For a more accurate documentation on regular expressions, please refer to the Python 're' module at: http://www.python.org/doc/2.3.2/lib/module-re.html
Spammers often use alternative characters to pass through filters. For example, @ for a, 1 for i, 0 for O... Therefore, the line:
Spammers often send their message to a list of users that belong to your domain. Use regular expressions for 'to:' and 'cc:'.