starting point for regexp based page search

find.js

// this is just the starting point. you can run it in the console. 
// there are numerous problems to solve, like working out whether 
// the text it finds is visible or not -- i.e. if a node has display: none,
// or is absolutely positioned offscreen or behind another element, or
// has visiblity: hidden, or has height: 0 and overflow: hidden, and so on.

function find (str, caseSensitive) {
    var head = (document.head || document.getElementsByTagName('head')[0]),
        body = (document.body || document.getElementsByTagName('body')[0]),
        re   = RegExp(str, 'g' + (caseSensitive ? '' : 'i')),
        nodes    = [],
        matches  = [],
        iterator = document.evaluate('//text()', body, null, XPathResult.ORDERED_NODE_ITERATOR_TYPE, null),
        node, highlights, nav, style, current, total, navCount, siv;

    while (node = iterator.iterateNext())
        nodes.push(node);

    nodes.filter(function (t) {
        if (t.parentNode.nodeType != 1)
            return false;

        var parentName = t.parentNode.nodeName.toLowerCase();
        return parentName != 'script' && parentName != 'style' && re.test(t.nodeValue);
    }).map(function (t) {
        var parent = t.parentNode,
            frags  = [],
            idx = 0,
            text, m, len, div, frag, next;

        text = t.nodeValue;
        
        while (m = re.exec(text)) {
            len = m[0].length;

            if (idx != m.index)
                frags.push(text.slice(idx, m.index));
            frags.push('<span class="nsf_highlight" tabindex="-1">' + text.slice(m.index, m.index + len) + '</span>');
            idx = m.index + len;
        }

        frags.push(text.slice(idx));

        div = document.createElement('div');
        div.innerHTML = frags.join('');
        frag = document.createDocumentFragment();

        while (div.firstChild)
            frag.appendChild(div.firstChild);

        return [parent, frag, t];
    }).forEach(function (x) {
        var parent = x.shift();
        parent.replaceChild.apply(parent, x);
        matches.push.apply(matches, [].slice.apply(parent.getElementsByClassName('nsf_highlight')));
    });

    highlights = [];

    for (var i = 0, l = matches.length; i < l; i++) {
        if (matches[i].id) {
            console.warn('highlight already in collection');
            continue;
        }
        matches[i].id = 'nsf_highlight-' + i;
        highlights.push(matches[i]);
    }
    total = highlights.length;

    if (!total)
        return;

    current = -1;

    nav = document.createElement('div');
    nav.innerHTML = '<a href="#">&laquo;</a> <span>-1</span> of <span>' + total + '</span> <a href="#">&raquo;</a>';
    nav.className = 'nsf_highlight-nav';
    navCount = nav.querySelector('span');

    style = document.createElement('style');
    style.innerHTML =   '.nsf_highlight {background: #FFA} .nsf_active {background: orange}' +
                        '.nsf_highlight-nav {font-size: 16px; background: #DDD; position: fixed; top: 0; right: 0; z-index: 100;  padding: 15px}';

    siv = typeof body.firstElementChild.scrollIntoViewIfNeeded != 'undefined' ? 'scrollIntoViewIfNeeded' : 'scrollIntoView';

    function move (back) {
        if (~current)
            highlights[current].className = 'nsf_highlight';
        back ? prev() : next();
        navCount.innerHTML = current + 1;
        highlights[current].className = 'nsf_highlight nsf_active';
        highlights[current][siv]();
        highlights[current].focus();
    }

    function prev () {
        --current;
        if (!~current)
            current = total - 1;
    }

    function next () {
        ++current;
        if (current >= total)
            current = 0; 
    }

    function keyHandler (e) {
        if (e.which == 27) {
            head.removeChild(style);

            document.removeEventListener('keydown', keyHandler);
            nav.removeEventListener('click', clickHandler);

            for (var i = 0; i < total; i++)
                highlights[i].parentNode.replaceChild(highlights[i].firstChild, highlights[i]);
        } else if (e.which == 78 && e.shiftKey || e.which == 37) {
            move(true);
        } else if (e.which == 78 || e.which == 39) {
            move(false);
        }
    }

    function clickHandler (e) {
        if (e.target != nav.firstChild && e.target != nav.lastChild)
            return;

        e.preventDefault();
        move(e.target == nav.firstChild);
    }

    document.addEventListener('keydown', keyHandler);
    nav.addEventListener('click', clickHandler);

    head.appendChild(style);
    move(false);
    body.appendChild(nav);
}

you can test it with something like

find('(?:is)|(?:if)')

to find all instances of is or if, case insensitive.