input: Write a python function to mask digits in a given string.
  assert mask_information('John Smith,123-456-7890') == 'John Smith,xxx-xxx-xxxx'
  assert mask_information('Alice,555-1234') == 'Alice,xxx-xxxx'
  assert mask_information('987 Elm St.') == 'xxx Elm St.'
output: def mask_information(s):
      import re
      return re.sub(r'\d', 'x', s)