Change Case

function changeCase(text) {
    var currentSymbol,
        openTag = '<',
        closeTag = '>',
        isTag = false,
        currentTag = '',
        result = '',
        tags = [];

    for (var i = 0, l = text.length; i < l; i += 1) {
        currentSymbol = text[i];

        if (currentSymbol === openTag) {
            isTag = true;
            continue;
        }

        if (currentSymbol === closeTag) {
            isTag = false;
            continue;
        }

        if (isTag) {
            switch (currentSymbol) {
                case 'u':
                    currentTag = '<upcase>';
                    tags.push(currentTag);
                    i += currentTag.length - 2;
                    break;
                case 'm':
                    currentTag = '<mixcase>';
                    tags.push(currentTag);
                    i += currentTag.length - 2;
                    break;
                case 'l':
                    currentTag = '<lowcase>';
                    tags.push(currentTag);
                    i += currentTag.length - 2;
                    break;
                case '/':
                    tags.pop();
                    i += currentTag.length - 2;
                    if (tags.length > 0) {
                        currentTag = tags[tags.length - 1];
                    } else {
                        currentTag = '';
                    }
                    break;
            }

            isTag = false;
            continue;
        }

        switch (currentTag) {
            case '<upcase>':
                result += currentSymbol.toUpperCase();
                break;
            case '<mixcase>':
                result += Math.random() < 0.5 ? currentSymbol.toUpperCase() : currentSymbol.toLowerCase();
                break;
            case '<lowcase>':
                result += currentSymbol.toLowerCase();
                break;
            default:
                result += currentSymbol;
                break;
        }
    }

    return result;
}

var text = 'We are <upcase><mixcase><lowcase></lowcase>living</mixcase> in </upcase> a <upcase>yellow submarine</upcase>. We <mixcase>don\'t</mixcase> have <lowcase>anything</lowcase> else.';

console.log(changeCase(text));