Check if character is a vowel
const isVowel = char => /[aeiou]/i.test(char);It could be used in a recursion to count vowels in a string
const countVowels = str => !str?.length ? 0 : isVowel(str[0]) + countVowels(str.slice(1));/regex boundary[]group bracket expression matching single chars it containsaeiouchars to test againsticase insensitive flag