Google Message Security for Google Apps Administration Guide - Manual (Page 220)

220 Message Security for Google Apps Administration Guide
Character Description
Anchors
^ (caret) Matches the start of the line or string of text that the
regular expression is searching. For example, a content rule
with a location Subject line and the following regular
expression:
^abc
captures any email message that has a subject line beginning
with the letters abc.
$ (dollar) Matches the end of the line or string of text that the
regular expression is searching. For example, a content rule
with a location Subject line and the following regular
expression:
xyz$
captures any email message that has a subject line ending with
the letters xyz.
Metacharacters
. (dot) Matches any single character, except a new line.
| (pipe) Indicates alternation—that is, an “or.” For example:
cat|dog matches the word cat or dog
\ Indicates that the next character is a literal rather than a special
character. For example:
\. matches a literal period, rather than any character (dot
character)
Character Classes
[...] Matches any character from a set of characters. Separate the
first and last character in a set with a dash. For example:
[123] matches the digit 1, 2, or 3
[a-f] matches any letter from a to f
Note: Regular expressions in Content Manager are not case
sensitive. Therefore, using this formatting to specify a character
set in lowercase letters also includes the equivalent uppercase
letters.