When trying to match tokens in the workspace to members of a class, you can use the $= operator. For example, consider the following rule:

R $+ < @ $={InternalHosts} >             $: $1 < @ mailhub >

Here, the workspace is expected to hold a focused address (such as gw<@wash.dc.gov>). The $={InternalHosts} expression causes sendmail to look up the host part of the address (the wash.dc.gov) in the class {InternalHosts}. If that host is found in that class, a match is made and the workspace is rewritten by the RHS to become gw<@mailhub>.

Class macros in general are described in Chapter 22, and the $= operator in particular is described in full in Section 22.2.1.

Note that the $= operator can be used only on the LHS of rules, and that the $= operator can be referenced by an RHS $digit operator.