Exploring Expect

What Happens To Things That Do Not Match

As characters are entered, they are compared to the patterns. Characters that do not match any of the patterns are sent to the spawned process. This includes characters before and after any matches. Any characters involved in a match are not sent to the spawned process. Consider the following interact command, which sends "hugs and kisses" if it sees XOX.

interact "XOX" {send "hugs and kisses"}

If the user types AXOXB, the spawned program receives "Ahugs and kissesB“. The A does not match so it is sent literally. The XOX is replaced by the phrase "hugs and kisses“. And the trailing B does not match so it is sent literally.

If the user types AXOYB, the spawned program receives AXOYB. The pattern XOX cannot match any part of what was entered, so it is all sent on.

If the user types XOXO, the spawned program receives "hugs and kissesO“. The first XOX matches so it is translated and sent on, but the O that remains cannot possibly match XOX so it is sent literally.

If the user types XOXOX, the spawned program receives "hugs and kissesO" following the logic of the previous case. The trailing X can match the pattern XOX if another OX is entered so the interact command waits for more characters to be entered before deciding whether to send on the X or not. Suppose the user instead enters Z. In this case, the X can no longer match so interact sends the string XZ to the spawned process. What would have happened if the user entered another X rather than ...