prex('ab_cd') generates %w(ab_cd ab_c abc a_cd a_c ac). It's useful for prefix match without regular expression. :-)

prex.rb

# usage: prex('ab_cd') generates %w(ab_cd ab_c abc a_cd a_c ac).
# It's useful for prefix match without regular expression. :-)

def xpand(array)
  first, *others = array
  return first if others == []
  first.map do |it|
    xpand(others).map do |jt|
      [it + '_' + jt, it + jt]
    end.flatten
  end.flatten
end

def prex(word)
  words = word.split('_')
  prexes = words.map do |w|
    len = w.length
    (1..len).to_a.reverse.map { |i| w[0, i] }
  end
  xpand prexes
end