# Postfix PCRE Support

## PCRE (Perl Compatible Regular Expressions) map support

The optional "pcre" map type allows you to specify regular expressions with the PERL style notation such as \s for space and \S for non-space. The main benefit, however, is that pcre lookups are often faster than regexp lookups. This is because the pcre implementation is often more efficient than the POSIX regular expression implementation that you find on many systems.

A description of how to use pcre tables, including examples, is given in the pcre_table(5) manual page. Information about PCRE itself can be found at https://www.pcre.org/.

## Using Postfix packages with PCRE support

To use pcre with Debian GNU/Linux's Postfix, or with Fedora or RHEL Postfix, all you need is to install the postfix-pcre package and you're done. There is no need to recompile Postfix.

## Building Postfix from source with PCRE support

These instructions assume that you build Postfix from source code as described in the INSTALL document.

To build Postfix from source with pcre support, you need a pcre library. Install a vendor package, or download the source code from locations in https://www.pcre.org/ and build that yourself.

Postfix can build with the pcre2 library or the legacy pcre library. It's probably easiest to let the Postfix build procedure pick one. The following commands will first discover if the pcre2 library is installed, and if that is not available, will discover if the legacy pcre library is installed.

>     $ make -f Makefile.init makefiles
>     $ make

To build Postfix explicitly with a pcre2 library (Postfix 3.7 and later):

>     $ make -f Makefile.init makefiles \
>         "CCARGS=-DHAS_PCRE=2 `pcre2-config --cflags`" \
>         "AUXLIBS_PCRE=`pcre2-config --libs8`"
>     $ make

To build Postfix explicitly with a legacy pcre library (all Postfix versions):

>     $ make -f Makefile.init makefiles \
>         "CCARGS=-DHAS_PCRE=1 `pcre-config --cflags`" \
>         "AUXLIBS_PCRE=`pcre-config --libs`"
>     $ make

Postfix versions before 3.0 use AUXLIBS instead of AUXLIBS_PCRE. With Postfix 3.0 and later, the old AUXLIBS variable still supports building a statically-loaded PCRE database client, but only the new AUXLIBS_PCRE variable supports building a dynamically-loaded or statically-loaded PCRE database client.

> Failure to use the AUXLIBS_PCRE variable will defeat the purpose of dynamic database client loading. Every Postfix executable file will have PCRE library dependencies. And that was exactly what dynamic database client loading was meant to avoid.

## Things to know

- When Postfix searches a pcre: or regexp: lookup table, each pattern is applied to the entire input string. Depending on the application, that string is an entire client hostname, an entire client IP address, or an entire mail address. Thus, no parent domain or parent network search is done, "user@domain" mail addresses are not broken up into their user and domain constituent parts, and "user+foo" is not broken up into user and foo.

- Regular expression tables such as pcre: or regexp: are not allowed to do $number substitution in lookup results that can be security sensitive: currently, that restriction applies to the local aliases(5) database or the virtual(8) delivery agent tables.