 | Chapter 28 Rules |  |
| Chapter 28 Rules | |
The LHS of any rule is compared to the current contents of the workspace to determine whether the two match. Table 28.1 displays a variety of special operators offered by sendmail that make comparisons easier and more versatile.
| Operator | Description or Use |
|---|---|
$* | Match zero or more tokens |
$+ | Match one or more tokens |
$- | Match exactly one token |
$@ | Match exactly zero tokens (V8 only) |
$= | Match any tokens in a classa |
$~ | Match any single token not in a class |
The first three operators in
Table 28.1
are wildcard operators,
which can be used to match arbitrary
sequences of tokens in the workspace.
Consider
the following rule, which employs the $- operator
(match any single token):
R$- fred.local
Here, a match is found only if the workspace contains a
single token (such as tom). If the workspace contains multiple
tokens (such as tom@host), the LHS does not match.
A match causes the workspace to be rewritten
by the RHS to become fred.local.
The rewritten workspace is then compared
again to the $-, but this time there is no match
because the workspace contains three tokens (fred,
a dot (.), and local).
Since there
is no match, the current workspace (fred.local) is
carried down to the next rule (if there is one).
Note that all comparisons of tokens in the LHS to tokens
in the workspace are done
in a case-insensitive manner. That is, tom in the LHS
matches TOM, Tom, and even ToM in the workspace.
When a pattern-matching operator can match multiple tokens ($+
and $*), sendmail
performs minimum matching. For example, given a workspace
of xxx.yyy.zzz and an LHS of
$+.$+
the first $+ matches only a single token (xxx),
but the second $+ matches three (yyy, a dot,
and zzz).
This is because the first $+ matches the minimum
number of tokens that it can while still allowing the whole
LHS to match the workspace. Shortly, when we discuss the RHS,
we'll show why this is important.
Multiple token-matching operators, such as $*, always try to match the fewest number of tokens that they can.
Such a simple-minded approach could lead to problems in
matching (or not matching) classes in the LHS. For example,
consider the following five tokens in the workspace:
A . B . C
Given the following LHS rule:
R$+.$=X$*
Because the $+ tries to match the minimum number of tokens,
it first matches only the A in the workspace. The $=X then
tries to match the B to the class X. If this
match fails, sendmail backs up and tries again.
The third time through, the $+ matches the
A.B, and the $=X tries to match the
C in the workspace. If C is not in the class
X, the entire LHS fails.
The ability of the sendmail program to back up and retry LHS matches eliminates much of the ambiguity from rule design. The multitoken matching operators try to match the minimum but match more if necessary for the whole LHS to match.
|  | |
| 28.4 The Behavior of a Rule |  | 28.6 The RHS |