input: Write a python function to check if a string is a valid numeric string using regular expression.
  assert is_valid_numeric('123456789') == True
  assert is_valid_numeric('abc123') == False
  assert is_valid_numeric('987654321') == True
  assert is_valid_numeric('123abc') == False
output: def is_valid_numeric(s):
      import re
      return bool(re.fullmatch(r'\d+', s))