<p>
<form>
<input type="text" size="15" id="viewer_replaceTextSpec" value="s/\n\n
++/\n&lt;p&gt;/g; s/\&#x5b/&amp;#x5b;/g; s/\&#x5d;/&amp;#x5d;/">
<input type="button" value="Subst" id="viewer_replaceTextButton">
</form>
<script type="text/javascript"><!--
function viewer_replaceText() {
    var spec = document.getElementById("viewer_replaceTextSpec").value
+;
    var parser = /\s*s\s*(\S)((?:[^\\]|\\.)*?)\1((?:[^\\]|\\.)*?)\1(\w
+*)\s*;?\s*|(.+)/g ;
    var parseE = {};
    var pairs = [];
    while (fret = parser.exec(spec)) {
        if (undefined != fret[5]) {
            window.alert("Error parsing substitution list, remaining: 
+" + fret[5]);
            return;
        } else {
            pairs.push([new RegExp(fret[2], fret[4]), fret[3]]);
        }
    };
    if (!pairs) {
        window.alert("Warning: no substitution patterns given.");
    }
    var textarea = document.getElementsByTagName("textarea")[0];
    var text = new String(textarea.value);
    var nmatches = 0;
    pairs.forEach(function(pair) {
        var lh = pair[0];
        var rh = pair[1];
        text = text.replace(lh, function(whole) {
            nmatches++;
            var captures = arguments;
            var rp = rh.replace( /(\\(?:u\w{1,4}|x\w{1,2}|\d{1,3}|.))|
+(\$\&)|\$(\d+)|\$\{(\d+)\}/g, function(rNone, rBs, rWhole, rCapture, 
+rCapture2) {
                var t;
                if (rBs.length) { t = eval('"'+rBs+'"'); }
                else if (rWhole.length) { t = whole; }
                else if (rCapture.length) { t = captures[rCapture-0]; 
+}
                else if (rCapture2.length) { t = captures[rCapture2-0]
+; }
                if (undefined == t) return "";
                return t;
            });
            return rp;
        });
    })
    //window.alert(text);
    textarea.value = text;
    window.alert("Replaced " + nmatches + " matches.");
}
var btn = document.getElementById("viewer_replaceTextButton");
var stdEventQueue = window.addEventListener;
if (stdEventQueue) {
    btn.addEventListener("click", function(e) { viewer_replaceText(); 
+}, false);
} else {
    btn.attachEvent("onclick", function(e) { viewer_replaceText(); });
+    
}
//-->
</script>
[download]

The input box allows more than one substitution operator separated by semicolons: these are performed one after another. Each substitution needs to have a syntax apparently similar to a perl substitution operator: it starts with s, followed by a delimiter, followed by a regular expression, followed by the same delimiter (nesting delimiters are not supported), followed by a replace expression, followed by the same delimiter, followed by optional sequence of flags. The regex is a javascript regex, which have syntax and semantics very similar to perl regexes: they support non-capturing parenthesis /(?:...) and they are backtracking the way perl regexen are. The replace can contain ordinary characters, backslash escapes (like the four escapes in "\n\\\x28\$"), and the following special variable references which are replaced by captures as in perl: $&; $1, $2… (even multi-digit); ${1}, ${2}… The captures that don't match are replaced by an empty string like in perl, but numbers that don't even have a capturing parenthesis in the regex can be replaced by strange things. The supported flags are that of javascript regexen as well: g for repeated match, i for case-insensitive match, m, and some browsers have a fourth as an extension.

The substitution input field is preloaded by an example. You can use this as is, or write new substitutions. Then, when you activate the Subst button besides the field, the substitutions are parsed, performed in place on the first input area on the current html page, and then you get a popup with an error or success message. Typically you would use this on a page where you edit a future writeup (like the comment on page, the preview page, or a section) or an existing writeup of yours, but you may use this on other pages as well. Then, you'd hit preview or submit to finalize the changes.