Always Count Parentheses Even Inside Of Alternatives
To decide which element of expect_out
to use, count the number of prior parenthesized expressions. The simplest way to do this is just to count the number of left parentheses. This works even if the parentheses occur in an alternative.
Consider the pattern "(a)|(b)
“. If the string is "a
“, the variable expect_out(1,string)
will be set to "a
“. If the string is "b
“, expect_out(2,string)
will be set to "b
“. In this way, it is possible that expect_out(2,string)
can be defined but not expect_out(1,string)
.
This behavior may seem to be a disadvantage—the limit of nine parentheses can be used up even when appearing in non-matching alternatives.[24] But the advantage is that you can know ahead of time where in expect_out
matching strings will be without worrying about whether other alternatives matched or not. For instance, the pattern a*((ab)*|b*)(c*)
is similar to the pattern from the previous example but with (c*)
appended. The variables expect_out(1,string)
and expect_out(2,string)
are set as before. The string matching (c*)
is stored in expect_out(3,string)
whether or not (ab)
is matched.
In cases like this one, it may not be immediately evident whether an element of expect_out
was written by the current expect
since the elements can retain values from prior expect
commands. If you need to be certain, set the element to an empty string (or some otherwise appropriate value) before the expect
command.
set expect_out(1,string) "unassigned" ...
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