path: root/docs/manual/mod/mod_substitute.xml

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<modulesynopsis metafile="mod_substitute.xml.meta">

<name>mod_substitute</name>
<description>Perform search and replace operations on response bodies</description>
<status>Extension</status>
<sourcefile>mod_substitute.c</sourcefile>
<identifier>substitute_module</identifier>

<summary>
    <p><module>mod_substitute</module> provides a mechanism to perform
    both regular expression and fixed string substitutions on
    response bodies.</p>
</summary>

<directivesynopsis>
<name>Substitute</name>
<description>Pattern to filter the response content</description>
<syntax>Substitute <var>s/pattern/substitution/[infq]</var></syntax>
<contextlist><context>directory</context>
<context>.htaccess</context></contextlist>
<override>FileInfo</override>
<compatibility>"expr=" substitution values were added in 2.5.1</compatibility>

<usage>
    <p>The <directive>Substitute</directive> directive specifies a
    search and replace pattern to apply to the response body.</p>

    <p>The meaning of the pattern can be modified by using any
    combination of these flags:</p>

    <dl>
        <dt><code>i</code></dt>
        <dd>Perform a case-insensitive match.</dd>
        <dt><code>n</code></dt>
        <dd>By default the pattern is treated as a regular expression.
        Using the <code>n</code> flag forces the pattern to be treated
        as a fixed string.</dd>
        <dt><code>f</code></dt>
        <dd>The <code>f</code> flag causes <code>mod_substitute</code> to flatten the
        result of a substitution allowing for later substitutions to
        take place on the boundary of this one. This is the default.</dd>
        <dt><code>q</code></dt>
        <dd>The <code>q</code> flag causes <code>mod_substitute</code> to not
        flatten the buckets after each substitution. This can
        result in much faster response and a decrease in memory
        utilization, but should only be used if there is no possibility
        that the result of one substitution will ever match a pattern
        or regex of a subsequent one.</dd>
    </dl>

    <p>The <var>substitution</var> may contain literal text and regular
    expression backreferences. If the substitution begins with the text 
    <code>expr=</code> it is intepreted as an <a href="../expr.html">
    expression</a> which allows access to environment variables and
    header values.  </p>

    <example><title>Example</title>
    <highlight language="config">
&lt;Location "/"&gt;
    AddOutputFilterByType SUBSTITUTE text/html
    Substitute "s/foo/bar/ni"
&lt;/Location&gt;
        </highlight>
    </example>

    <p>The character which is used to separate (or "delimit") the
    various parts of the substituion string is referred to as the
    "delimiter", and it is most common to use a slash for this
    purpose.</p>

    <p>If either the pattern or the substitution contain a slash
    character then an alternative delimiter may be used to make the
    directive more readable:</p>

    <example><title>Example of using an alternate delimiter</title>
    <highlight language="config">
&lt;Location "/"&gt;
    AddOutputFilterByType SUBSTITUTE text/html
    Substitute "s|&lt;BR */?&gt;|&lt;br /&gt;|i"
&lt;/Location&gt;
        </highlight>
    </example>

    <p>Backreferences can be used in the comparison and in the substitution,
    when regular expressions are used, as illustrated in the following example: </p>
    <example><title>Example of using backreferences and captures</title>
    <highlight language="config">
&lt;Location "/"&gt;
    AddOutputFilterByType SUBSTITUTE text/html
    # "foo=k,bar=k" -> "foo/bar=k"
    Substitute "s|foo=(\w+),bar=\1|foo/bar=$1|"
&lt;/Location&gt;
    </highlight>
    </example>

    <p> When using an <a href="../expr.html">expression</a> for the 
    <var>substitution</var>, regular expression backreferences must be 
    backslash ('\') escaped as illustrated in the example below:</p> 
    <example><title>Expression Example</title>
    <highlight language="config">
&lt;Location "/"&gt;
    AddOutputFilterByType SUBSTITUTE text/html
    Substitute "s/example.com/expr=%{HTTP:HOST}/i"
    Substitute "s/Hello, (\S+)/expr=Hello from %{REQUEST_URI}, \$1/i"
&lt;/Location&gt;
        </highlight>
    </example>

    <note type="warning"><title>Expressions and caching</title>
    <p>Caution must be exercised when performing substitutions that reference
    HTTP request headers.  Because this module operates after response headers
    have been sent, the <a href="../expr.html">expression parser</a> cannot add
    referenced HTTP request headers to the outgoing Vary header. </p>
    </note>

    <p>A common use scenario for <code>mod_substitute</code> is the
    situation in which a front-end server proxies requests to a back-end
    server which returns HTML with hard-coded embedded URLs that refer
    to the back-end server. These URLs don't work for the end-user,
    since the back-end server is unreachable.</p>

    <p>In this case, <code>mod_substitute</code> can be used to rewrite
    those URLs into something that will work from the front end:</p>

    <example><title>Rewriting URLs embedded in proxied content</title>
    <highlight language="config">
ProxyPass        "/blog/" "http://internal.blog.example.com/"
ProxyPassReverse "/blog/" "http://internal.blog.example.com/"

Substitute "s|http://internal.blog.example.com/|http://www.example.com/blog/|i"
    </highlight>
    </example>

    <p><directive module="mod_proxy">ProxyPassReverse</directive>
    modifies any <code>Location</code> (redirect) headers that are sent
    by the back-end server, and, in this example,
    <directive>Substitute</directive> takes care of the rest of the problem by
    fixing up the HTML response as well.</p>

</usage>
</directivesynopsis>

<directivesynopsis>
<name>SubstituteMaxLineLength</name>
<description>Set the maximum line size</description>
<syntax>SubstituteMaxLineLength <var>bytes</var>(b|B|k|K|m|M|g|G)</syntax>
<default>SubstituteMaxLineLength 1m</default>
<contextlist><context>directory</context>
<context>.htaccess</context></contextlist>
<override>FileInfo</override>
<compatibility>Available in httpd 2.4.11 and later</compatibility>

<usage>
    <p>The maximum line size handled by <module>mod_substitute</module>
    is limited to restrict memory use. The limit can be configured
    using <directive>SubstituteMaxLineLength</directive>.
    The value can be given as the number of bytes and can be suffixed
    with a single letter <code>b</code>, <code>B</code>, <code>k</code>,
    <code>K</code>, <code>m</code>, <code>M</code>, <code>g</code>,
    <code>G</code> to provide the size in bytes, kilobytes, megabytes
    or gigabytes respectively.</p>

    <example><title>Example</title>
    <highlight language="config">
&lt;Location "/"&gt;
    AddOutputFilterByType SUBSTITUTE text/html
    SubstituteMaxLineLength 10m
    Substitute "s/foo/bar/ni"
&lt;/Location&gt;
        </highlight>
    </example>

</usage>
</directivesynopsis>

<directivesynopsis>
<name>SubstituteInheritBefore</name>
<description>Change the merge order of inherited patterns</description>
<syntax>SubstituteInheritBefore on|off</syntax>
<default>SubstituteInheritBefore on</default>
<contextlist><context>directory</context>
<context>.htaccess</context></contextlist>
<override>FileInfo</override>
<compatibility>Available in httpd 2.4.17 and later</compatibility>

<usage>
    <p>Whether to apply the inherited <directive module="mod_substitute">Substitute</directive>
    patterns first (<code>on</code>), or after the ones of the current
    context (<code>off</code>).
    The latter was the default in versions 2.4 and earlier, but changed
    starting with 2.5, hence <directive>SubstituteInheritBefore</directive>
    set to <code>off</code> allows to restore the legacy behaviour.
    <directive>SubstituteInheritBefore</directive> is itself inherited,
    hence contexts that inherit it (those that don't specify their own
    <directive>SubstituteInheritBefore</directive> value) will apply the
    closest defined merge order.</p>
</usage>
</directivesynopsis>

</modulesynopsis>