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Regular Expression Backends

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.

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.

Current Solution

So we have now by default:

  • Library netstring, module Netstring_str: This is a thread-safe API around Str. All uses of regular expressions inside the OCamlnet code base call functions of this module only.
You can enable PCRE in the configure script with -enable-pcre or -enable-full-pcre. The effects are as follows:

  • Library netstring, module Netstring_str: If only -enable-pcre is configured, this is still an API around Str. If, however, -enable-full-pcre 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.
  • Library netstring-pcre, module Netstring_pcre: This module is now available for both configure options. Netstring_pcre 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 netstring library, but it is now in the new netstring-pcre library.
The meanings of the configuration options are thus:

  • -enable-pcre: The default regexp engine is still Str, and Netstring_pcre is available in the separate library netstring-pcre
  • -enable-full-pcre: The regexp engine is changed to PCRE, and Netstring_pcre is also available in the separate library netstring-pcre

Previous Solutions

Actually, Ocamlnet stepped away from PCRE in several phases.

  • Before release 3.3: Ocamlnet always used PCRE as backend. Netstring_str used Str syntax, but mapped this to PCRE syntax.
  • Release 3.3.1: Optionally, Netstring_str can now use Str as backend. There are still thread-safety issues, so this was not made the default, and only active after -disable-pcre.
  • Release 3.6: This version fixed the thread-safety problems, and Str was now the default backend. The option -enable-pcre reverted to the previous behavior: PCRE backed Netstring_str, and Netstring_pcre was built. The latter module was moved to its own library netstring-pcre.
  • Release 3.6.4 (current): Adding -enable-full-pcre with the meaning that PCRE becomes the backend, and -enable-pcre has now the weaker meaning that netstring-pcre is built.


For packagers of Ocamlnet, the recommendation is to build Ocamlnet with Str as default engine, and to offer PCRE as option (i.e. -enable-pcre). The latter allows it to easily port other software to the new Ocamlnet style.

It is discouraged to switch completely to PCRE (-enable-full-pcre).

For users of Ocamlnet, the recommendations are:

  • Use Str as regular expression engine, and if thread-safety is an issue, prefer Netstring_str over the default Str API.
  • If there are still good reasons to use PCRE, you have either the choice to use PCRE directly, or via Netstring_pcre. This module will not go away (it is not deprecated).

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