View on
MetaCPAN is shutting down
For details read Perl NOC. After June 25th this page will redirect to
José Alves de Castro > Text-Ngram-0.08 > Text::Ngram



Annotate this POD

Related Modules



Open  1
View/Report Bugs
Module Version: 0.08   Source  


Text::Ngram - Ngram analysis of text


  use Text::Ngram qw(ngram_counts add_to_counts);
  my $text   = "abcdefghijklmnop";
  my $hash_r = ngram_counts($text, 3); # Window size = 3
  # $hash_r => { abc => 1, bcd => 1, ... }

  add_to_counts($more_text, 3, $hash_r);


n-Gram analysis is a field in textual analysis which uses sliding window character sequences in order to aid topic analysis, language determination and so on. The n-gram spectrum of a document can be used to compare and filter documents in multiple languages, prepare word prediction networks, and perform spelling correction.

The neat thing about n-grams, though, is that they're really easy to determine. For n=3, for instance, we compute the n-gram counts like so:

    the cat sat on the mat
    ---                     $counts{"the"}++;
     ---                    $counts{"he "}++;
      ---                   $counts{"e c"}++;

This module provides an efficient XS-based implementation of n-gram spectrum analysis.

There are two functions which can be imported:


This first function returns a hash reference with the n-gram histogram of the text for the given window size. The default window size is 5.

    $href = ngram_counts(\%config, $text, $window_size);

The only necessary parameter is $text.

The possible value for \%config are:


If set to 1 (default), breaks are flanked by spaces; if set to 0, they're not. Breaks are punctuation and other non-alfabetic characters, which, unless you use punctuation = 0> in your configuration, do not make it into the returned hash.

Here's an example, supposing you're using the default value for punctuation (1):

  my $text = "hello, world";
  my $hash = ngram_counts($text, 5);

That produces the following ngrams:

    'hello' => 1,
    'ello ' => 1,
    ' worl' => 1,
    'world' => 1,

On the other hand, this:

  my $text = "hello, world";
  my $hash = ngram_counts({flankbreaks => 0}, $text, 5);

Produces the following ngrams:

    'hello' => 1,
    ' worl' => 1,
    'world' => 1,


If set to 0, casing is preserved. If set to 1, all letters are lowercased before counting ngrams. Default is 1.

    # Get all ngrams of size 4 preserving case
    $href_p = ngram_counts( {lowercase => 0}, $text, 4 );


If set to 0 (default), punctuation is removed before calculating the ngrams. Set to 1 to preserve it.

    # Get all ngrams of size 2 preserving punctuation
    $href_p = ngram_counts( {punctuation => 1}, $text, 2 );


If set to 0 default is 1, no ngrams contaning spaces will be returned.

   # Get all ngrams of size 3 that do not contain spaces
   $href = ngram_counts( {spaces => 0}, $text, 3);

If you're going to request both types of ngrams, than the best way to avoid calculating the same thing twice is probably this:

    $href_with_spaces = ngram_counts($text[, $window]);
    $href_no_spaces = $href_with_spaces;
    for (keys %$href_no_spaces) { delete $href->{$_} if / / }


This incrementally adds to the supplied hash; if $window is zero or undefined, then the window size is computed from the hash keys.

    add_to_counts($more_text, $window, $href)



Cavnar, W. B. (1993). N-gram-based text filtering for TREC-2. In D. Harman (Ed.), Proceedings of TREC-2: Text Retrieval Conference 2. Washington, DC: National Bureau of Standards.

Shannon, C. E. (1951). Predication and entropy of printed English. The Bell System Technical Journal, 30. 50-64.

Ullmann, J. R. (1977). Binary n-gram technique for automatic correction of substitution, deletion, insert and reversal errors in words. Computer Journal, 20. 141-147.


Maintained by Jose Castro,

Originally created by Simon Cozens,


Copyright 2004 by Jose Castro

Copyright 2003 by Simon Cozens

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

syntax highlighting: