input: Write a function to implement regular expression matching for numeric strings. The function should check if the given input string consists entirely of numeric characters and return a corresponding message.
  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("123abc")=="String isn't a valid numeric string"
  assert match_numeric_string("")=="String isn't a valid numeric string"
  assert match_numeric_string("0")=="String is a valid numeric string"
  assert match_numeric_string("9876543210")=="String is a valid numeric string"
output: def match_numeric_string(input_string):
      if input_string.isdigit():
          return "String is a valid numeric string"
      else:
          return "String isn't a valid numeric string"