input: Write a function to develop a regular expression in Python to match all numeric strings. The function should validate if a given string is a numeric string and print the appropriate message. If the given string is a numeric string, it should print "String is a valid numeric string"; otherwise, it should print "String isn't a valid numeric string".
  import re 
    
  def validate_numeric_string(string):
      # Regular expression to match all numeric strings
      pattern = '^[0-9]+$'
      # Matching a pattern 
      result = re.match(pattern, string) 
      if result: 
          print("String is a valid numeric string") 
      else: 
          print("String isn't a valid numeric string")
  
  assert validate_numeric_string('123456789') == None
  assert validate_numeric_string('abc123') == None
  assert validate_numeric_string('') == None
  assert validate_numeric_string('0') == None
  assert validate_numeric_string('123abc456') == None
output: import re 
    
  def validate_numeric_string(string):
      pattern = '^[0-9]+$'
      result = re.match(pattern, string) 
      if result: 
          print("String is a valid numeric string") 
      else: 
          print("String isn't a valid numeric string")