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<manualpage metafile="intro.xml.meta">
<parentdocument href="./">Rewrite</parentdocument>

  <title>Apache mod_rewrite Introduction</title>

<summary>
<p>This document supplements the <module>mod_rewrite</module>
<a href="../mod/mod_rewrite.html">reference documentation</a>. It
describes the basic concepts necessary for use of
<module>mod_rewrite</module>. Other documents go into greater detail,
but this doc should help the beginner get their feet wet.
</p>
</summary>

<seealso><a href="../mod/mod_rewrite.html">Module documentation</a></seealso>
<!-- <seealso><a href="intro.html">mod_rewrite introduction</a></seealso> -->
<seealso><a href="remapping.html">Redirection and remapping</a></seealso>
<seealso><a href="access.html">Controlling access</a></seealso>
<seealso><a href="vhosts.html">Virtual hosts</a></seealso>
<seealso><a href="proxy.html">Proxying</a></seealso>
<seealso><a href="rewritemap.html">Using RewriteMap</a></seealso>
<seealso><a href="advanced.html">Advanced techniques</a></seealso>
<seealso><a href="avoid.html">When not to use mod_rewrite</a></seealso>

<section id="introduction"><title>Introduction</title>
<p>The Apache module <module>mod_rewrite</module> is a very powerful and
sophisticated module which provides a way to do URL manipulations. With
it, you can do nearly all types of URL rewriting that you may need. It
is, however, somewhat complex, and may be intimidating to the beginner.
There is also a tendency to treat rewrite rules as magic incantation,
using them without actually understanding what they do.</p>

<p>This document attempts to give sufficient background so that what
follows is understood, rather than just copied blindly.
</p>

<p>Remember that many common URL-manipulation tasks don't require the
full power and complexity of <module>mod_rewrite</module>. For simple
tasks, see <module>mod_alias</module> and the documentation
on <a href="../urlmapping.html">mapping URLs to the
filesystem</a>.</p>

<p>Finally, before proceeding, be sure to configure
<module>mod_rewrite</module>'s log level to one of the trace levels using
the <directive module="core">LogLevel</directive> directive. Although this
can give an overwhelming amount of information, it is indispensable in
debugging problems with <module>mod_rewrite</module> configuration, since
it will tell you exactly how each rule is processed.</p>

</section>

<section id="regex"><title>Regular Expressions</title>

<p>mod_rewrite uses the <a href="http://pcre.org/">Perl Compatible
Regular Expression</a> vocabulary. In this document, we do not attempt
to provide a detailed reference to regular expressions. For that, we
recommend the <a href="http://pcre.org/pcre.txt">PCRE man pages</a>, the
<a href="http://perldoc.perl.org/perlre.html">Perl regular
expression man page</a>, and <a
href="http://shop.oreilly.com/product/9780596528126.do">Mastering
Regular Expressions, by Jeffrey Friedl</a>.</p>

<p>In this document, we attempt to provide enough of a regex vocabulary
to get you started, without being overwhelming, in the hope that
<directive module="mod_rewrite">RewriteRule</directive>s will be scientific
formulae, rather than magical incantations.</p>

<section id="regexvocab"><title>Regex vocabulary</title>

<p>The following are the minimal building blocks you will need, in order
to write regular expressions and <directive
module="mod_rewrite">RewriteRule</directive>s. They certainly do not
represent a complete regular expression vocabulary, but they are a good
place to start, and should help you read basic regular expressions, as
well as write your own.</p>

<table>
<tr>
<th>Character</th>
<th>Meaning</th>
<th>Example</th>
</tr>

<tr><td><code>.</code></td><td>Matches any single
character</td><td><code>c.t</code> will match <code>cat</code>,
<code>cot</code>, <code>cut</code>, etc.</td></tr>
<tr><td><code>+</code></td><td>Repeats the previous match one or more
times</td><td><code>a+</code> matches <code>a</code>, <code>aa</code>,
<code>aaa</code>, etc</td></tr>
<tr><td><code>*</code></td><td>Repeats the previous match zero or more
times.</td><td><code>a*</code> matches all the same things
<code>a+</code> matches, but will also match an empty string.</td></tr>
<tr><td><code>?</code></td><td>Makes the match optional.</td><td>
<code>colou?r</code> will match <code>color</code> and <code>colour</code>.</td>
</tr>
<tr><td><code>^</code></td><td>Called an anchor, matches the beginning
of the string</td><td><code>^a</code> matches a string that begins with
<code>a</code></td></tr>
<tr><td><code>$</code></td><td>The other anchor, this matches the end of
the string.</td><td><code>a$</code> matches a string that ends with
<code>a</code>.</td></tr>
<tr><td><code>( )</code></td><td>Groups several characters into a single
unit, and captures a match for use in a backreference.</td><td><code>(ab)+</code>
matches <code>ababab</code> - that is, the <code>+</code> applies to the group.
For more on backreferences see <a href="#InternalBackRefs">below</a>.</td></tr>
<tr><td><code>[ ]</code></td><td>A character class - matches one of the
characters</td><td><code>c[uoa]t</code> matches <code>cut</code>,
<code>cot</code> or <code>cat</code>.</td></tr>
<tr><td><code>[^ ]</code></td><td>Negative character class - matches any character not specified</td><td><code>c[^/]t</code> matches <code>cat</code> or <code>c=t</code> but not <code>c/t</code></td></tr>
</table>

<p>In <module>mod_rewrite</module> the <code>!</code> character can be
used before a regular expression to negate it. This is, a string will
be considered to have matched only if it does not match the rest of
the expression.</p>

</section>

<section id="InternalBackRefs"><title>Regex Back-Reference Availability</title>

      <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> as 
      outlined in the following chapters. Figure 1 shows to which 
      locations the back-references are transferred for expansion as 
      well as illustrating the flow of the RewriteRule, RewriteCond 
      matching. In the next chapters, we will be exploring how to use 
      these back-references, so do not fret if it seems a bit alien 
      to you at first.
      </p>

<p class="figure">
      <img src="../images/rewrite_backreferences.png"
      alt="Flow of RewriteRule and RewriteCond matching" /><br />
      <dfn>Figure 1:</dfn> The back-reference flow through a rule.<br />
      In this example, a request for <code>/test/1234</code> would be transformed into <code>/admin.foo?page=test&amp;id=1234&amp;host=admin.example.com</code>.
</p>

</section>
</section>

<section id="rewriterule"><title>RewriteRule Basics</title>
<p>A <directive module="mod_rewrite">RewriteRule</directive> consists
of three arguments separated by spaces. The arguments are</p>
<ol>
<li><var>Pattern</var>: which incoming URLs should be affected by the rule;</li>
<li><var>Substitution</var>: where should the matching requests be sent;</li>
<li><var>[flags]</var>: options affecting the rewritten request.</li>
</ol>

<p>The <var>Pattern</var> is a <a href="#regex">regular expression</a>. 
It is initially (for the first rewrite rule or until a substitution occurs) 
matched against the URL-path of the incoming request (the part after the 
hostname but before any question mark indicating the beginning of a query 
string) or, in per-directory context, against the request's path relative 
to the directory for which the rule is defined. Once a substitution has 
occurred, the rules that follow are matched against the substituted
value.
</p>

<p class="figure">
      <img src="../images/syntax_rewriterule.png"
      alt="Syntax of the RewriteRule directive" /><br />
      <dfn>Figure 2:</dfn> Syntax of the RewriteRule directive.
</p>


<p>The <var>Substitution</var> can itself be one of three things:</p>

<dl>
<dt>1. A full filesystem path to a resource</dt>
<dd>
<highlight language="config">
RewriteRule "^/games" "/usr/local/games/web/puzzles.html"
</highlight>
<p>This maps a request to an arbitrary location on your filesystem, much
like the <directive module="mod_alias">Alias</directive> directive.</p>
</dd>

<dt>2. A web-path to a resource</dt>
<dd>
<highlight language="config">
RewriteRule "^/games$" "/puzzles.html"
</highlight>
<p>If <directive module="core">DocumentRoot</directive> is set
to <code>/usr/local/apache2/htdocs</code>, then this directive would
map requests for <code>http://example.com/games</code> to the
path <code>/usr/local/apache2/htdocs/puzzles.html</code>.</p>

</dd>

<dt>3. An absolute URL</dt>
<dd>
<example>
RewriteRule ^/product/view$ http://site2.example.com/seeproduct.html [R]
</example>
<p>This tells the client to make a new request for the specified URL.</p>
</dd>
</dl>

<note type="warning">Note that <strong>1</strong> and <strong>2</strong> have exactly the same syntax. The difference between them is that in the case of <strong>1</strong>, the top level of the target path (i.e., <code>/usr/</code>) exists on the filesystem, where as in the case of <strong>2</strong>, it does not. (i.e., there's no <code>/bar/</code> as a root-level directory in the filesystem.)</note>

<p>The <var>Substitution</var> can also
contain <em>back-references</em> to parts of the incoming URL-path
matched by the <var>Pattern</var>. Consider the following:</p>
<example>
RewriteRule ^/product/(.*)/view$ /var/web/productdb/$1
</example>
<p>The variable <code>$1</code> will be replaced with whatever text
was matched by the expression inside the parenthesis in
the <var>Pattern</var>. For example, a request
for <code>http://example.com/product/r14df/view</code> will be mapped
to the path <code>/var/web/productdb/r14df</code>.</p>

<p>If there is more than one expression in parenthesis, they are
available in order in the
variables <code>$1</code>, <code>$2</code>, <code>$3</code>, and so
on.</p>


</section>

<section id="flags"><title>Rewrite Flags</title>
<p>The behavior of a <directive
module="mod_rewrite">RewriteRule</directive> can be modified by the
application of one or more flags to the end of the rule. For example, the
matching behavior of a rule can be made case-insensitive by the
application of the <code>[NC]</code> flag:
</p>
<example>
RewriteRule ^puppy.html smalldog.html [NC]
</example>

<p>For more details on the available flags, their meanings, and
examples, see the <a href="flags.html">Rewrite Flags</a> document.</p>

</section>


<section id="rewritecond"><title>Rewrite Conditions</title>
<p>One or more <directive module="mod_rewrite">RewriteCond</directive>
directives can be used to restrict the types of requests that will be
subject to the
following <directive module="mod_rewrite">RewriteRule</directive>. The
first argument is a variable describing a characteristic of the
request, the second argument is a <a href="#regex">regular
expression</a> that must match the variable, and a third optional
argument is a list of flags that modify how the match is evaluated.</p>

<p class="figure">
      <img src="../images/syntax_rewritecond.png"
      alt="Syntax of the RewriteCond directive" /><br />
      <dfn>Figure 3:</dfn> Syntax of the RewriteCond directive
</p>

<p>For example, to send all requests from a particular IP range to a
different server, you could use:</p>
<example>
RewriteCond %{REMOTE_ADDR} ^10\.2\.<br />
RewriteRule (.*) http://intranet.example.com$1
</example>

<p>When more than
one <directive module="mod_rewrite">RewriteCond</directive> is
specified, they must all match for
the <directive module="mod_rewrite">RewriteRule</directive> to be
applied. For example, to deny requests that contain the word "hack" in
their query string, unless they also contain a cookie containing
the word "go", you could use:</p>
<example>
RewriteCond %{QUERY_STRING} hack<br />
RewriteCond %{HTTP_COOKIE} !go<br />
RewriteRule .* - [F]
</example>
<p>Notice that the exclamation mark specifies a negative match, so the rule is only applied if the cookie does not contain "go".</p>

<p>Matches in the regular expressions contained in
the <directive module="mod_rewrite">RewriteCond</directive>s can be
used as part of the <var>Substitution</var> in
the <directive module="mod_rewrite">RewriteRule</directive> using the
variables <code>%1</code>, <code>%2</code>, etc. For example, this
will direct the request to a different directory depending on the
hostname used to access the site:</p>
<example>
RewriteCond %{HTTP_HOST} (.*)<br />
RewriteRule ^/(.*) /sites/%1/$1
</example>
<p>If the request was for <code>http://example.com/foo/bar</code>,
then <code>%1</code> would contain <code>example.com</code>
and <code>$1</code> would contain <code>foo/bar</code>.</p>



</section>

<section id="rewritemap"><title>Rewrite maps</title>

<p>The <directive module="mod_rewrite">RewriteMap</directive> directive
provides a way to call an external function, so to speak, to do your
rewriting for you. This is discussed in greater detail in the <a
href="rewritemap.html">RewriteMap supplementary documentation</a>.</p>
</section>

<section id="htaccess"><title>.htaccess files</title>

<p>Rewriting is typically configured in the main server configuration
setting (outside any <directive type="section"
module="core">Directory</directive> section) or
inside <directive type="section" module="core">VirtualHost</directive>
containers. This is the easiest way to do rewriting and is
recommended. It is possible, however, to do rewriting
inside <directive type="section" module="core">Directory</directive>
sections or <a href="../howto/htaccess.html"><code>.htaccess</code>
files</a> at the expense of some additional complexity. This technique
is called per-directory rewrites.</p>

<p>The main difference with per-server rewrites is that the path
prefix of the directory containing the <code>.htaccess</code> file is
stripped before matching in
the <directive module="mod_rewrite">RewriteRule</directive>. In addition, the <directive module="mod_rewrite">RewriteBase</directive> should be used to assure the request is properly mapped.</p>

</section>

</manualpage>