input: Write a function to create a regular expression pattern that matches all numeric strings and test it with various input strings to check if they are valid numeric strings. The function should return "String is a valid numeric string" if the input matches the pattern, otherwise it should return "String isn't a valid numeric string".
  
  assert match_numeric_string('123456789')==True
  assert match_numeric_string('123a456')==False
  assert match_numeric_string('0')==True
  assert match_numeric_string('abc')==False
  assert match_numeric_string("")==False
output: def match_numeric_string(input_string):
      import re
      pattern = r'^\d+$'
      if re.match(pattern, input_string):
          return "String is a valid numeric string"
      else:
          return "String isn't a valid numeric string"