function user_validate_name($name) {
if (!strlen($name)) return t('You must enter a username.');
if (substr($name, 0, 1) == ' ') return t('The username cannot begin with a space.');
if (substr($name, -1) == ' ') return t('The username cannot end with a space.');
if (strpos($name, ' ') !== FALSE) return t('The username cannot contain multiple spaces in a row.');
if (ereg("[^\x80-\xF7 [:alnum:]_.-]", $name)) return t('The username contains an illegal character.');
if (preg_match('/[\x{80}-\x{A0}'. // Non-printable ISO-8859-1 + NBSP
'\x{AD}'. // Soft-hyphen
'\x{2000}-\x{200F}'. // Various space characters
'\x{2028}-\x{202F}'. // Bidirectional text overrides
'\x{205F}-\x{206F}'. // Various text hinting characters
'\x{FEFF}'. // Byte order mark
'\x{FF01}-\x{FF60}'. // Full-width latin
'\x{FFF9}-\x{FFFD}'. // Replacement characters
'\x{0}]/u', // NULL byte
$name)) {
return t('The username contains an illegal character.');
}
if (strpos($name, '@') !== FALSE && !eregi('@([0-9a-z](-?[0-9a-z])*.)+[a-z]{2}([zmuvtg]|fo|me)?$', $name)) return t('The username is not a valid authentication ID.');
if (strlen($name) > USERNAME_MAX_LENGTH) return t('The username %name is too long: it must be %max characters or less.', array('%name' => $name, '%max' => USERNAME_MAX_LENGTH));
}

Это из модуля user - править надо в ней.