sendmail, 4th Edition

Copy by Position: $digit

The $digit operator in the RHS is used to copy tokens from the LHS into the workspace. The digit refers to positions of LHS wildcard operators in the LHS:

R $+ @ $*    $2!$1
  ↑    ↑
  $1         $2

Here, the $1 in the RHS indicates tokens matched by the first wildcard operator in the LHS (in this case, the $+), and the $2 in the RHS indicates tokens matched by the second wildcard operator in the LHS (the $*). In this example, if the workspace contains A@B.C, it will be rewritten by the RHS as follows (note that the order is defined by the RHS):

$* matches    B.C     so  $2 copies  it to workspace
        !    explicitly added to the workspace
$+ matches    A       so  $1 adds  it to workspace

The $digit copies all the tokens matched by its corresponding wildcard operator. For the $+ wildcard operator, only a single token (A) is matched and copied with $1. The ! is copied as is. For the $* wildcard operator, three tokens are matched (B.C), so $2 copies all three. Thus, this rule rewrites A@B.C into B.C!A.

Not all LHS operators need to be referenced with a $digit in the RHS. Consider the following:

R $* < $* > $*   <$2>

Here, only the middle LHS operator (the second one) is required to rewrite the workspace. So, only the $2 is needed in the RHS ($1 and $3 are not needed and are not present in the RHS).

Although macros appear to be operators in the LHS, they are not. Recall that macros are expanded when the configuration file is read (Macros in Rules on page 650). As a consequence, although they appear as $letter ...