input: Write a function to validate if a given string is a valid numeric string using a regular expression. If the string matches the regular expression, return True, otherwise return False. In case of an empty string, return True.
  def is_numeric_string(string):
      import re
      pattern = '^[0-9]+$'
      if not string:
          return True
      result = re.match(pattern, string)
      return bool(result)
  
  assert is_numeric_string("123456789")==True
  assert is_numeric_string("0123456789")==True
  assert is_numeric_string("abc123")==False
  assert is_numeric_string("abc")==False
  assert is_numeric_string("")==True
output: def is_numeric_string(string):
      import re
      pattern = '^[0-9]+$'
      if not string:
          return True
      result = re.match(pattern, string)
      return bool(result)