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an <span class="QUOTE">"action"</span>.</p>
<p><span class="APPLICATION">Privoxy</span> supports three different
- filter actions: <tt class="LITERAL"><a href=
+
pcrs-based filter actions: <tt class="LITERAL"><a href=
"actions-file.html#FILTER">filter</a></tt> to rewrite the content that is
send to the client, <tt class="LITERAL"><a href=
"actions-file.html#CLIENT-HEADER-FILTER">client-header-filter</a></tt> to
used to change the applying actions through sections with <a href=
"actions-file.html#TAG-PATTERN">tag-patterns</a>.</p>
+ <p>Finally <span class="APPLICATION">Privoxy</span> supports the
+ <tt class="LITERAL"><a href=
+ "actions-file.html#EXTERNAL-FILTER">external-filter</a></tt> action to
+ enable <tt class="LITERAL"><a href=
+ "filter-file.html#EXTERNAL-FILTER-SYNTAX">external filters</a></tt>
+ written in proper programming languages.</p>
+
<p>Multiple filter files can be defined through the <tt class=
"LITERAL"><a href="config.html#FILTERFILE">filterfile</a></tt> config
directive. The filters as supplied by the developers are located in
"_top">Perl</a>'s <tt class="LITERAL">s///</tt> operator. If you are
familiar with Perl, you will find this to be quite intuitive, and may
want to look at the PCRS documentation for the subtle differences to Perl
- behaviour. Most notably, the non-standard option letter <tt class=
+ behaviour.</p>
+
+ <p>Most notably, the non-standard option letter <tt class=
"LITERAL">U</tt> is supported, which turns the default to ungreedy
- matching.</p>
+ matching (add <tt class="LITERAL">?</tt> to quantifiers to turn them
+ greedy again).</p>
+
+ <p>The non-standard option letter <tt class="LITERAL">D</tt> (dynamic)
+ allows to use the variables $host, $origin (the IP address the request
+ came from), $path and $url. They will be replaced with the value they
+ refer to before the filter is executed.</p>
+
+ <p>Note that '$' is a bad choice for a delimiter in a dynamic filter as
+ you might end up with unintended variables if you use a variable name
+ directly after the delimiter. Variables will be resolved without escaping
+ anything, therefore you also have to be careful not to chose delimiters
+ that appear in the replacement text. For example '<' should be save,
+ while '?' will sooner or later cause conflicts with $url.</p>
+
+ <p>The non-standard option letter <tt class="LITERAL">T</tt> (trivial)
+ prevents parsing for backreferences in the substitute. Use it if you want
+ to include text like '$&' in your substitute without quoting.</p>
<p>If you are new to <a href=
"http://en.wikipedia.org/wiki/Regular_expressions" target=
started.</p>
<div class="SECT2">
- <h2 class="SECT2"><a name="AEN5185" id="AEN5185">9.1. Filter File
+ <h2 class="SECT2"><a name="AEN5020" id="AEN5020">9.1. Filter File
Tutorial</a></h2>
<p>Now, let's complete our <span class="QUOTE">"foo"</span> content
</dl>
</div>
</div>
+
+ <div class="SECT2">
+ <h2 class="SECT2"><a name="EXTERNAL-FILTER-SYNTAX" id=
+ "EXTERNAL-FILTER-SYNTAX">9.3. External filter syntax</a></h2>
+
+ <p>External filters are scripts or programs that can modify the content
+ in case common <tt class="LITERAL"><a href=
+ "actions-file.html#FILTER">filters</a></tt> aren't powerful enough.</p>
+
+ <p>External filters can be written in any language the platform
+ <span class="APPLICATION">Privoxy</span> runs on supports.</p>
+
+ <p>They are controlled with the <tt class="LITERAL"><a href=
+ "actions-file.html#EXTERNAL-FILTER">external-filter</a></tt> action and
+ have to be defined in the <tt class="LITERAL"><a href=
+ "config.html#FILTERFILE">filterfile</a></tt> first.</p>
+
+ <p>The header looks like any other filter, but instead of pcrs jobs,
+ external filters contain a single job which can be a program or a shell
+ script (which may call other scripts or programs).</p>
+
+ <p>External filters read the content from STDIN and write the rewritten
+ content to STDOUT. The environment variables PRIVOXY_URL, PRIVOXY_PATH,
+ PRIVOXY_HOST, PRIVOXY_ORIGIN can be used to get some details about the
+ client request.</p>
+
+ <p><span class="APPLICATION">Privoxy</span> will temporary store the
+ content to filter in the <tt class="LITERAL"><a href=
+ "config.html#TEMPORARY-DIRECTORY">temporary-directory</a></tt>.</p>
+
+ <table border="0" bgcolor="#E0E0E0" width="100%">
+ <tr>
+ <td>
+ <pre class="SCREEN">
+EXTERNAL-FILTER: cat Pointless example filter that doesn't actually modify the content
+/bin/cat
+
+# Incorrect reimplementation of the filter above in POSIX shell.
+#
+# Note that it's a single job that spans multiple lines, the line
+# breaks are not passed to the shell, thus the semicolons are required.
+#
+# If the script isn't trivial, it is recommended to put it into an external file.
+#
+# In general, writing external filters entirely in POSIX shell is not
+# considered a good idea.
+EXTERNAL-FILTER: cat2 Pointless example filter that despite its name may actually modify the content
+while read line; \
+do \
+ echo "$line"; \
+done
+
+EXTERNAL-FILTER: rotate-image Rotate an image by 180 degree. Test filter with limited value.
+/usr/local/bin/convert - -rotate 180 -
+
+EXTERNAL-FILTER: citation-needed Adds a "[citation needed]" tag to an image. The coordinates may need adjustment.
+/usr/local/bin/convert - -pointsize 16 -fill white -annotate +17+418 "[citation needed]" -
+</pre>
+ </td>
+ </tr>
+ </table>
+
+ <div class="WARNING">
+ <table class="WARNING" border="1" width="100%">
+ <tr>
+ <td align="center"><b>Warning</b></td>
+ </tr>
+
+ <tr>
+ <td align="left">
+ <p>Currently external filters are executed with <span class=
+ "APPLICATION">Privoxy</span>'s privileges! Only use external
+ filters you understand and trust.</p>
+ </td>
+ </tr>
+ </table>
+ </div>
+
+ <p>External filters are experimental and the syntax may change in the
+ future.</p>
+ </div>
</div>
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