input: Write a function to develop a regular expression in Python to match all numeric strings. If the input string is a valid numeric string, return "String is a valid numeric string"; otherwise, return "String isn't a valid numeric string".
  
  assert check_numeric_string("123456789")=="String is a valid numeric string"
  assert check_numeric_string("abc123")=="String isn't a valid numeric string"
  assert check_numeric_string("")=="String isn't a valid numeric string"
  assert check_numeric_string("123.45")=="String isn't a valid numeric string"
  assert check_numeric_string("0")=="String is a valid numeric string"
output: import re
  
  def check_numeric_string(s):
      if re.fullmatch(r'\d+', s):
          return "String is a valid numeric string"
      else:
          return "String isn't a valid numeric string"