input: Write a function to check if a given string is a valid numeric string using a regular expression. If the string is valid return "String is a valid numeric string" else return "String isn't a valid numeric string". 
  
  import re 
    
  def check_numeric_string(string): 
      pattern = '^[0-9]+$'
      result = re.match(pattern, string) 
      if result: 
          return "String is a valid numeric string" 
      else: 
          return "String isn't a valid numeric string"
  
  assert check_numeric_string('123456789')=="String is a valid numeric string"
  assert check_numeric_string('123abc456')=="String isn't a valid numeric string"
  assert check_numeric_string('')=="String isn't a valid numeric string"
  assert check_numeric_string('9876543210')=="String is a valid numeric string"
  assert check_numeric_string('0')=="String is a valid numeric string"
output: import re 
    
  def check_numeric_string(string): 
      pattern = '^[0-9]+$'
      result = re.match(pattern, string) 
      if result: 
          return "String is a valid numeric string" 
      else: 
          return "String isn't a valid numeric string"