif" /> <a href="#Internal">Internal Processing</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#EnvVar">Environment Variables</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#Solutions">Practical Solutions</a></li>
</ul></div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="Internal" id="Internal">Internal Processing</a></h2>

      <p>The internal processing of this module is very complex but
      needs to be explained once even to the average user to avoid
      common mistakes and to let you exploit its full
      functionality.</p>

<h3><a name="InternalAPI" id="InternalAPI">API Phases</a></h3>

      <p>First you have to understand that when Apache processes a
      HTTP request it does this in phases. A hook for each of these
      phases is provided by the Apache API. Mod_rewrite uses two of
      these hooks: the URL-to-filename translation hook which is
      used after the HTTP request has been read but before any
      authorization starts and the Fixup hook which is triggered
      after the authorization phases and after the per-directory
      config files (<code>.htaccess</code>) have been read, but
      before the content handler is activated.</p>

      <p>So, after a request comes in and Apache has determined the
      corresponding server (or virtual server) the rewriting engine
      starts processing of all mod_rewrite directives from the
      per-server configuration in the URL-to-filename phase. A few
      steps later when the final data directories are found, the
      per-directory configuration directives of mod_rewrite are
      triggered in the Fixup phase. In both situations mod_rewrite
      rewrites URLs either to new URLs or to filenames, although
      there is no obvious distinction between them. This is a usage
      of the API which was not intended to be this way when the API
      was designed, but as of Apache 1.x this is the only way
      mod_rewrite can operate. To make this point more clear
      remember the following two points:</p>

      <ol>
        <li>Although mod_rewrite rewrites URLs to URLs, URLs to
        filenames and even filenames to filenames, the API
        currently provides only a URL-to-filename hook. In Apache
        2.0 the two missing hooks will be added to make the
        processing more clear. But this point has no drawbacks for
        the user, it is just a fact which should be remembered:
        Apache does more in the URL-to-filename hook than the API
        intends for it.</li>

        <li>
          Unbelievably mod_rewrite provides URL manipulations in
          per-directory context, <em>i.e.</em>, within
          <code>.htaccess</code> files, although these are reached
          a very long time after the URLs have been translated to
          filenames. It has to be this way because
          <code>.htaccess</code> files live in the filesystem, so
          processing has already reached this stage. In other
          words: According to the API phases at this time it is too
          late for any URL manipulations. To overcome this chicken
          and egg problem mod_rewrite uses a trick: When you
          manipulate a URL/filename in per-directory context
          mod_rewrite first rewrites the filename back to its
          corresponding URL (which is usually impossible, but see
          the <code>RewriteBase</code> directive below for the
          trick to achieve this) and then initiates a new internal
          sub-request with the new URL. This restarts processing of
          the API phases. 

          <p>Again mod_rewrite tries hard to make this complicated
          step totally transparent to the user, but you should
          remember here: While URL manipulations in per-server
          context are really fast and efficient, per-directory
          rewrites are slow and inefficient due to this chicken and
          egg problem. But on the other hand this is the only way
          mod_rewrite can provide (locally restricted) URL
          manipulations to the average user.</p>
        </li>
      </ol>

      <p>Don't forget these two points!</p>


<h3><a name="InternalRuleset" id="InternalRuleset">Ruleset Processing</a></h3>
 
      <p>Now when mod_rewrite is triggered in these two API phases, it
      reads the configured rulesets from its configuration
      structure (which itself was either created on startup for
      per-server context or during the directory walk of the Apache
      kernel for per-directory context). Then the URL rewriting
      engine is started with the contained ruleset (one or more
      rules together with their conditions). The operation of the
      URL rewriting engine itself is exactly the same for both
      configuration contexts. Only the final result processing is
      different. </p>

      <p>The order of rules in the ruleset is important because the
      rewriting engine processes them in a special (and not very
      obvious) order. The rule is this: The rewriting engine loops
      through the ruleset rule by rule (<code class="directive"><a href="#rewriterule">RewriteRule</a></code> directives) and
      when a particular rule matches it optionally loops through
      existing corresponding conditions (<code>RewriteCond</code>
      directives). For historical reasons the conditions are given
      first, and so the control flow is a little bit long-winded. See
      Figure 1 for more details.</p>
<p class="figure">
      <img src="../images/mod_rewrite_fig1.gif" width="428" height="385" alt="[Needs graphics capability to display]" /><br />
      <dfn>Figure 1:</dfn>The control flow through the rewriting ruleset
</p>
      <p>As you can see, first the URL is matched against the
      <em>Pattern</em> of each rule. When it fails mod_rewrite
      immediately stops processing this rule and continues with the
      next rule. If the <em>Pattern</em> matches, mod_rewrite looks
      for corresponding rule conditions. If none are present, it
      just substitutes the URL with a new value which is
      constructed from the string <em>Substitution</em> and goes on
      with its rule-looping. But if conditions exist, it starts an
      inner loop for processing them in the order that they are
      listed. For conditions the logic is different: we don't match
      a pattern against the current URL. Instead we first create a
      string <em>TestString</em> by expanding variables,
      back-references, map lookups, <em>etc.</em> and then we try
      to match <em>CondPattern</em> against it. If the pattern
      doesn't match, the complete set of conditions and the
      corresponding rule fails. If the pattern matches, then the
      next condition is processed until no more conditions are
      available. If all conditions match, processing is continued
      with the substitution of the URL with
      <em>Substitution</em>.</p>



<h3><a name="quoting" id="quoting">Quoting Special Characters</a></h3>

      <p>As of Apache 1.3.20, special characters in
      <em>TestString</em> and <em>Substitution</em> strings can be
      escaped (that is, treated as normal characters without their
      usual special meaning) by prefixing them with a slosh ('\')
      character. In other words, you can include an actual
      dollar-sign character in a <em>Substitution</em> string by
      using '<code>\$</code>'; this keeps mod_rewrite from trying
      to treat it as a backreference.</p>


<h3><a name="InternalBackRefs" id="InternalBackRefs">Regex Back-Reference Availability</a></h3>

      <p>One important thing here has to be remembered: Whenever you
      use parentheses in <em>Pattern</em> or in one of the
      <em>CondPattern</em>, back-references are internally created
      which can be used with the strings <code>$N</code> and
      <code>%N</code> (see below). These are available for creating
      the strings <em>Substitution</em> and <em>TestString</em>.
      Figure 2 shows to which locations the back-references are
      transfered for expansion.</p>

<p class="figure">
      <img src="../images/mod_rewrite_fig2.gif" width="381" height="179" alt="[Needs graphics capability to display]" /><br />
      <dfn>Figure 2:</dfn> The back-reference flow through a rule.
</p>
      <p>We know this was a crash course on mod_rewrite's internal
      processing. But you will benefit from this knowledge when
      reading the following documentation of the available
      directives.</p>


</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="EnvVar" id="EnvVar">Environment Variables</a></h2>

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

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

<div class="example"><h3>Example</h3><pre>
SCRIPT_NAME=/sw/lib/w3s/tree/global/u/rse/.www/index.html
SCRIPT_FILENAME=/u/rse/.www/index.html
SCRIPT_URL=/u/rse/
SCRIPT_URI=http://en1.engelschall.com/u/rse/
</pre></div>

</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="Solutions" id="Solutions">Practical Solutions</a></h2>

      <p>We also have an <a href="../misc/rewriteguide.html">URL
      Rewriting Guide</a> available, which provides a collection of
      practical solutions for URL-based problems. There you can
      find real-life rulesets and additional information about
      mod_rewrite.</p>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="RewriteBase" id="RewriteBase">RewriteBase</a> <a name="rewritebase" id="rewritebase">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Sets the base URL for per-directory rewrites</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>RewriteBase <em>URL-path</em></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>See usage for information.</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Override">Override:</a></th><td>FileInfo</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_rewrite</td></tr>
</table>
      <p>The <code class="directive">RewriteBase</code> directive explicitly
      sets the base URL for per-directory rewrites. As you will see
      below, <code class="directive"><a href="#rewriterule">RewriteRule</a></code>
      can be used in per-directory config files
      (<code>.htaccess</code>). There it will act locally,
      <em>i.e.</em>, 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 back to the
      path. The default setting is; <code class="directive">RewriteBase</code> <em>physical-directory-path</em></p>

      <p>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. <strong>But at most websites URLs are NOT
      directly related to physical filename paths, so this
      assumption will usually be wrong!</strong> There you have to
      use the <code>RewriteBase</code> directive to specify the
      correct URL-prefix.</p>

<div class="note"> If your webserver's URLs are <strong>not</strong> directly
related to physical file paths, you have to use
<code class="directive">RewriteBase</code> in every <code>.htaccess</code>
files where you want to use <code class="directive"><a href="#rewriterule">RewriteRule</a></code> directives.
</div>

        <p> For example, assume the following per-directory config file:</p>

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

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
</pre></div>

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

<div class="note"><h3>For Apache Hackers</h3>
<p>The following list gives detailed information about
              the internal processing steps:</p>
<pre>
Request:
  /xyz/oldstuff.html

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

Result:
  /abc/def/newstuff.html
</pre>
              <p>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
              internally 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.</p>
</div>


</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="RewriteCond" id="RewriteCond">RewriteCond</a> <a name="rewritecond" id="rewritecond">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Defines a condition under which rewriting will take place
</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code> RewriteCond
      <em>TestString</em> <em>CondPattern</em></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>None</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Override">Override:</a></th><td>FileInfo</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_rewrite</td></tr>
</table>
      <p>The <code class="directive">RewriteCond</code> directive defines a
      rule condition. Precede a <code class="directive"><a href="#rewriterule">RewriteRule</a></code> directive with one
      or more <code class="directive">RewriteCond</code> directives. The following
      rewriting rule is only used if its pattern matches the current
      state of the URI <strong>and</strong> if these additional
      conditions apply too.</p>

      <p><em>TestString</em> is a string which can contains the
      following expanded constructs in addition to plain text:</p>

      <ul>
        <li>
          <strong>RewriteRule backreferences</strong>: These are
          backreferences of the form 

          <p class="indent">
            <strong><code>$N</code></strong>
          </p>
          (0 &lt;= N &lt;= 9) which provide access to the grouped
          parts (parenthesis!) of the pattern from the
          corresponding <code>RewriteRule</code> directive (the one
          following the current bunch of <code>RewriteCond</code>
          directives).
        </li>

        <li>
          <strong>RewriteCond backreferences</strong>: These are
          backreferences of the form 

          <p class="indent">
            <strong><code>%N</code></strong>
          </p>
          (1 &lt;= N &lt;= 9) which provide access to the grouped
          parts (parentheses!) of the pattern from the last matched
          <code>RewriteCond</code> directive in the current bunch
          of conditions.
        </li>

        <li>
          <strong>RewriteMap expansions</strong>: These are
          expansions of the form 

          <p class="indent">
            <strong><code>${mapname:key|default}</code></strong>
          </p>
          See <a href="#mapfunc">the documentation for
          RewriteMap</a> for more details.
        </li>

        <li>
          <strong>Server-Variables</strong>: These are variables of
          the form 

          <p class="indent">
            <strong><code>%{</code> <em>NAME_OF_VARIABLE</em>
            <code>}</code></strong>
          </p>
          where <em>NAME_OF_VARIABLE</em> can be a string taken
          from the following list: 

          <table>
            <tr>
              <th>HTTP headers:</th> <th>connection &amp; request:</th> <th />
	    </tr>

            <tr>
	      <td>
		 HTTP_USER_AGENT<br />
                 HTTP_REFERER<br />
                 HTTP_COOKIE<br />
                 HTTP_FORWARDED<br />
                 HTTP_HOST<br />
                 HTTP_PROXY_CONNECTION<br />
                 HTTP_ACCEPT<br />
              </td>

              <td>
                 REMOTE_ADDR<br />
                 REMOTE_HOST<br />
                 REMOTE_USER<br />
                 REMOTE_IDENT<br />
                 REQUEST_METHOD<br />
                 SCRIPT_FILENAME<br />
                 PATH_INFO<br />
                 QUERY_STRING<br />
                 AUTH_TYPE<br />
              </td>
	      
	      <td />
            </tr>

            <tr>
              <th>server internals:</th> <th>system stuff:</th> <th>specials:</th>
	    </tr>

            <tr>
	      <td>
	         DOCUMENT_ROOT<br />
                 SERVER_ADMIN<br />
                 SERVER_NAME<br />
                 SERVER_ADDR<br />
                 SERVER_PORT<br />
                 SERVER_PROTOCOL<br />
                 SERVER_SOFTWARE<br />
              </td>

              <td>
                 TIME_YEAR<br />
                 TIME_MON<br />
                 TIME_DAY<br />
                 TIME_HOUR<br />
                 TIME_MIN<br />
                 TIME_SEC<br />
                 TIME_WDAY<br />
                 TIME<br />
              </td>

              <td>
                 API_VERSION<br />
                 THE_REQUEST<br />
                 REQUEST_URI<br />
                 REQUEST_FILENAME<br />
                 IS_SUBREQ<br />
              </td>
            </tr>
          </table>

<div class="note">
                <p>These variables all
                correspond to the similarly named HTTP
                MIME-headers, C variables of the Apache server or
                <code>struct tm</code> 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:</p>

                <dl>
                  <dt><code>IS_SUBREQ</code></dt>

                  <dd>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.</dd>

                  <dt><code>API_VERSION</code></dt>

                  <dd>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.</dd>

                  <dt><code>THE_REQUEST</code></dt>

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

                  <dt><code>REQUEST_URI</code></dt>

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

                  <dt><code>REQUEST_FILENAME</code></dt>

                  <dd>The full local filesystem path to the file or
                  script matching the request.</dd>
                </dl>
</div>
        </li>
      </ul>

      <p>Special Notes:</p>

      <ol>
        <li>The variables SCRIPT_FILENAME and REQUEST_FILENAME
        contain the same value, <em>i.e.</em>, the value of the
        <code>filename</code> field of the internal
        <code>request_rec</code> 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
        <code>uri</code> field of <code>request_rec</code>).</li>

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

        <li>There is the special format:
        <code>%{HTTP:header}</code> where <em>header</em> can be
        any HTTP MIME-header name. This is looked-up from the HTTP
        request. Example: <code>%{HTTP:Proxy-Connection}</code> is
        the value of the HTTP header
        ``<code>Proxy-Connection:</code>''.</li>

        <li>There is the special format
        <code>%{LA-U:variable}</code> for look-aheads which perform
        an internal (URL-based) sub-request to determine the final
        value of <em>variable</em>. 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
        <code>REMOTE_USER</code> variable from within the
        per-server context (<code>httpd.conf</code> file) you have
        to use <code>%{LA-U:REMOTE_USER}</code> because this
        variable is set by the authorization phases which come
        <em>after</em> the URL translation phase where mod_rewrite
        operates. On the other hand, because mod_rewrite implements
        its per-directory context (<code>.htaccess</code> file) via
        the Fixup phase of the API and because the authorization
        phases come <em>before</em> this phase, you just can use
        <code>%{REMOTE_USER}</code> there.</li>

        <li>There is the special format:
        <code>%{LA-F:variable}</code> which performs an internal
        (filename-based) sub-request to determine the final value
        of <em>variable</em>. Most of the time this is the same as
        LA-U above.</li>
      </ol>

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

      <p><strong>Remember:</strong> <em>CondPattern</em> is a
      <em>perl compatible regular expression</em> with some
      additions:</p>

      <ol>
        <li>You can prefix the pattern string with a
        '<code>!</code>' character (exclamation mark) to specify a
        <strong>non</strong>-matching pattern.</li>

        <li>
          There are some special variants of <em>CondPatterns</em>.
          Instead of real regular expression strings you can also
          use one of the following: 

          <ul>
            <li>'<strong>&lt;CondPattern</strong>' (is lexically
            lower)<br />
             Treats the <em>CondPattern</em> as a plain string and
            compares it lexically to <em>TestString</em>. True if
            <em>TestString</em> is lexically lower than
            <em>CondPattern</em>.</li>

            <li>'<strong>&gt;CondPattern</strong>' (is lexically
            greater)<br />
             Treats the <em>CondPattern</em> as a plain string and
            compares it lexically to <em>TestString</em>. True if
            <em>TestString</em> is lexically greater than
            <em>CondPattern</em>.</li>

            <li>'<strong>=CondPattern</strong>' (is lexically
            equal)<br />
             Treats the <em>CondPattern</em> as a plain string and
            compares it lexically to <em>TestString</em>. True if
            <em>TestString</em> is lexically equal to
            <em>CondPattern</em>, i.e the two strings are exactly
            equal (character by character). If <em>CondPattern</em>
            is just <code>""</code> (two quotation marks) this
            compares <em>TestString</em> to the empty string.</li>

            <li>'<strong>-d</strong>' (is
            <strong>d</strong>irectory)<br />
             Treats the <em>TestString</em> as a pathname and tests
            if it exists and is a directory.</li>

            <li>'<strong>-f</strong>' (is regular
            <strong>f</strong>ile)<br />
             Treats the <em>TestString</em> as a pathname and tests
            if it exists and is a regular file.</li>

            <li>'<strong>-s</strong>' (is regular file with
            <strong>s</strong>ize)<br />
             Treats the <em>TestString</em> as a pathname and tests
            if it exists and is a regular file with size greater
            than zero.</li>

            <li>'<strong>-l</strong>' (is symbolic
            <strong>l</strong>ink)<br />
             Treats the <em>TestString</em> as a pathname and tests
            if it exists and is a symbolic link.</li>

            <li>'<strong>-F</strong>' (is existing file via
            subrequest)<br />
             Checks if <em>TestString</em> 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!</li>

            <li>'<strong>-U</strong>' (is existing URL via
            subrequest)<br />
             Checks if <em>TestString</em> 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!</li>
          </ul>

<div class="note"><h3>Notice</h3>
              All of these tests can
              also be prefixed by an exclamation mark ('!') to
              negate their meaning.
</div>
        </li>
      </ol>

      <p>Additionally you can set special flags for
      <em>CondPattern</em> by appending</p>

      <p class="indent">
        <strong><code>[</code><em>flags</em><code>]</code></strong>
      </p>

      <p>as the third argument to the <code>RewriteCond</code>
      directive. <em>Flags</em> is a comma-separated list of the
      following flags:</p>

      <ul>
        <li>'<strong><code>nocase|NC</code></strong>'
        (<strong>n</strong>o <strong>c</strong>ase)<br />
         This makes the test case-insensitive, <em>i.e.</em>, there
        is no difference between 'A-Z' and 'a-z' both in the
        expanded <em>TestString</em> and the <em>CondPattern</em>.
        This flag is effective only for comparisons between
        <em>TestString</em> and <em>CondPattern</em>. It has no
        effect on filesystem and subrequest checks.</li>

        <li>
          '<strong><code>ornext|OR</code></strong>'
          (<strong>or</strong> next condition)<br />
           Use this to combine rule conditions with a local OR
          instead of the implicit AND. Typical example: 

<div class="example"><pre>
RewriteCond %{REMOTE_HOST}  ^host1.*  [OR]
RewriteCond %{REMOTE_HOST}  ^host2.*  [OR]
RewriteCond %{REMOTE_HOST}  ^host3.*
RewriteRule ...some special stuff for any of these hosts...
</pre></div>

          Without this flag you would have to write the cond/rule
          three times.
        </li>
      </ul>

      <p><strong>Example:</strong></p>

       <p>To rewrite the Homepage of a site according to the
        ``<code>User-Agent:</code>'' header of the request, you can
        use the following: </p>

<div class="example"><pre>
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]
</pre></div>

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


</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="RewriteEngine" id="RewriteEngine">RewriteEngine</a> <a name="rewriteengine" id="rewriteengine">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Enables or disables runtime rewriting engine</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>RewriteEngine on|off</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>RewriteEngine off</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Override">Override:</a></th><td>FileInfo</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_rewrite</td></tr>
</table>       

      <p>The <code class="directive">RewriteEngine</code> directive enables or
      disables the runtime rewriting engine. If it is set to
      <code>off</code> this module does no runtime processing at
      all. It does not even update the <code>SCRIPT_URx</code>
      environment variables.</p>

      <p>Use this directive to disable the module instead of
      commenting out all the <code class="directive"><a href="#rewriterule">RewriteRule</a></code> directives!</p>

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

</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="RewriteLock" id="RewriteLock">RewriteLock</a> <a name="rewritelock" id="rewritelock">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Sets the name of the lock file used for RewriteMap
synchronization</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>RewriteLock <em>file-path</em></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>None</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_rewrite</td></tr>
</table>
      <p>This directive sets the filename for a synchronization
      lockfile which mod_rewrite needs to communicate with <code class="directive"><a href="#rewritemap">RewriteMap</a></code>
      <em>programs</em>. 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.</p>

</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="RewriteLog" id="RewriteLog">RewriteLog</a> <a name="rewritelog" id="rewritelog">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Sets the name of the file used for logging rewrite engine
processing</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>RewriteLog <em>file-path</em></code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_rewrite</td></tr>
</table>
      <p>The <code class="directive">RewriteLog</code> 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
      ('<code>/</code>') then it is assumed to be relative to the
      <em>Server Root</em>. The directive should occur only once per
      server config.</p>

<div class="note">    To disable the logging of
          rewriting actions it is not recommended to set
          <em>Filename</em> to <code>/dev/null</code>, because
          although the rewriting engine does not then output to a
          logfile it still creates the logfile output internally.
          <strong>This will slow down the server with no advantage
          to the administrator!</strong> To disable logging either
          remove or comment out the <code class="directive">RewriteLog</code>
          directive or use <code>RewriteLogLevel 0</code>!
</div>

<div class="note"><h3>Security</h3>

See the <a href="../misc/security_tips.html">Apache Security Tips</a>
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.
</div>

<div class="example"><h3>Example</h3><p><code>
RewriteLog "/usr/local/var/apache/logs/rewrite.log"
</code></p></div>


</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="RewriteLogLevel" id="RewriteLogLevel">RewriteLogLevel</a> <a name="rewriteloglevel" id="rewriteloglevel">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Sets the verbosity of the log file used by the rewrite
engine</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>RewriteLogLevel <em>Level</em></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>RewriteLogLevel 0</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_rewrite</td></tr>
</table>
      <p>The <code class="directive">RewriteLogLevel</code> 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.</p>

      <p>To disable the logging of rewriting actions simply set
      <em>Level</em> to 0. This disables all rewrite action
      logs.</p>

<div class="note"> Using a high value for
          <em>Level</em> will slow down your Apache server
          dramatically! Use the rewriting logfile at a
          <em>Level</em> greater than 2 only for debugging!
</div>

<div class="example"><h3>Example</h3><p><code>
RewriteLogLevel 3
</code></p></div>


</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="RewriteMap" id="RewriteMap">RewriteMap</a> <a name="rewritemap" id="rewritemap">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Defines a mapping function for key-lookup</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>RewriteMap <em>MapName</em> <em>MapType</em>:<em>MapSource</em>
</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>None</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_rewrite</td></tr>
<tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>The choice of different dbm types is available in
Apache 2.0.41 and later</td></tr>
</table>
      <p>The <code class="directive">RewriteMap</code> directive defines a
      <em>Rewriting Map</em> 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.</p>

      <p>The <a id="mapfunc" name="mapfunc"><em>MapName</em></a> 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:</p>

      <p class="indent">
        <strong><code>${</code> <em>MapName</em> <code>:</code>
        <em>LookupKey</em> <code>}</code><br />
         <code>${</code> <em>MapName</em> <code>:</code>
        <em>LookupKey</em> <code>|</code> <em>DefaultValue</em>
        <code>}</code></strong>
      </p>

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

      <p>The following combinations for <em>MapType</em> and
      <em>MapSource</em> can be used:</p>

      <ul>
        <li>
          <strong>Standard Plain Text</strong><br />
           MapType: <code>txt</code>, MapSource: Unix filesystem
          path to valid regular file 

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

          <p class="indent">
            <strong><em>MatchingKey</em>
            <em>SubstValue</em></strong>
          </p>

<div class="example"><h3>Example</h3><pre>
##
##  map.txt -- rewriting map
##

Ralf.S.Engelschall    rse   # Bastard Operator From Hell
Mr.Joe.Average        joe   # Mr. Average
</pre></div>

<div class="example"><p><code>
RewriteMap real-to-user txt:/path/to/file/map.txt
</code></p></div>
        </li>

        <li>
          <strong>Randomized Plain Text</strong><br />
           MapType: <code>rnd</code>, MapSource: Unix filesystem
          path to valid regular file 

          <p>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
          ``<code>|</code>'' 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. Although this sounds crazy and useless,
          it was actually designed for load balancing in a reverse
          proxy situation where the looked up values are server
          names. Example:</p>

<div class="example"><pre>
##
##  map.txt -- rewriting map
##

static   www1|www2|www3|www4
dynamic  www5|www6
</pre></div>

<div class="example"><p><code>
RewriteMap servers rnd:/path/to/file/map.txt
</code></p></div>
        </li>

        <li>
          <strong>Hash File</strong><br /> MapType:
          <code>dbm[=<em>type</em>]</code>, MapSource: Unix filesystem
          path to valid regular file

          <p>Here the source is a binary format DBM file containing
          the same contents as a <em>Plain Text</em> format file, but
          in a special representation which is optimized for really
          fast lookups. The <em>type</em> can be sdbm, gdbm, ndbm, or
          db depending on <a href="../install.html#dbm">compile-time
          settings</a>.  If the <em>type</em> is ommitted, 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.</p>

<div class="example"><pre>
#!/path/to/bin/perl
##
##  txt2dbm -- convert txt map to dbm format
##

use NDBM_File;
use Fcntl;

($txtmap, $dbmmap) = @ARGV;

open(TXT, "&lt;$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 (&lt;TXT&gt;) {
  next if (/^\s*#/ or /^\s*$/);
  $DB{$1} = $2 if (/^\s*(\S+)\s+(\S+)/);
}

untie %DB;
close(TXT);
</pre></div>

<div class="example"><p><code>
$ txt2dbm map.txt map.db
</code></p></div>
        </li>

        <li>
          <strong>Internal Function</strong><br />
           MapType: <code>int</code>, MapSource: Internal Apache
          function 

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

          <ul>
            <li><strong>toupper</strong>:<br />
             Converts the looked up key to all upper case.</li>

            <li><strong>tolower</strong>:<br />
             Converts the looked up key to all lower case.</li>

            <li><strong>escape</strong>:<br />
             Translates special characters in the looked up key to
            hex-encodings.</li>

            <li><strong>unescape</strong>:<br />
             Translates hex-encodings in the looked up key back to
            special characters.</li>
          </ul>
        </li>

        <li>
          <strong>External Rewriting Program</strong><br />
           MapType: <code>prg</code>, MapSource: Unix filesystem
          path to valid regular file 

          <p>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 (<em>i.e.</em>, either
          object-code or a script with the magic cookie trick
          '<code>#!/path/to/interpreter</code>' as the first
          line).</p>

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

<div class="example"><pre>
#!/usr/bin/perl
$| = 1;
while (&lt;STDIN&gt;) {
    # ...put here any transformations or lookups...
    print $_;
}
</pre></div>

          <p>But be very careful:</p>

          <ol>
            <li>``<em>Keep it simple, stupid</em>'' (KISS), because
            if this program hangs it will hang the Apache server
            when the rule occurs.</li>

            <li>Avoid one common mistake: never do buffered I/O on
            <code>stdout</code>! This will cause a deadloop! Hence
            the ``<code>$|=1</code>'' in the above example...</li>

            <li>Use the <code class="directive"><a href="#rewritelock">RewriteLock</a></code> directive to
            define a lockfile mod_rewrite can use to synchronize the
            communication to the program. By default no such
            synchronization takes place.</li>
          </ol>
        </li>
      </ul>
      <p>The <code class="directive">RewriteMap</code> directive can occur more than
      once. For each mapping-function use one
      <code class="directive">RewriteMap</code> directive to declare its rewriting
      mapfile. While you cannot <strong>declare</strong> a map in
      per-directory context it is of course possible to
      <strong>use</strong> this map in per-directory context. </p>

<div class="note"><h3>Note</h3> For plain text and DBM format files the
looked-up keys are cached in-core until the <code>mtime</code> of the
mapfile changes or the server does a restart. This way you can have
map-functions in rules which are used for <strong>every</strong>
request.  This is no problem, because the external lookup only happens
once!
</div>


</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="RewriteOptions" id="RewriteOptions">RewriteOptions</a> <a name="rewriteoptions" id="rewriteoptions">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Sets some special options for the rewrite engine</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>RewriteOptions <em>Options</em></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>None</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Override">Override:</a></th><td>FileInfo</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_rewrite</td></tr>
</table>

      <p>The <code class="directive">RewriteOptions</code> directive sets some
      special options for the current per-server or per-directory
      configuration. The <em>Option</em> strings can be one of the
      following:</p>

      <ul>
        <li>'<strong><code>inherit</code></strong>'<br />
         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
        <code>.htaccess</code> configuration are inherited.</li>
      </ul>

</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="RewriteRule" id="RewriteRule">RewriteRule</a> <a name="rewriterule" id="rewriterule">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Defines rules for the rewriting engine</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>RewriteRule
      <em>Pattern</em> <em>Substitution</em></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>None</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Override">Override:</a></th><td>FileInfo</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_rewrite</td></tr>
<tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>The cookie-flag is available in Apache 2.0.40 and later.</td></tr>
</table>
      <p>The <code class="directive">RewriteRule</code> directive is the real
      rewriting workhorse. The directive can occur more than once.
      Each directive then defines one single rewriting rule. The
      <strong>definition order</strong> of these rules is
      <strong>important</strong>, because this order is used when
      applying the rules at run-time.</p>

      <p><a id="patterns" name="patterns"><em>Pattern</em></a> is
      a perl compatible <a id="regexp" name="regexp">regular
      expression</a> 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.</p>

      <p>Some hints about the syntax of regular expressions:</p>

<div class="note"><pre>
<strong>Text:</strong>
  <strong><code>.</code></strong>           Any single character
  <strong><code>[</code></strong>chars<strong><code>]</code></strong>     Character class: One  of chars
  <strong><code>[^</code></strong>chars<strong><code>]</code></strong>    Character class: None of chars
  text1<strong><code>|</code></strong>text2 Alternative: text1 or text2

<strong>Quantifiers:</strong>
  <strong><code>?</code></strong>           0 or 1 of the preceding text
  <strong><code>*</code></strong>           0 or N of the preceding text (N &gt; 0)
  <strong><code>+</code></strong>           1 or N of the preceding text (N &gt; 1)

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

<strong>Anchors:</strong>
  <strong><code>^</code></strong>           Start of line anchor
  <strong><code>$</code></strong>           End   of line anchor

<strong>Escaping:</strong>
  <strong><code>\</code></strong>char       escape that particular char
              (for instance to specify the chars "<code>.[]()</code>" <em>etc.</em>)
</pre></div>

      <p>For more information about regular expressions have a look at the
      perl regular expression manpage ("<a href="http://www.perldoc.com/perl5.6.1/pod/perlre.html">perldoc
      perlre</a>"). If you are interested in more detailed
      information about regular expressions and their variants
      (POSIX regex <em>etc.</em>) have a look at the
      following dedicated book on this topic:</p>

      <p class="indent">
        <em>Mastering Regular Expressions</em><br />
         Jeffrey E.F. Friedl<br />
         Nutshell Handbook Series<br />
         O'Reilly &amp; Associates, Inc. 1997<br />
         ISBN 1-56592-257-3<br />
      </p>

      <p>Additionally in mod_rewrite the NOT character
      ('<code>!</code>') is a possible pattern prefix. This gives
      you the ability to negate a pattern; to say, for instance:
      ``<em>if the current URL does <strong>NOT</strong> match this
      pattern</em>''. This can be used for exceptional cases, where
      it is easier to match the negative pattern, or as a last
      default rule.</p>

<div class="note"><h3>Notice</h3>
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 <code>$N</code> in the substitution
          string!
</div>

      <p><a id="rhs" name="rhs"><em>Substitution</em></a> of a
      rewriting rule is the string which is substituted for (or
      replaces) the original URL for which <em>Pattern</em>
      matched. Beside plain text you can use</p>

      <ol>
        <li>back-references <code>$N</code> to the RewriteRule
        pattern</li>

        <li>back-references <code>%N</code> to the last matched
        RewriteCond pattern</li>

        <li>server-variables as in rule condition test-strings
        (<code>%{VARNAME}</code>)</li>

        <li><a href="#mapfunc">mapping-function</a> calls
        (<code>${mapname:key|default}</code>)</li>
      </ol>
      <p>Back-references are <code>$</code><strong>N</strong>
      (<strong>N</strong>=0..9) identifiers which will be replaced
      by the contents of the <strong>N</strong>th group of the
      matched <em>Pattern</em>. The server-variables are the same
      as for the <em>TestString</em> of a <code>RewriteCond</code>
      directive. The mapping-functions come from the
      <code>RewriteMap</code> directive and are explained there.
      These three types of variables are expanded in the order of
      the above list. </p>

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

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

      <p>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.</p>

<div class="note"><h3>Note</h3>
There is a special feature:
          When you prefix a substitution field with
          <code>http://</code><em>thishost</em>[<em>:thisport</em>]
          then <strong>mod_rewrite</strong> 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.
</div>

<div class="note"><h3>Remember</h3>
 An unconditional external
          redirect to your own server will not work with the prefix
          <code>http://thishost</code> because of this feature. To
          achieve such a self-redirect, you have to use the
          <strong>R</strong>-flag (see below).
</div>

      <p>Additionally you can set special flags for
      <em>Substitution</em> by appending</p>

      <p class="indent">
        <strong><code>[</code><em>flags</em><code>]</code></strong>
      </p>
      <p>
      as the third argument to the <code>RewriteRule</code>
      directive. <em>Flags</em> is a comma-separated list of the
      following flags: </p>

      <ul>
        <li>
          '<strong><code>redirect|R</code>
          [=<em>code</em>]</strong>' (force <a id="redirect" name="redirect"><strong>r</strong>edirect</a>)<br />
           Prefix <em>Substitution</em> with
          <code>http://thishost[:thisport]/</code> (which makes the
          new URL a URI) to force a external redirection. If no
          <em>code</em> 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:
          <code>temp</code> (default), <code>permanent</code>,
          <code>seeother</code>. Use it for rules which should
          canonicalize the URL and give it back to the client,
          <em>e.g.</em>, translate ``<code>/~</code>'' into
          ``<code>/u/</code>'' or always append a slash to
          <code>/u/</code><em>user</em>, etc.<br />
           

          <p><strong>Note:</strong> 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
          <code>http://thishost[:thisport]/</code>, 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.</p>
        </li>

        <li>'<strong><code>forbidden|F</code></strong>' (force URL
        to be <strong>f</strong>orbidden)<br />
         This forces the current URL to be forbidden,
        <em>i.e.</em>, it immediately sends back a HTTP response of
        403 (FORBIDDEN). Use this flag in conjunction with
        appropriate RewriteConds to conditionally block some
        URLs.</li>

        <li>'<strong><code>gone|G</code></strong>' (force URL to be
        <strong>g</strong>one)<br />
         This forces the current URL to be gone, <em>i.e.</em>, it
        immediately sends back a HTTP response of 410 (GONE). Use
        this flag to mark pages which no longer exist as gone.</li>

        <li>
          '<strong><code>proxy|P</code></strong>' (force
          <strong>p</strong>roxy)<br />
           This flag forces the substitution part to be internally
          forced as a proxy request and immediately (<em>i.e.</em>,
          rewriting rule processing stops here) put through the <a href="mod_proxy.html">proxy module</a>. You have to make
          sure that the substitution string is a valid URI
          (<em>e.g.</em>, typically starting with
          <code>http://</code><em>hostname</em>) 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 <a href="mod_proxy.html#proxypass">ProxyPass</a> directive,
          to map some remote stuff into the namespace of the local
          server. 

          <p>Notice: To use this functionality make sure you have
          the proxy module compiled into your Apache server
          program. If you don't know please check whether
          <code>mod_proxy.c</code> is part of the ``<code>httpd
          -l</code>'' output. If yes, this functionality is
          available to mod_rewrite. If not, then you first have to
          rebuild the ``<code>httpd</code>'' program with mod_proxy
          enabled.</p>
        </li>

        <li>'<strong><code>last|L</code></strong>'
        (<strong>l</strong>ast rule)<br />
         Stop the rewriting process here and don't apply any more
        rewriting rules. This corresponds to the Perl
        <code>last</code> command or the <code>break</code> 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
        ('<code>/</code>') to a real one, <em>e.g.</em>,
        '<code>/e/www/</code>'.</li>

        <li>'<strong><code>next|N</code></strong>'
        (<strong>n</strong>ext round)<br />
         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 <code>next</code> command or
        the <code>continue</code> command from the C language. Use
        this flag to restart the rewriting process, <em>i.e.</em>,
        to immediately go to the top of the loop.<br />
         <strong>But be careful not to create an infinite
        loop!</strong></li>

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

        <li>
        '<strong><code>type|T</code></strong>=<em>MIME-type</em>'
        (force MIME <strong>t</strong>ype)<br />
         Force the MIME-type of the target file to be
        <em>MIME-type</em>. For instance, this can be used to
        simulate the <code>mod_alias</code> directive
        <code>ScriptAlias</code> which internally forces all files
        inside the mapped directory to have a MIME type of
        ``<code>application/x-httpd-cgi</code>''.</li>

        <li>
          '<strong><code>nosubreq|NS</code></strong>' (used only if
          <strong>n</strong>o internal
          <strong>s</strong>ub-request)<br />
           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 <code>mod_include</code> tries to find out
          information about possible directory default files
          (<code>index.xxx</code>). 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.<br />
           

          <p>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.</p>
        </li>

        <li>'<strong><code>nocase|NC</code></strong>'
        (<strong>n</strong>o <strong>c</strong>ase)<br />
         This makes the <em>Pattern</em> case-insensitive,
        <em>i.e.</em>, there is no difference between 'A-Z' and
        'a-z' when <em>Pattern</em> is matched against the current
        URL.</li>

        <li>'<strong><code>qsappend|QSA</code></strong>'
        (<strong>q</strong>uery <strong>s</strong>tring
        <strong>a</strong>ppend)<br />
         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.</li>

        <li>
          '<strong><code>noescape|NE</code></strong>'
          (<strong>n</strong>o URI <strong>e</strong>scaping of
          output)<br />
           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 
<div class="example"><p><code>
    RewriteRule /foo/(.*) /bar?arg=P1\%3d$1 [R,NE]
</code></p></div>

          which would turn '<code>/foo/zed</code>' into a safe
          request for '<code>/bar?arg=P1=zed</code>'. 
        </li>

        <li>
          '<strong><code>passthrough|PT</code></strong>'
          (<strong>p</strong>ass <strong>t</strong>hrough to next
          handler)<br />
           This flag forces the rewriting engine to set the
          <code>uri</code> field of the internal
          <code>request_rec</code> structure to the value of the
          <code>filename</code> field. This flag is just a hack to
          be able to post-process the output of
          <code>RewriteRule</code> directives by
          <code>Alias</code>, <code>ScriptAlias</code>,
          <code>Redirect</code>, <em>etc.</em> directives from
          other URI-to-filename translators. A trivial example to
          show the semantics: If you want to rewrite
          <code>/abc</code> to <code>/def</code> via the rewriting
          engine of <code>mod_rewrite</code> and then
          <code>/def</code> to <code>/ghi</code> with
          <code>mod_alias</code>: 
<div class="example"><p><code>
    RewriteRule ^/abc(.*)  /def$1 [PT]<br />
    Alias       /def       /ghi
</code></p></div>
          If you omit the <code>PT</code> flag then
          <code>mod_rewrite</code> will do its job fine,
          <em>i.e.</em>, it rewrites <code>uri=/abc/...</code> to
          <code>filename=/def/...</code> as a full API-compliant
          URI-to-filename translator should do. Then
          <code>mod_alias</code> comes and tries to do a
          URI-to-filename transition which will not work. 

          <p>Note: <strong>You have to use this flag if you want to
          intermix directives of different modules which contain
          URL-to-filename translators</strong>. The typical example
          is the use of <code>mod_alias</code> and
          <code>mod_rewrite</code>..</p>

<div class="note"><h3>For Apache hackers</h3>
               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 a hook in
              Apache version 2.0.
</div>
        </li>

        <li>'<strong><code>skip|S</code></strong>=<em>num</em>'
        (<strong>s</strong>kip next rule(s))<br />
         This flag forces the rewriting engine to skip the next
        <em>num</em> 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
        <code>skip=N</code> where N is the number of rules in the
        else-clause. (This is <strong>not</strong> the same as the
        'chain|C' flag!)</li>

        <li>
        '<strong><code>env|E=</code></strong><em>VAR</em>:<em>VAL</em>'
        (set <strong>e</strong>nvironment variable)<br />
         This forces an environment variable named <em>VAR</em> to
        be set to the value <em>VAL</em>, where <em>VAL</em> can
        contain regexp backreferences <code>$N</code> and
        <code>%N</code> which will be expanded. You can use this
        flag more than once to set more than one variable. The
        variables can be later dereferenced in many situations, but
        usually from within XSSI (via <code>&lt;!--#echo
        var="VAR"--&gt;</code>) or CGI (<em>e.g.</em>
        <code>$ENV{'VAR'}</code>). Additionally you can dereference
        it in a following RewriteCond pattern via
        <code>%{ENV:VAR}</code>. Use this to strip but remember
        information from URLs.</li>

        <li>
		'<strong><code>cookie|CO=</code></strong><em>NAME</em>:<em>VAL</em>:<em>domain</em>[:<em>lifetime</em>[:<em>path</em>]]'
        (set <strong>co</strong>cookie)<br />
        This sets a cookie on the client's browser.  The cookie's name
        is specified by <em>NAME</em> and the value is
        <em>VAL</em>. The <em>domain</em> field is the domain of the
        cookie, such as '.apache.org',the optional <em>lifetime</em>
	is the lifetime of the cookie in minutes, and the optional 
	<em>path</em> is the path of the cookie</li>
 
      </ul>

<div class="note"><h3>Note</h3> Never forget that <em>Pattern</em> is
applied to a complete URL in per-server configuration
files. <strong>But in per-directory configuration files, the
per-directory prefix (which always is the same for a specific
directory!) is automatically <em>removed</em> for the pattern matching
and automatically <em>added</em> after the substitution has been
done.</strong> 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.

            <p>There is one exception: If a substitution string
            starts with ``<code>http://</code>'' then the directory
            prefix will <strong>not</strong> be added and an
            external redirect or proxy throughput (if flag
            <strong>P</strong> is used!) is forced!</p>
</div>

<div class="note"><h3>Note</h3>
 To enable the rewriting engine
          for per-directory configuration files you need to set
          ``<code>RewriteEngine On</code>'' in these files
          <strong>and</strong> ``<code>Options
          FollowSymLinks</code>'' must be enabled. If your
          administrator has disabled override of
          <code>FollowSymLinks</code> for a user's directory, then
          you cannot use the rewriting engine. This restriction is
          needed for security reasons.
</div>

      <p>Here are all possible substitution combinations and their
      meanings:</p>

      <p><strong>Inside per-server configuration
      (<code>httpd.conf</code>)<br />
       for request ``<code>GET
      /somepath/pathinfo</code>'':</strong><br />
      </p>

<div class="note"><pre>
<strong>Given Rule</strong>                                      <strong>Resulting Substitution</strong>
----------------------------------------------  ----------------------------------
^/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
</pre></div>

      <p><strong>Inside per-directory configuration for
      <code>/somepath</code><br />
       (<em>i.e.</em>, file <code>.htaccess</code> in dir
      <code>/physical/path/to/somepath</code> containing
      <code>RewriteBase /somepath</code>)<br />
       for request ``<code>GET
      /somepath/localpath/pathinfo</code>'':</strong><br /> 
     </p>

<div class="note"><pre>
<strong>Given Rule</strong>                                      <strong>Resulting Substitution</strong>
----------------------------------------------  ----------------------------------
^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
</pre></div>

      <p><strong>Example:</strong></p>

      <p>We want to rewrite URLs of the form </p>

        <p class="indent">
          <code>/</code> <em>Language</em> <code>/~</code>
          <em>Realname</em> <code>/.../</code> <em>File</em>
        </p>

        <p>into </p>

        <p class="indent">
          <code>/u/</code> <em>Username</em> <code>/.../</code>
          <em>File</em> <code>.</code> <em>Language</em>
        </p>

        <p>We take the rewrite mapfile from above and save it under
        <code>/path/to/file/map.txt</code>. Then we only have to
        add the following lines to the Apache server configuration
        file:</p>

<div class="example"><pre>
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
</pre></div>
  
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