input: Write a function to develop a regular expression in Python to match all numeric strings. The function should return a message indicating whether the input string is a valid numeric string or not. If the input string is a numeric string, return "String is a valid numeric string"; otherwise, return "String isn't a valid numeric string".
  assert validate_numeric_string('123456789')=="String is a valid numeric string"
  assert validate_numeric_string('abc123')=="String isn't a valid numeric string"
  assert validate_numeric_string('')=="String isn't a valid numeric string"
  assert validate_numeric_string('123.456')=="String isn't a valid numeric string"
  assert validate_numeric_string('0')=="String is a valid numeric string"
output: def validate_numeric_string(input_string):
      if input_string.isdigit():
          return "String is a valid numeric string"
      else:
          return "String isn't a valid numeric string"