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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 http:/
/www.pcre.org/.

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These instructions assume that you build Postfix from source code as described
in the INSTALL document. Some modification may be required if you build Postfix
from a vendor-specific source package.

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

In some future, Postfix will have a plug-in interface for adding map types.
Until then, you need to compile PCRE support into Postfix.

First of all, you need the PCRE library (Perl Compatible Regular Expressions),
which can be obtained from:

    ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/.

NOTE: pcre versions prior to 2.06 cannot be used.

In order to build Postfix with PCRE support you need to add -DHAS_PCRE and a -
I option for the PCRE include file to CCARGS, and add the path to the PCRE
library to AUXLIBS_PCRE, for example:

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

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.

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  * 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.