input: Write a function to determine if a given string is a valid numeric string using regular expressions. If the string matches the numeric pattern, return True; otherwise, return False. If the given string is empty, return True.
  assert is_numeric_string("123456789")==True
  assert is_numeric_string("1234abc")==False
  assert is_numeric_string("")==True
  assert is_numeric_string("0")==True
  assert is_numeric_string("123.456")==False
output: import re
  
  def is_numeric_string(s):
      if s == "":
          return True
      return bool(re.fullmatch(r'\d+', s))