ensureValidHandle function

void ensureValidHandle(String handle)

Handle constraints, in English:

• must be a possible domain name
  • RFC-1035 is commonly referenced, but has been updated. eg, RFC-3696, section 2. and RFC-3986, section 3. can now have leading numbers (eg, 4chan.org)
  • "labels" (sub-names) are made of ASCII letters, digits, hyphens
  • can not start or end with a hyphen
  • TLD (last component) should not start with a digit
  • can't end with a hyphen (can end with digit)
  • each segment must be between 1 and 63 characters (not including any periods)
  • overall length can't be more than 253 characters
  • separated by (ASCII) periods; does not start or end with period
  • case insensitive
  • domains (handles) are equal if they are the same lower-case
  • punycode allowed for internationalization
• no whitespace, null bytes, joining chars, etc
• does not validate whether domain or TLD exists, or is a reserved or special TLD (eg, .onion or .local)

Implementation

//  - does not validate punycode
void ensureValidHandle(final String handle) {
  // check that all chars are boring ASCII
  if (!RegExp(r'^[a-zA-Z0-9.-]*$').hasMatch(handle)) {
    throw InvalidHandleError(
      'Disallowed characters in handle '
      '(ASCII letters, digits, dashes, periods only)',
    );
  }

  if (handle.length > 253) {
    throw InvalidHandleError('Handle is too long (253 chars max)');
  }

  final labels = handle.split('.');
  if (labels.length < 2) {
    throw InvalidHandleError('Handle domain needs at least two parts');
  }

  for (int i = 0; i < labels.length; i++) {
    final label = labels[i];

    if (label.isEmpty) {
      throw InvalidHandleError('Handle parts can not be empty');
    }

    if (label.length > 63) {
      throw InvalidHandleError('Handle part too long (max 63 chars)');
    }

    if (label.endsWith('-') || label.startsWith('-')) {
      throw InvalidHandleError(
        'Handle parts can not start or end with hyphens',
      );
    }

    if (i + 1 == labels.length && !RegExp(r'^[a-zA-Z]').hasMatch(label)) {
      throw InvalidHandleError(
        'Handle final component (TLD) must start with ASCII letter',
      );
    }
  }
}