input: Write a function to validate if a given string contains only numeric characters using Python's regular expressions. The function should take a single string as input and return True if the string is a valid numeric string, otherwise return False.
  
  def is_numeric_string(string):
      pattern = '^[0-9]+$'
      result = re.match(pattern, string)
      return bool(result)
  
  # Test cases
  assert is_numeric_string('123456789')==True
  assert is_numeric_string('abc123')==False
  assert is_numeric_string('123.45')==False
  assert is_numeric_string('')==False
  assert is_numeric_string('0')==True
output: def is_numeric_string(string):
      pattern = '^[0-9]+$'
      result = re.match(pattern, string)
      return bool(result)