input: Write a function to match all numeric strings using a regular expression. The function should return 'String is a valid numeric string' if the input string is a numeric string, otherwise, it should return 'String isn't a valid numeric string'. If the input string is empty, it should return 'String isn't a valid numeric string'.
  assert match_numeric_string('123456789')=='String is a valid numeric string'
  assert match_numeric_string('000000')=='String is a valid numeric string'
  assert match_numeric_string('123abc')=='String isn't a valid numeric string'
  assert match_numeric_string('')=='String isn't a valid numeric string'
  assert match_numeric_string('123.45')=='String isn't 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'