Google Message Security for Google Apps Administration Guide - Manual (Page 253)

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About the Credit Card Numbers Lexicon
The Credit Card Numbers lexicon is a predefined filter that finds credit card
numbers in email messages. The following table describes how this lexicon
works:
Lexicon pattern
matching This lexicon matches sequences of 16 digits. The digits
in a valid sequence can be separated by spaces or
dashes. The following are examples of the patterns
that this lexicon matches:
• nnnn-nnnn-nnnn-nnnn
• nnnn nnnn nnnn nnnn
• nnnn nnnn-nnnn nnnn
• nnnnnnnnnnnnnnnn
Number validity
checking This lexicon also checks sequences of 16 digits to
determine whether they meet the requirements for a
valid credit card number. However, it does not
determine whether a number is for a valid or active
credit card account.
To verify the validity of a credit card number, this
lexicon uses the LUNH formula (a type of
mathematical calculation) to determine whether the 16-
digit sequence matches the requirements from the
following card issuers:
• Visa
• Mastercard
• Discover
• JCB
Important: This lexicon does not match or verify the
validity of credit card numbers that contain more or
fewer than 16 digits, such as those issued by American
Express or Diner’s Club.
Filtering Accuracy The Credit Card Numbers lexicon is designed to
provide a high level of filtering accuracy, to help reduce
the risk of exposure of personal information. However,
please note the following:
• Credit card numbers can be represented or
formatted in various ways; therefore, this lexicon
may not capture all messages that contain credit
card numbers.
• Because this lexicon looks for specific patterns of
numbers, it may match numerical patterns that are
not credit card numbers.