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Chatbot::Eliza(3pm) User Contributed Perl Documentation Chatbot::Eliza(3pm)


Chatbot::Eliza - A clone of the classic Eliza program


  use Chatbot::Eliza;
  $mybot = new Chatbot::Eliza;
  # see below for details


This module implements the classic Eliza algorithm. The original Eliza program was written by Joseph Weizenbaum and described in the Communications of the ACM in 1966. Eliza is a mock Rogerian psychotherapist. It prompts for user input, and uses a simple transformation algorithm to change user input into a follow-up question. The program is designed to give the appearance of understanding.

This program is a faithful implementation of the program described by Weizenbaum. It uses a simplified script language (devised by Charles Hayden). The content of the script is the same as Weizenbaum's.

This module encapsulates the Eliza algorithm in the form of an object. This should make the functionality easy to incorporate in larger programs.


The current version of is available on CPAN:

To install this package, just change to the directory which you created by untarring the package, and type the following:

        perl Makefile.PL
        make test
        make install

This will copy to your perl library directory for use by all perl scripts. You probably must be root to do this, unless you have installed a personal copy of perl.


This is all you need to do to launch a simple Eliza session:

        use Chatbot::Eliza;
        $mybot = new Chatbot::Eliza;

You can also customize certain features of the session:

        $myotherbot = new Chatbot::Eliza;
        $myotherbot->name( "Hortense" );
        $myotherbot->debug( 1 );

These lines set the name of the bot to be "Hortense" and turn on the debugging output.

When creating an Eliza object, you can specify a name and an alternative scriptfile:

        $bot = new Chatbot::Eliza "Brian", "myscript.txt";

You can also use an anonymous hash to set these parameters. Any of the fields can be initialized using this syntax:

        $bot = new Chatbot::Eliza {
                name       => "Brian", 
                scriptfile => "myscript.txt",
                debug      => 1,
                prompts_on => 1,
                memory_on  => 0,
                myrand     => 
                        sub { my $N = defined $_[0] ? $_[0] : 1;  rand($N); },

If you don't specify a script file, then the new object will be initialized with a default script. The module contains this script within itself.

You can use any of the internal functions in a calling program. The code below takes an arbitrary string and retrieves the reply from the Eliza object:

        my $string = "I have too many problems.";
        my $reply  = $mybot->transform( $string );

You can easily create two bots, each with a different script, and see how they interact:

        use Chatbot::Eliza
        my ($harry, $sally, $he_says, $she_says);
        $sally = new Chatbot::Eliza "Sally", "histext.txt";
        $harry = new Chatbot::Eliza "Harry", "hertext.txt";
        $he_says  = "I am sad.";
        # Seed the random number generator.
        srand( time ^ ($$ + ($$ << 15)) );      
        while (1) {
                $she_says = $sally->transform( $he_says );
                print $sally->name, ": $she_says \n";
                $he_says  = $harry->transform( $she_says );
                print $harry->name, ": $he_says \n";

Mechanically, this works well. However, it critically depends on the actual script data. Having two mock Rogerian therapists talk to each other usually does not produce any sensible conversation, of course.

After each call to the transform() method, the debugging output for that transformation is stored in a variable called $debug_text.

        my $reply      = $mybot->transform( "My foot hurts" );
        my $debugging  = $mybot->debug_text;

This feature always available, even if the instance's $debug variable is set to 0.

Calling programs can specify their own random-number generators. Use this syntax:

        $chatbot = new Chatbot::Eliza;
                sub {
                        #function goes here!

The custom random function should have the same prototype as perl's built-in rand() function. That is, it should take a single (numeric) expression as a parameter, and it should return a floating-point value between 0 and that number.

What this code actually does is pass a reference to an anonymous subroutine ("code reference"). Make sure you've read the perlref manpage for details on how code references actually work.

If you don't specify any custom rand function, then the Eliza object will just use the built-in rand() function.


Each Eliza object uses the following data structures to hold the script data in memory:


Hash: the set of keywords; Values: strings containing the decomposition rules.


Hash: a set of values which are each the join of a keyword and a corresponding decomposition rule; Values: the set of possible reassembly statements for that keyword and decomposition rule.


This structure is identical to %reasmblist, except that these rules are only invoked when a user comment is being retrieved from memory. These contain comments such as "Earlier you mentioned that...," which are only appropriate for remembered comments. Rules in the script must be specially marked in order to be included in this list rather than %reasmblist. The default script only has a few of these rules.


A list of user comments which an Eliza instance is remembering for future use. Eliza does not remember everything, only some things. In this implementation, Eliza will only remember comments which match a decomposition rule which actually has reassembly rules that are marked with the keyword "reasm_for_memory" rather than the normal "reasmb". The default script only has a few of these.


Hash: the set of keywords; Values: the ranks for each keyword


"quit" words -- that is, words the user might use to try to exit the program.


Possible greetings for the beginning of the program.


Possible farewells for the end of the program.


Hash: words which are replaced before any transformations; Values: the respective replacement words.


Hash: words which are replaced after the transformations and after the reply is constructed; Values: the respective replacement words.


Hash: words which are found in decomposition rules; Values: words which are treated just like their corresponding synonyms during matching of decomposition rules.

Other data members

There are several other internal data members. Hopefully these are sufficiently obvious that you can learn about them just by reading the source code.



    my $chatterbot = new Chatbot::Eliza;

new() creates a new Eliza object. This method also calls the internal _initialize() method, which in turn calls the parse_script_data() method, which initializes the script data.

    my $chatterbot = new Chatbot::Eliza 'Ahmad', 'myfile.txt';

The eliza object defaults to the name "Eliza", and it contains default script data within itself. However, using the syntax above, you can specify an alternative name and an alternative script file.

See the method parse_script_data(). for a description of the format of the script file.



command_interface() opens an interactive session with the Eliza object, just like the original Eliza program.

If you want to design your own session format, then you can write your own while loop and your own functions for prompting for and reading user input, and use the transform() method to generate Eliza's responses. (Note: you do not need to invoke preprocess() and postprocess() directly, because these are invoked from within the transform() method.)

But if you're lazy and you want to skip all that, then just use command_interface(). It's all done for you.

During an interactive session invoked using command_interface(), you can enter the word "debug" to toggle debug mode on and off. You can also enter the keyword "memory" to invoke the _debug_memory() method and print out the contents of the Eliza instance's memory.


    $string = preprocess($string);

preprocess() applies simple substitution rules to the input string. Mostly this is to catch varieties in spelling, misspellings, contractions and the like.

preprocess() is called from within the transform() method. It is applied to user-input text, BEFORE any processing, and before a reassebly statement has been selected.

It uses the array %pre, which is created during the parse of the script.


    $string = postprocess($string);

postprocess() applies simple substitution rules to the reassembly rule. This is where all the "I"'s and "you"'s are exchanged. postprocess() is called from within the transform() function.

It uses the array %post, created during the parse of the script.


     if ($self->_testquit($user_input) ) { ... }

_testquit() detects words like "bye" and "quit" and returns true if it finds one of them as the first word in the sentence.

These words are listed in the script, under the keyword "quit".



_debug_memory() is a special function which returns the contents of Eliza's memory stack.


    $reply = $chatterbot->transform( $string, $use_memory );

transform() applies transformation rules to the user input string. It invokes preprocess(), does transformations, then invokes postprocess(). It returns the transformed output string, called $reasmb.

The algorithm embedded in the transform() method has three main parts:

Search the input string for a keyword.
If we find a keyword, use the list of decomposition rules for that keyword, and pattern-match the input string against each rule.
If the input string matches any of the decomposition rules, then randomly select one of the reassembly rules for that decomposition rule, and use it to construct the reply.

transform() takes two parameters. The first is the string we want to transform. The second is a flag which indicates where this sting came from. If the flag is set, then the string has been pulled from memory, and we should use reassembly rules appropriate for that. If the flag is not set, then the string is the most recent user input, and we can use the ordinary reassembly rules.

The memory flag is only set when the transform() function is called recursively. The mechanism for setting this parameter is embedded in the transoform method itself. If the flag is set inappropriately, it is ignored.

How memory is used

In the script, some reassembly rules are special. They are marked with the keyword "reasm_for_memory", rather than just "reasm". Eliza "remembers" any comment when it matches a docomposition rule for which there are any reassembly rules for memory. An Eliza object remembers up to $max_memory_size (default: 5) user input strings.

If, during a subsequent run, the transform() method fails to find any appropriate decomposition rule for a user's comment, and if there are any comments inside the memory array, then Eliza may elect to ignore the most recent comment and instead pull out one of the strings from memory. In this case, the transform method is called recursively with the memory flag.

Honestly, I am not sure exactly how this memory functionality was implemented in the original Eliza program. Hopefully this implementation is not too far from Weizenbaum's.

If you don't want to use the memory functionality at all, then you can disable it:


You can also achieve the same effect by making sure that the script data does not contain any reassembly rules marked with the keyword "reasm_for_memory". The default script data only has 4 such items.


    $self->parse_script_data( $script_file );

parse_script_data() is invoked from the _initialize() method, which is called from the new() function. However, you can also call this method at any time against an already-instantiated Eliza instance. In that case, the new script data is added to the old script data. The old script data is not deleted.

You can pass a parameter to this function, which is the name of the script file, and it will read in and parse that file. If you do not pass any parameter to this method, then it will read the data embedded at the end of the module as its default script data.

If you pass the name of a script file to parse_script_data(), and that file is not available for reading, then the module dies.

Format of the script file

This module includes a default script file within itself, so it is not necessary to explicitly specify a script file when instantiating an Eliza object.

Each line in the script file can specify a key, a decomposition rule, or a reassembly rule.

  key: remember 5
    decomp: * i remember *
      reasmb: Do you often think of (2) ?
      reasmb: Does thinking of (2) bring anything else to mind ?
    decomp: * do you remember *
      reasmb: Did you think I would forget (2) ?
      reasmb: What about (2) ?
      reasmb: goto what
  pre: equivalent alike
  synon: belief feel think believe wish

The number after the key specifies the rank. If a user's input contains the keyword, then the transform() function will try to match one of the decomposition rules for that keyword. If one matches, then it will select one of the reassembly rules at random. The number (2) here means "use whatever set of words matched the second asterisk in the decomposition rule."

If you specify a list of synonyms for a word, the you should use a "@" when you use that word in a decomposition rule:

  decomp: * i @belief i *
    reasmb: Do you really think so ?
    reasmb: But you are not sure you (3).

Otherwise, the script will never check to see if there are any synonyms for that keyword.

Reassembly rules should be marked with reasm_for_memory rather than reasmb when it is appropriate for use when a user's comment has been extracted from memory.

  key: my 2
    decomp: * my *
      reasm_for_memory: Let's discuss further why your (2).
      reasm_for_memory: Earlier you said your (2).
      reasm_for_memory: But your (2).
      reasm_for_memory: Does that have anything to do with the fact that your (2) ?

How the script file is parsed

Each line in the script file contains an "entrytype" (key, decomp, synon) and an "entry", separated by a colon. In turn, each "entry" can itself be composed of a "key" and a "value", separated by a space. The parse_script_data() function parses each line out, and splits the "entry" and "entrytype" portion of each line into two variables, $entry and $entrytype.

Next, it uses the string $entrytype to determine what sort of stuff to expect in the $entry variable, if anything, and parses it accordingly. In some cases, there is no second level of key-value pair, so the function does not even bother to isolate or create $key and $value.

$key is always a single word. $value can be null, or one single word, or a string composed of several words, or an array of words.

Based on all these entries and keys and values, the function creates two giant hashes: %decomplist, which holds the decomposition rules for each keyword, and %reasmblist, which holds the reassembly phrases for each decomposition rule. It also creates %keyranks, which holds the ranks for each key.

Six other arrays are created: "%reasm_for_memory, %pre, %post, %synon, @initial," and @final.


This software is copyright (c) 2003 by John Nolan <>.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.


John Nolan January 2003.

Implements the classic Eliza algorithm by Prof. Joseph Weizenbaum. Script format devised by Charles Hayden.

2022-10-13 perl v5.34.0