input: 
  def is_valid_numeric_string(string: str) -> bool:
      """ Checks if the given string is a valid numeric string.
      >>> is_valid_numeric_string('123456789')
      True
      >>> is_valid_numeric_string('123a456')
      False
      """
      import re
      pattern = '^[0-9]+$'
      result = re.match(pattern, string)
      return result is not None
  
output:     return string.isdigit()