Ruby Coleman-Liau string extension

coleman_liau_string_ext.rb

# Code to calculate the Coleman-Liau index, which measures the understandability of a text.
# See http://en.wikipedia.org/wiki/Coleman-Liau_Index for more information

class String
  def words
    self.split(/[\W]? /)
  end
  
  def letters
    self.scan(/\w/)
  end
  
  def sentences
    # Rules: [.!?] end sentences, repetition is ignored
    # Next sentence must be followed by a space and a capital letter or number
    # Sentence will continue if last word is an acronym such as e.g. or U.S. 
    # Trailing non-word characters are removed from end of last sentence
    
    self.split(/[\.!?]+\s+(?=[A-Z0-9])/).inject([]) do |sentences, partial|
      sentences.last =~ /\.\w/ ? sentences.last << " #{partial.sub(/\W+$/,'')}" : sentences << partial.sub(/\W+$/,'')
      sentences
    end
  end
  
  def coleman_liau_index
    (5.88 * letters.size / words.size) - (29.6 * sentences.size / words.size) - 15.8
  end
end