input: Write a python function to tokenize a string into words using the nltk library and filter out only alphanumeric tokens using regex.
  assert tokenize_and_filter('Hello, world! This is a test.')==['Hello', 'world', 'This', 'is', 'a', 'test']
  assert tokenize_and_filter('Python3.8 is great, isn't it?')==['Python3', '8', 'is', 'great', 'isn', 't', 'it']
  assert tokenize_and_filter('1234 5678 90')==['1234', '5678', '90']
output: import nltk
  import re
  
  def tokenize_and_filter(text):
      nltk.download('punkt')
      tokens = nltk.word_tokenize(text)
      filtered_tokens = [token for token in tokens if re.match(r'^[a-zA-Z0-9]+$', token)]
      return filtered_tokens