input: Write a function to develop a regular expression in Python to match all numeric strings. The function should take a string as input and return True if the string is a valid numeric string, otherwise return False.
  assert is_numeric_string("123456789")==True
  assert is_numeric_string("123abc456")==False
  assert is_numeric_string("")==False
  assert is_numeric_string("0")==True
  assert is_numeric_string("123.456")==False
output: def is_numeric_string(s):
      return s.isdigit()