Exploring Expect

Nested Parentheses

The pattern a*((ab)*|b*) has nested parentheses. How are the results stored in the array expect_out? They are determined by counting left parentheses: the subexpression which starts with the Nth left parenthesis corresponds with expect_out(N,...). For example, the string that matches ((ab)*|b*) is stored in expect_out(1,string), and the string that matches (ab) is stored in expect_out(2,string). Of course, the only string that could possibly match "ab" is "ab“. If you want to record the string of ab’s that corresponds to (ab)* then you have to put another pair of parentheses around that, as in "((ab)*)“.

Figure 5-1. Strings that match parenthesized subpatterns are stored in the expect_out array

The string matched by the whole pattern is stored in expect_out(0,string). This makes sense if you imagine the whole pattern is wrapped in another set of parentheses. And you can also imagine the whole pattern prefaced by a .* pattern^[23] and wrapped in yet another pair of parentheses to determine the value of expect_out(buffer).

Figure 5-2. The original pattern is shaded. Text that matches the imaginary pattern (.*(...)) is stored in expect(0,string) and expect_out(buffer).

^[23] Unlike the usual meaning of .* which matches as many characters as possible, this imaginary .* matches ...