input: Write a function to determine if a given string is a valid numeric string using a regular expression. If the string contains only numeric characters, return True, otherwise return False. If the string is empty, return False.
  assert is_numeric_string("123456789")==True
  assert is_numeric_string("abc123")==False
  assert is_numeric_string("123.456")==False
  assert is_numeric_string("")==False
  assert is_numeric_string("0")==True
output: def is_numeric_string(s):
      import re
      return bool(re.fullmatch(r'\d+', s)) if s else False