Google Message Security for Google Apps Administration Guide - Manual (Page 230)

230 Message Security for Google Apps Administration Guide
Match Any Email Address from a Specific Domain
Notes • [\w.+\-] matches any word character (a-z, A-Z, 0-9, or
an underscore), a period, a plus sign, a percent sign, or
a hyphen. These are the only valid characters in a URL.
Note that the
\- (which indicates a hyphen) must occur
last in the list of characters within the square brackets.
• {0,25} indicates that from 0 to 25 characters in the
preceding character set can occur after the text badmail.
Content Manager supports matching of up to 25
characters for each character set in a regular
expression.
• The \ before the dash and period “escapes” these
characters—that is, it indicates that the dash and period
are not a regex special characters themselves. Note that
there is no need to escape the period within the square
brackets.
Usage example Match any email address from the domains yahoo.com,
hotmail.com, and gmail.com.
Regex example (\W|^)[\w.+\-]{0,25}@(yahoo|hotmail|gmail)
\.com(\W|$)