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/.
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:
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 for the PCRE include file to CCARGS, and add the path to the PCRE library to AUXLIBS, for example:
make -f Makefile.init makefiles \ "CCARGS=-DHAS_PCRE -I/usr/local/include" \ "AUXLIBS=-L/usr/local/lib -lpcre"
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.