Rules are the building blocks of SpamAssassin. Every test performed on a message is based upon a rule, with an associated score. Rules were covered briefly in Chapter 7.
User-definable rules are based on a Perl regular expression, also called a regex. Some knowledge of regexes is required to write new rules. There are many good sources of information on Perl regular expressions. The standard Perl distribution contains a quick-start document on regular expressions, a longer regular expressions tutorial, and a syntax definition. To access these, use the
$ perldoc perlrequick $ perldoc perlretut $ perldoc perlre
Other sources of regular expression material include most beginners' books on Perl. An Internet search ...