code: function validateFilename(input) {
      let isValidCharacterString = validateCharacterString(input);
  
      if (isValidCharacterString !== true) {
          return isValidCharacterString;
      }
  
      return (input.search(/ /) === -1) ? true : 'Must be a valid POSIX.1-2013 3.170 Filename!';
  }
docs: Validates that a string is a valid POSIX.1-2013 3.170 filename.
  
  @see http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html#tag_03_170
  
  @param {?string} input
  The string to validate.
  
  @return {boolean|string}
  Will either be true, or a string.  If it is a string, validation failed.