The Eliza algorithm is actually relatively straightforward. It consists of three steps:

As we saw above, Eliza read the input string He says I am too lazy and found the keyword i. It ran through its entire list of keywords, but i is the only one that matched. The keywords are ranked; if more than one keyword matches, it picks the most salient.

Next, it applied all the decomposition rules for the keyword i (e.g., * i was *, * i am * @happy *, and so on) to see if any matched. One rule did: * i am *. Using this rule, we isolate parts of the input string around i am: the two phrases He says and too lazy.

Next we randomly select a reassemble rule: Is it because you are (2) that you came to me. We use this rule to construct the reply. We replace (2) with the text that matched the second asterisk in the decomposition rule—in our example, the string too lazy. Finally, Eliza replies with, Is it because you are too lazy that you came to me?

The Eliza algorithm has pre- and post-processing steps as well. These handle the transformation of words like I and you; you can read the documentation embedded in the module to learn more.

You can also access all of the module’s internal functions from your program. For example, using the transform method, you can feed a string to Eliza and fetch its response:

$string   = "I'm sad.";
$response = $mybot->transform( $string );

The Eliza bot is an object, and its configuration data is encapsulated, which means that you can instantiate other Eliza bots, each with their own distinct configurations.

Here, we create two bots and make them talk to one another:

use Chatbot::Eliza

my ($harry, $sally, $he_says, $she_says);

$sally = new Chatbot::Eliza "Sally";
$harry = new Chatbot::Eliza "Harry";

$he_says  = "I am sad.";

srand( time ^ ($$ + ($$ << 15)) );    # Seed the random number generator.

print $harry->name, ": $he_says 
";

while (1) {
    $she_says = $sally->transform( $he_says );
    print $sally->name, ": $she_says 
";

    $he_says  = $harry->transform( $she_says );
    print $harry->name, ": $he_says 
";
}

Two bots conversing produces interesting results only if we have clever scripts. Below, we see sample output from the program, with both bots using the default Eliza script:

Harry:  I am sad.
Sally:  I'm sure it's not pleasant to be sad.
Harry:  Do you think coming here will help you not to be sad?
Sally:  You're not really talking about me -- are you?
Harry:  Perhaps I am in your fantasies.
Sally:  What makes you think of my fantasies?
Harry:  Your fantasies?
Sally:  Are you worried about someone else's fantasies?
Harry:  Would you prefer if I weren't worried about someone else's fantasies?

In general, the default Eliza script does not produce any sensible conversation when interacting with itself. (In fairness, people who talk to themselves often don’t make much sense either.)

One of the reasons the original Eliza was so successful was Weizenbaum’s clever rules and limited domain; the role of a mock Rogerian psychotherapist doesn’t require much intelligence, and a relatively simple algorithm can pull it off. But as Weizenbaum found, even this relatively simple program fooled naive users into believing that it understood what they were saying. Scarier still, some people actually felt more comfortable talking to Eliza than to a flesh-and-blood psychiatrist.

Here’s an interactive session, this time with a human deliberately trying to expose Eliza’s weaknesses:

Eliza:  Please tell me what's been bothering you.
you:    Why are you asking me that?
Eliza:  Do you sometimes think I am asking you that?
you:    What time is it?
Eliza:  Why do you ask?
you:    You don't understand.
Eliza:  We were discussing you -- not me.
you:    Who are you?
Eliza:  What answer would please you most?
you:    I have asdwet in the asdgasdg.
Eliza:  Do you say you have asdwet in the asdgasdg for some special reason?
you:    my am we are is in the always my brother
Eliza:  Your brother?
you:    You your in I my me mine.
Eliza:  Your you mine?
you:    I have a gun.  Give me your money!
Eliza:  Can you elaborate on that?
you:    Do you remember the Kennedy assassination?
Eliza:  Did you think I would forget the Kennedy assassination?
you:    Stop asking questions.
Eliza:  That is interesting. Please continue.

The CPAN includes many modules that allow a script to interact easily with resources on the Internet. For example, it is straightforward to write scripts that combine the Chatbot::Eliza module with the Net::IRC module or the CGI module. (The CPAN distribution of Chatbot::Eliza includes a sample CGI script—a Web-based Eliza.)