To selectively uppercase parts of a string via mb_eregi_replace

    $str = mb_eregi_replace('\b([0-9]{1,4}[a-z]{1,2})\b', "strtoupper
('\\1')", $str, 'e');

Full example, how to fix an address manually typed, uppercasing the first letter of a words and keeping uppercase roman numerals and the letters A,B,C after the house number):

function ucAddress($str) {
// first lowercase all and use the default ucwords
    $str = ucwords(strtolower($str));
// let's fix the default ucwords...
// uppercase letters after house number (was lowercased by the strtolower above)
    $str = mb_eregi_replace('\b([0-9]{1,4}[a-z]{1,2})\b', "strtoupper
('\\1')", $str, 'e');
// the same for roman numerals
    $str = mb_eregi_replace('\bM{0,4}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})\b', "strtoupper('\\0')", $str, 'e');
    return $str;
}