input: Write a function to define a regular expression in Python to match all numeric strings. If the input string matches the pattern, return "String is a valid numeric string", otherwise return "String isn't a valid numeric string".
  assert numeric_string_match('123456789')=="String is a valid numeric string"
  assert numeric_string_match('abc123')=="String isn't a valid numeric string"
  assert numeric_string_match('')=="String isn't a valid numeric string"
  assert numeric_string_match('0')=="String is a valid numeric string"
  assert numeric_string_match('9876543210')=="String is a valid numeric string"
output: import re
  
  def numeric_string_match(s):
      if re.fullmatch(r'\d+', s):
          return "String is a valid numeric string"
      else:
          return "String isn't a valid numeric string"