input: from typing import List
  
  def matches_numeric_strings(strings: List[str]) -> List[str]:
      """
      Given a list of strings, return a list of strings that are valid numeric strings according to the regular expression '^[0-9]+$'.
      
      >>> matches_numeric_strings(['123456789', 'abc123', '123a456', ''])
      ['123456789']
      >>> matches_numeric_strings(['0', '100', '10.5', ' 123', '321 ', 'helloWorld'])
      ['0', '100']
      """
      
      pattern = '^[0-9]+$'
      numeric_strings = []
      for string in strings:
          if re.match(pattern, string):
              numeric_strings.append(string)
      return numeric_strings
output:     import re