input: Write a function to validate if a given string is a valid numeric string using a regular expression. If it is valid, return "String is a valid numeric string"; otherwise, return "String isn't a valid numeric string". If the string is empty, return "String isn't a valid numeric string".
  assert is_numeric("")=="String isn't a valid numeric string"
  assert is_numeric("123a45")=="String isn't a valid numeric string"
  assert is_numeric("123")=="String is a valid numeric string"
  assert is_numeric("000000")=="String is a valid numeric string"
  assert is_numeric("123456.789")=="String isn't a valid numeric string"
output: import re
  
  def is_numeric(s):
      if not s:
          return "String isn't a valid numeric string"
      if re.fullmatch(r'\d+', s):
          return "String is a valid numeric string"
      return "String isn't a valid numeric string"