Prefix tree find-and-replace.

function buildTrie(hash) {
    "use strict";
    // A very simple function to build a Trie
    // we could compress this later, but simplicity
    // is better for this example. If we don't
    // perform well, we'll try to optimize this a bit
    // there is a room for optimization here.
    var p, result = {}, leaf, i;
    for (p in hash) {
        if (hash.hasOwnProperty(p)) {
            leaf = result;
            i = 0;
            do {
                if (p[i] in leaf) {
                    leaf = leaf[p[i]];
                } else {
                    leaf = leaf[p[i]] = {};
                }
                i += 1;
            } while (i < p.length);
            // since, obviously, no character
            // equals to empty character, we'll
            // use it to store the reference to the
            // original value
            leaf[""] = hash[p];
        }
    }
    return result;
}

function prefixReplaceHtml(html, trie) {
    "use strict";
    var i, len = html.length, result = [], lastMatch = 0,
        current, leaf, match, matched, replacement;
    for (i = 0; i < len; i += 1) {
        current = html[i];
        if (current === "<") {
            // don't check for out of bounds access
            // assume we never face a situation, when
            // "<" is the last character in an HTML
            if (match) {
                result.push(
                    html.substring(lastMatch, i - matched.length),
                    "<a href=\"", match, "\">", replacement, "</a>");
                lastMatch = i - matched.length + replacement.length;
                i = lastMatch - 1;
            } else {
                if (matched) {
                    // go back to the second character of the
                    // matched string and try again
                    i = i - matched.length;
                }
            }
            matched = match = replacement = leaf = "";
            if (html[i + 1] === "a") {
                // we want to skip replacing inside
                // anchor tags. We also assume they
                // are never nested, as valid HTML is
                // against that idea
                if (html[i + 2] in
                    { " " : 1, "\t" : 1, "\r" : 1, "\n" : 1 }) {
                    // this is certainly an anchor
                    i = html.indexOf("</a", i + 3) + 3;
                    continue;
                }
            }
            // if we got here, it's a regular tag, just look
            // for terminating ">"
            i = html.indexOf(">", i + 1);
            continue;
        }
        // if we got here, we need to start checking
        // for the match in the trie
        if (!leaf) {
            leaf = trie;
        }
        leaf = leaf[current];
        // we prefer longest possible match, just like POSIX
        // regular expressions do
        if (leaf && ("" in leaf)) {
            match = leaf[""];
            replacement = html.substring(
                i - (matched ? matched.length : 0), i + 1);
        }
        if (!leaf) {
            // newby-style inline (all hand work!) pay extra
            // attention, this code is duplicated few lines above
            if (match) {
                result.push(
                    html.substring(lastMatch, i - matched.length),
                    "<a href=\"", match, "\">", replacement, "</a>");
                lastMatch = i - matched.length + replacement.length;
                i = lastMatch - 1;
            } else {
                if (matched) {
                    // go back to the second character of the
                    // matched string and try again
                    i = i - matched.length;
                }
            }
            matched = match = replacement = "";
        } else if (matched) {
            // perhaps a bit premature, but we'll try to avoid
            // string concatenation, when we can.
            matched = html.substring(i - matched.length, i + 1);
        } else {
            matched = current;
        }
    }
    return result.join("");
}

function testPrefixReplace() {
    "use strict";
    var trie = buildTrie(
        { "x" : "www.xxx.com", "yyy" : "www.y.com",
          "xy" : "www.xy.com", "yy" : "www.why.com" });
    // return trie["y"]["y"]["y"];
    return prefixReplaceHtml(
        "<html><head>x</head><body><a >yyy</a><p>" +
            "xyyy yy x xy</p><abrval><yy>xxy</yy>", trie);
}