The result, an indexed file in dbm or db format, is used for fast searching by the mail system. When the table is provided via other means such as NIS, LDAP or SQL, the same lookups are done as for ordinary indexed files. In that case, the lookups are done in a slightly different way as described below.
Table lookups are case insensitive.
This form has precedence over all other forms. This form has the lowest precedence. This section describes how the table lookups change when the table is given in the form of regular expressions. Each pattern is a regular expression that is applied to the entire address being looked up. Patterns are applied in the order as specified in the table, until a pattern is found that matches the search string.