Javascript regex to test if a language code is RFC5646 compliant [locale] [I18N]

gistfile1.java

private static RegExp buildRegexpLangRfc5646()
    {
        String extLang = "([A-Za-z]{3}(-[A-Za-z]{3}){0,2})";
        String language =
                "(([a-zA-Z]{2,3}(-" + extLang + ")?)|([a-zA-Z]{5,8}))";
        String script = "([A-Za-z]{4})";
        String region = "([A-Za-z]{2}|\\d{3})";
        String variant = "([A-Za-z0-9]{5,8}|(\\d[A-Z-a-z0-9]{3}))";
        String singleton = "(\\d|[A-W]|[Y-Z]|[a-w]|[y-z])";
        String extension = "(" + singleton + "(-[A-Za-z0-9]{2,8})+)";
        String privateUse = "(x(-[A-Za-z0-9]{1,8})+)";

        String langTag =
                language + "(-" + script + ")?(-" + region + ")?(-" + variant
                        + ")*(-" + extension + ")*(-" + privateUse + ")?";

        String irregular =
                "((en-GB-oed)|(i-ami)|(i-bnn)|(i-default)|(i-enochian)|(i-hak)|(i-klingon)|(i-lux)|(i-mingo)|(i-navajo)|(i-pwn)|(i-tao)|(i-tay)|(i-tsu)|(sgn-BE-FR)|(sgn-BE-NL)|(sgn-CH-DE))";
        String regular =
                "((art-lojban)|(cel-gaulish)|(no-bok)|(no-nyn)|(zh-guoyu)|(zh-hakka)|(zh-min)|(zh-min-nan)|(zh-xiang))";
        String grandFathered = "(" + irregular + "|" + regular + ")";

        StringBuffer languageTag = new StringBuffer();
        languageTag.append("(^").append(privateUse).append("$)");
        languageTag.append('|');
        languageTag.append("(^").append(grandFathered).append("$)");
        languageTag.append('|');
        languageTag.append("(^").append(langTag).append("$)");

        return RegExp.compile(languageTag.toString());
    }