Exploring Expect

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" ...