excerpts.rb

# loosely based on Rails' excerpt helper
# http://rails.rubyonrails.org/classes/ActionView/Helpers/TextHelper.html#M001713

# TODO: don't break in the middle of words
def excerpts(text, word, options={})
  omission = options[:omission] || '...'
  radius = options[:radius] || 5
  word.class == String ? re = /(#{Regexp.escape(word)})/ : re = word
  return nil unless text =~ re
  ans = ""; start = 0; last_range = -2..-1
  while text[start..-1] =~ re
    match_index = start + (text[start..-1] =~ re)
    start_ind = [0, match_index - radius].max
    end_ind = [match_index + text[start..-1].match(re)[0].length + radius, text.length - 1].min
	  unless last_range.include? start_ind-1
      ans.rstrip!; ans << omission unless start_ind == 0
      last_range = start_ind..end_ind
      ans << text[last_range].lstrip
	  else
	    ans << text[last_range.end+1..end_ind]
      last_range = last_range.begin..end_ind
	  end
    start = match_index + text[start..-1].match(re)[0].length
  end
  ans.rstrip!; ans << omission unless last_range.end == text.length-1
  options[:highlight].nil? ? ans : ans.gsub(re,options[:highlight])
end

puts excerpts('a string which contains many instances of the word string. the end','string',:highlight => "[\\1]")