10.2.1 The LHS

The wildcard operator in this LHS, the $*, is different from the $+ wildcard operator that you saw in rule set 0. Recall that the $+ wildcard operator matches one or more tokens in the workspace. To review, consider the LHS rule:

$+ @ $+

This LHS easily matches an address like you@here.us.edu in the workspace:

workspace                     LHS
you               $+        match one or more              
@                 @         match exactly              
here              $+        match one              
.                               or more
us
.
edu

This same LHS, however, does not match an address like @here.us.edu:

workspace                  LHS
@               $+        match one              
here                          or more              
.
us
.
edu             
                @         match exactly, fails!
                $+

Because the $+ wildcard operator needs to match one or more tokens, it fails when there is nothing in front of the @.

The $* wildcard operator is just like the $+ wildcard operator, except that it will match nothing (zero tokens). If the LHS had used $* instead of $+, an address like @here.us.edu would be matched:

workspace                   LHS
                 $*        match zero or more (matches zero)              
@                @         match exactly              
here             $*        match zero              
.                              or more
us
.
edu

The LHS in rule set 3 matches anything or nothing, provided that there is a pair of angle brackets in the workspace somewhere:

R$* < $* > $*   $2             basic RFC822 parsing

For example, consider an address that might be given to sendmail by your MUA:

Your Fullname <you@here.us.edu>

This address is tokenized and placed into the workspace. That workspace is then compared to the LHS:

workspace                   LHS
Your             $*        match zero              
Fullname                       or more              
<                <         match exactly              
you              $*        match zero              
@                              or more
here
.
us
.
edu
>                >         match exactly
                 $*        match zero or more

10.2.2 The RHS

Recall that the objective of rule set 3 is to strip everything but the address part (the text between the angle brackets). That stripping is accomplished by rewriting the workspace using the $2 positional operator in the RHS:

R$* < $* > $*   $2             basic RFC822 parsing
                
                strip all but the address

Remember, when a $digit appears in the RHS, that digit is used as a count into the wildcard operators of the LHS.

$* < $* > $*
        
$1   $2   $3

$1 refers to the first $*, $2 refers to the second, and $3 to the third. Comparing this ordering of operators to the test address, you see

workspace            LHS                                                RHS
Your         $*     match zero          $1       
Fullname                 or more              
<            <      match exactly              
you          $*     match zero          $2         
@                        or more
here
.
us
.
edu
>            >      match exactly
             $*     match zero or more   $3

This illustrates that the middle (second) $* matches the you@here.us.edu part of the workspace. When the RHS rewrites the workspace, it does so by copying the tokens matched by the second wildcard operator (specified in the RHS with the $2 positional operator).

10.2.3 Test Rule Set 3

Take a few moments to experiment. Observe the transformation of a user-specified address into one that sendmail can use. Add the following new rule set 3 to the rule sets in the client.cf file:

S3 # preprocessing for all rule sets                                   new
R$* < $* > $*       $2                      basic RFC822 parsing       new

Now run sendmail again. Up to now, you have been testing rule set 0, so you have specified a 0 following the > prompt. Instead, you will now specify a 3 because you are testing rule set 3:

% ./sendmail -Cclient.cf -bt
ADDRESS TEST MODE (ruleset 3 NOT automatically invoked)
Enter <ruleset> <address>
> 3 Your Fullname <you@here>
rewrite: ruleset  3   input: Your Fullname < you @ here >
rewrite: ruleset  3 returns: you @ here

As expected, the new rule causes everything except the "good" email address, the address between the angle brackets, to be thrown away.

Before we improve rule set 3, take a few moments to experiment. Experiment by putting the fullname last. Try omitting the email address between the angle brackets. Try nesting angle brackets in an address, like <a<b>c>.

As a closing note, recall that sendmail does the minimum matching possible when comparing operators to the workspace. Although $*, for example, can match zero or more, it prefers to match zero if possible and, if not, to match the fewest tokens possible. A LHS of $*@$+, for example, will match as shown in Table 10.1.

Expecting operators to match more than they do can cause you to misunderstand the effect of rules.