A regular expression is a set of symbols and syntactic elements that Mail Security uses to match patterns of text. Mail Security matches regular expressions on a line-by-line basis. It does not evaluate the line feed (newline) character at the end of each input expression phrase.

You can build regular expressions using a combination of normal alphanumeric characters and metacharacters. For example, some email messages contain a trailing number at the end of the subject line text. Trailing numbers often indicate that a message is spam. Consider the following sample subject line:

Here's a hot stock pick!43234

To write a rule to match email subject lines that have trailing numbers, compare the subject against the following regular expression:

^.+![0-9]+$

This regular expression contains the normal alphanumeric characters 0-9 and the following metacharacters: circumflex (^), period (.), plus (+), and open and close brackets ([ , ]). By using the subject attribute, the = operator, and the regular expression as the value, you can build a content filtering rule to catch any email messages whose subject lines end with a trailing number.

See Regular expressions.