input: Write a function to validate if a given string consists of only numeric characters using a regular expression in Python. If the string is a valid numeric string, return "String is a valid numeric string", otherwise return "String isn't a valid numeric string".
  
  assert validate_numeric_string("123456789") == "String is a valid numeric string"
  assert validate_numeric_string("abc123") == "String isn't a valid numeric string"
  assert validate_numeric_string("12-34") == "String isn't a valid numeric string"
  assert validate_numeric_string("0") == "String is a valid numeric string"
  assert validate_numeric_string("") == "String isn't a valid numeric string"
output: import re
  
  def validate_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"