input: Write a function to check if a given string is a valid numeric string using a regular expression. If the string matches the numeric pattern, return True; otherwise, return False. If the input 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
output: import re
  
  def is_numeric_string(s):
      if not s:
          return False
      return bool(re.fullmatch(r'^\d+$', s))