RegionsCasting

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

var closeIndex = text.indexOf("</");
while (closeIndex >= 0) {
    text = implementTags(text, closeIndex);
    closeIndex = text.indexOf("</");
}
console.log(text);

function implementTags(str, closeStartIndex) {
    var tag, tagLen, indexOpenBracketStart, closeEndIndex, textToReplace;
    closeEndIndex = str.indexOf(">", closeStartIndex);
    tag = str.substring(closeStartIndex + 2, closeEndIndex);
    tagLen = tag.length;
    indexOpenBracketStart = (str.substring(0, closeStartIndex)).lastIndexOf("<" + tag + ">");
    textToReplace = str.substring(indexOpenBracketStart, closeEndIndex+1);
    return str.replace(textToReplace, textManipulation(textToReplace, tag, tagLen));
}

function textManipulation(text, tagType, len) {
    var replacementText = text.substring(len + 2, text.length - len - 3);
    switch (tagType) {
        case "upcase": replacementText = replacementText.toUpperCase(); break;
        case "lowcase": replacementText = replacementText.toLowerCase(); break;
        case "mixcase": replacementText = executeMixcase(replacementText); break;
    }
    return replacementText;
}

function executeMixcase(text) {
    var result = "",
        option = 0;
    for (var i = 0; i < text.length; i++) {
        option = Math.random();
        result += (option < 0.5 ? text[i].toLowerCase() : text[i].toUpperCase());
    }
    return result;
}