sendmail, 4th Edition

Rewrite Once Prefix: $:

Ordinarily, the RHS rewrites the workspace as long as the workspace continues to match the LHS. This looping behavior can be useful. Consider the need to strip extra trailing dots off an address in the workspace:

R $* ..        $1.

Here, the $* matches any address that has two or more trailing dots. The $1. in the RHS then strips one of those two trailing dots when rewriting the workspace. For example:

xxx . . . . .     becomes  →  xxx . . . .
xxx . . . .       becomes  →  xxx . . .
xxx . .           becomes  → xxx . .
xxx . .           becomes  → xxx .
xxx .             ← match fails

Although this looping behavior of rules can be handy, for most rules it can be dangerous. Consider the following example:

R $*       <$1>

The intention of this rule is to cause whatever is in the workspace to become surrounded with angle brackets. But after the workspace is rewritten, the LHS again checks for a match; and because the $* matches anything, the match succeeds, the RHS rewrites the workspace again, and again the LHS checks for a match:

xxx               becomes  →  < xxx  >
< xxx >           becomes → < < xxx  > >
< < xxx > >       becomes  → < < < xxx  > > >
↓

     and so on, until ...
     ↓

sendmail prints: rewrite: expansion too long

In this case,sendmail catches the problem because the workspace has become too large. It prints the preceding error message and skips that and all further rules in the rule set. If you are running sendmail in test mode, this fatal error would also be printed:

= = Ruleset 0 (0) status 65

Unfortunately, not all such endless looping produces a visible ...