Google Message Security for Google Apps Administration Guide - Manual (Page 223)

Content Manager 223
Group
(...) Groups parts of an expression. Use grouping to apply a
quantifier to a group or to match a character class before or
after a group. For example, in the following expression:
\W(dog|cat|mouse)\W
the \W character class applies before and after each word in the
group. The expression would match things like dog+cat or
*dog*.
Quantifiers
{n} Match the preceding expression exactly n times. For example:
[a-c]{2} matches any letter from a to c only if two letters occur
in a row. Thus, the expression would match ab and ac but not
abc or aabbc.
{n,m} Match the preceding expression a minimum of n times and a
maximum of m times. For example:
[a-c]{2,4} matches any letter from a to c only if the letters
occur a minimum of 2 times and a maximum of 4 times in a row.
Thus, the expression would match ab and abc but not aabbc.
? Indicates that the preceding character or expression can match
0 or 1 times. Equivalent to the range
{0,1} . For example, the
following regular expression:
colou?r
matches either colour or color, because the ? makes the letter
u optional.
Character Description