input: Write a function to determine if a given string is a valid numeric string using regular expression. If it is a valid numeric string, return True; otherwise, return False. If the input string is empty, return False.
  import re 
    
  # Function to match all numeric strings 
  def is_numeric_string(string):
      pattern = '^[0-9]+$'
      result = re.match(pattern, string) 
      return bool(result)
  
  assert is_numeric_string('123456789')==True
  assert is_numeric_string('12a34')==False
  assert is_numeric_string('')==False
  assert is_numeric_string('0')==True
  assert is_numeric_string('3.14')==False
output: import re 
    
  # Function to match all numeric strings 
  def is_numeric_string(string):
      pattern = '^[0-9]+$'
      result = re.match(pattern, string) 
      return bool(result)