Exploring Expect

Pattern Matching Strategy

At the beginning of the previous chapter was an example where the string "philosophic\n" was matched against the pattern "*hi*“. This pattern can be rewritten as a regular expression:

expect -re ".*hi.*"

Adding parentheses around various pieces of the pattern makes it possible to see exactly how the string is matched. Here is a snapshot of Expect running interactively. First I entered the expect command to wait for a string. Then I entered philosophic and pressed return. Finally I printed the first four elements of expect_out surrounded by angle brackets.

expect1.1> expect -re "(.*)(hi)(.*)"philosophic
expect1.2> for {set i 0} {$i<4} {incr i} {
+> send "<$expect_out($i,string)>\n"}
<philosophic
>
<philosop>
<hi>
<c
>
expect1.3>

You can see that the entire string matched was "philosophic\n“. The first .* matched philosop while the second .* matched "c\n“. "hi“, of course, matched "hi" but notice that it was the second "hi" in the string, not the first "hi“. This is similar to the way the analogous glob pattern worked in the previous chapter, but I will go over it again in the context of regular expressions.

Each piece of the pattern attempts to match as many characters as possible. Pattern pieces are matched before remaining subpatterns to the right in the pattern. So if there is any question about which pattern piece matches a string, it is resolved in such a way that the leftmost piece matches the longest run of characters. For example, the pattern .* always ...