When Postfix searches a
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
not allowed to do $number substitution in lookup results that can
be security sensitive: currently, that restriction applies to the
aliases(5) database or the
virtual(8) delivery agent tables.