$#
Match a literal $# LHS operator
Because the RHS can return a delivery agent
specification, it is sometimes desirable to check
for the $#
operator on the LHS of a rule. Consider, for
example, the following rule:
R $+ $| $# OK $@ $1
The LHS looks for anything (the $+) followed by a
$| operator,
and then $# OK.
This might match a workspace that was set up by a
database-map lookup or a call to another rule set.
The $# OK means
the address was OK as is, and so should be placed
back into the workspace. The RHS does just that by
returning (the $@
prefix) the original address (the $1 references the LHS
$+, which
contained the original address).
Note that the $#
operator has no special meaning in the LHS. It is
used only to detect a delivery agent-like
specification made by an earlier rule on the RHS.
The next two sections reveal how this is
done.
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