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<h1>Regexp</h1>
<div class="info-desc">
<h2 id="1_RegularExpressionBackends">Regular Expression Backends</h2>
<p>Unfortunately, there is a bit of confusion about the regular expression
backend used by Ocamlnet. Traditionally, this was always PCRE, and not
the Str library coming with the OCaml distribution. The reason was
that Str did not have a thread-safe API, but it was required to make
Ocamlnet thread-safe.</p>
<p>However, recent OCaml releases ship with a version of Str that can
be easily made thread-safe. Because of this, the default regexp backend is
now Str, with an option to use PCRE instead.</p>
<h3 id="2_CurrentSolution">Current Solution</h3>
<p>So we have now by default:</p>
<ul>
<li>Library <code class="code">netstring</code>, module <a href="Netstring_str.html"><code class="code">Netstring_str</code></a>: This is a thread-safe
API around Str. All uses of regular expressions inside the OCamlnet
code base call functions of this module only.</li>
</ul>
<p>You can enable PCRE in the configure script with <code class="code">-enable-pcre</code> or
<code class="code">-enable-full-pcre</code>. The effects are as follows:</p>
<ul>
<li>Library <code class="code">netstring</code>, module <a href="Netstring_str.html"><code class="code">Netstring_str</code></a>: If only <code class="code">-enable-pcre</code>
is configured, this is still an API around Str. If, however,
<code class="code">-enable-full-pcre</code> is configured, the module maps the functions
to PCRE calls. Note that the regular expressions still use Str-compatible
syntax, so this switch of the backend is normally invisible to the caller.</li>
<li>Library <code class="code">netstring-pcre</code>, module <a href="Netstring_pcre.html"><code class="code">Netstring_pcre</code></a>: This module
is now available for both configure options. <a href="Netstring_pcre.html"><code class="code">Netstring_pcre</code></a> provides
the same API, but uses PCRE-style regular expression syntax. We
provide this module solely for backward compatibility with older
versions of Ocamlnet. Note that previous versions of Ocamlnet added
this module to the <code class="code">netstring</code> library, but it is now in the new
<code class="code">netstring-pcre</code> library.</li>
</ul>
<p>The meanings of the configuration options are thus:</p>
<ul>
<li><code class="code">-enable-pcre</code>: The default regexp engine is still Str, and
<a href="Netstring_pcre.html"><code class="code">Netstring_pcre</code></a> is available in the separate library <code class="code">netstring-pcre</code></li>
<li><code class="code">-enable-full-pcre</code>: The regexp engine is changed to PCRE, and
<a href="Netstring_pcre.html"><code class="code">Netstring_pcre</code></a> is also available in the separate library <code class="code">netstring-pcre</code></li>
</ul>
<p><b>OPAM users</b>: Note that the OPAM package for OCamlnet does not
build with PCRE support by default. The trigger for this is the presence
of the <code class="code">pcre</code> OPAM package, i.e. do <code class="code">opam install pcre</code>
to include <code class="code">netstring-pcre</code> in a rebuild.</p>
<h3 id="2_PreviousSolutions">Previous Solutions</h3>
<p>Actually, Ocamlnet stepped away from PCRE in several phases.</p>
<ul>
<li>Before release 3.3: Ocamlnet always used PCRE as backend. <a href="Netstring_str.html"><code class="code">Netstring_str</code></a>
used Str syntax, but mapped this to PCRE syntax.</li>
<li>Release 3.3.1: Optionally, <a href="Netstring_str.html"><code class="code">Netstring_str</code></a> can now use Str as backend.
There are still thread-safety issues, so this was not made the default,
and only active after <code class="code">-disable-pcre</code>.</li>
<li>Release 3.6: This version fixed the thread-safety problems, and Str
was now the default backend. The option <code class="code">-enable-pcre</code> reverted to
the previous behavior: PCRE backed <a href="Netstring_str.html"><code class="code">Netstring_str</code></a>, and <a href="Netstring_pcre.html"><code class="code">Netstring_pcre</code></a>
was built. The latter module was moved to its own library <code class="code">netstring-pcre</code>.</li>
<li>Release 3.6.4 (current): Adding <code class="code">-enable-full-pcre</code> with the meaning
that PCRE becomes the backend, and <code class="code">-enable-pcre</code> has now the weaker
meaning that <code class="code">netstring-pcre</code> is built.</li>
</ul>
<h3 id="2_Recommendations">Recommendations</h3>
<p>For <b>packagers</b> of Ocamlnet, the recommendation is to build Ocamlnet
with Str as default engine, and to offer PCRE as option (i.e.
<code class="code">-enable-pcre</code>). The latter allows it to easily port other software
to the new Ocamlnet style.</p>
<p>It is discouraged to switch completely to PCRE (<code class="code">-enable-full-pcre</code>).</p>
<p>For <b>users</b> of Ocamlnet, the recommendations are:</p>
<ul>
<li>Use Str as regular expression engine, and if thread-safety is an
issue, prefer <a href="Netstring_str.html"><code class="code">Netstring_str</code></a> over the default Str API.</li>
<li>If there are still good reasons to use PCRE, you have either
the choice to use PCRE directly, or via <a href="Netstring_pcre.html"><code class="code">Netstring_pcre</code></a>. This
module will not go away (it is <b>not deprecated</b>).
</li>
</ul>
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