input: 
  import re
  
  def match_numeric_strings(string: str) -> bool:
      """Match if the input string is a valid numeric string using a regular expression.
      >>> match_numeric_strings('123456789')
      True
      >>> match_numeric_strings('123ABC456')
      False
      >>> match_numeric_strings('')
      False
      >>> match_numeric_strings('0')
      True
      """
      pattern = '^[0-9]+$'
      result = re.match(pattern, string)
      return bool(result)
  
output:     pattern = '^[0-9]+$'
      result = re.match(pattern, string)
      return bool(result)