def matching_lines(re)
      @lines.each_with_index.select{|text, linenum| re.match(text)}.map{
        |i| i[1]+1}
    end
def matching_lines(re)
      @lines.each_with_index.select{|text, linenum| re.match(text)}.map{
        |i| i[1]+1}
    end
Returns line numbers for lines that match the given regular expression