Few ways to do this, but lowercase the sentence, remove all whitespace and punctuation and then union the alphabet string and the sentence string and compare length. I'm sure there's a more performant way to do it. Another way might be just to loop through each character in the input, check to see if the alphabet array contains the character and then increment a counter if you find a match. Once you're done iterating you compare the counter to the length of the alphabet array/string to see if they match.