input: 
  def match_numeric_strings(string: str) -> bool:
      """ Check if the input string is a valid numeric string using a regular expression.
      >>> match_numeric_strings('123456789')
      True
      >>> match_numeric_strings('abc123')
      False
      >>> match_numeric_strings('123.456')
      False
      """
      import re
      pattern = '^[0-9]+$'
      result = re.match(pattern, string)
      return bool(result)
  
output:     import re
      pattern = '^[0-9]+$'
      result = re.match(pattern, string)
      return bool(result)