input: Write a function to develop a regular expression in Python that matches all numeric strings. The function should take a string as input and evaluate whether it is a valid numeric string. If it is, the function should return "String is a valid numeric string"; otherwise, it should return "String isn't a valid numeric string".
  import re
  
  def is_numeric_string(string):
      pattern = '^[0-9]+$'
      result = re.match(pattern, string)
      if result:
          return "String is a valid numeric string"
      else:
          return "String isn't a valid numeric string"
  
  assert is_numeric_string('123456789')=="String is a valid numeric string"
  assert is_numeric_string('123a456')=="String isn't a valid numeric string"
  assert is_numeric_string('')=="String isn't a valid numeric string"
  assert is_numeric_string('0123')=="String is a valid numeric string"
output: import re
  
  def is_numeric_string(string):
      pattern = '^[0-9]+$'
      result = re.match(pattern, string)
      if result:
          return "String is a valid numeric string"
      else:
          return "String isn't a valid numeric string"