input: Write a function to develop a regular expression in Python that matches all numeric strings. If the string matches the numeric pattern, return "String is a valid numeric string". Otherwise, return "String isn't a valid numeric string".
  
  import re
  
  def match_numeric_string(string):
      # Regular expression to match all numeric strings
      pattern = '^[0-9]+$'
      
      # Matching a pattern
      result = re.match(pattern, string)
      if result:
          return "String is a valid numeric string"
      else:
          return "String isn't a valid numeric string"
  
  assert match_numeric_string('123456789') == "String is a valid numeric string"
  assert match_numeric_string('abc123') == "String isn't a valid numeric string"
  assert match_numeric_string('') == "String isn't a valid numeric string"
  assert match_numeric_string('123.456') == "String isn't a valid numeric string"
  assert match_numeric_string('9876543210') == "String is a valid numeric string"
output: 
  def match_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"
  