input: Write a function to create a regular expression pattern that matches all numeric strings in Python. If the input string matches the pattern, return "String is a valid numeric string", otherwise 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('0') == "String is a valid numeric string"
output: def match_numeric_string(s):
      if s.isdigit():
          return "String is a valid numeric string"
      else:
          return "String isn't a valid numeric string"