def common_sub(*strings)
  reg = Regexp.new strings.shift.split(//).map { |i| "#{i}?" }.join("")
  c = nil
  strings.each do |s|
    return nil unless m = reg.match(s)

    if c
      c = m[0] if c.size > m[0].size
    else
      c = m[0]
    end
  end

  return c
end

p common_sub("foobar", "foobaz", "foowhoo")