input: Write a function to replace non-alphanumeric characters with asterisks.
  assert replaceNonAlphanum("Hello* World_$!") == "Hello** World**$*"
  assert replaceNonAlphanum("Python 3.8 is cool!") == "Python 3*8 is cool*"
  assert replaceNonAlphanum("Regex#rocks!") == "Regex*rocks*"
output: def replaceNonAlphanum(s):
      return ''.join(c if c.isalnum() or c.isspace() else '*' for c in s)