Add Whitespace based on validity o the words

gistfile1.txt

def repair_whitespace(str)

    buff = []
    final_words = []


    str.each_char do |char|
        buff << char
        if valid_word?(buff.join)
            final_words << buff.join
            buff = []
        end
    end
    final_words.join(' ') end

def valid_word?(word)
    %w(this is a valid string).include?(word)
end

puts repair_whitespace('thisis')
puts repair_whitespace('thisisavalidstring')