JS port of django-tagging's tagging.utils.parse_tag_input(), licensed MIT. See http://code.google.com/p/django-tagging/source/browse/trunk/tagging/utils.py for original Python.

parseTags.js

// Output should pass all tests in tagging.tests.tests.TestParseTagInput except '"one two' 
// and '"one two three'. Not sure why those fail, but I figured I should go ahead and 
// post what I've got, anyway.

function parseTagInput(tagText) {
    function eliminateDuplicates(arr) {
        // From Rhett Anderson (@nosredna): 
        // http://dreaminginjavascript.wordpress.com/2008/08/22/eliminating-duplicates/
        // Modified to fit my coding style, but that should be about it.
        var out = [];
        var obj = {};
        
        for (var i = 0, j = arr.length; i < j; i++) {obj[arr[i]]=0;}
        for (var i in obj) {out.push(i);}
        return out;
    }
    
    if (typeof(tagText) == 'undefined' || tagText == null || tagText.length == 0) {return [];}
    
    if (tagText.indexOf(',') == -1 && tagText.indexOf('"') == -1) {
        return eliminateDuplicates(tagText.split(' ')).sort();
    }
    
    var tags = [];
    var buffer = [];
    
    var toBeSplit = [];
    var sawLooseComma = false;
    var openQuote = false;
    
    for (var i = 0, j = tagText.length; i < j; i++) {
        var c = tagText[i];
        if (c == '"') {
            if (buffer.length) {
                toBeSplit.push(buffer.join(''));
                buffer = [];
            }
            openQuote = true;
            c = tagText[++i];
            while (c != '"' && c != undefined) {
                buffer.push(c);
                c = tagText[++i];
            }
            if (buffer.length) {
                var tag = buffer.join('').trim();
                if (tag.length) {tags.push(tag);}
                buffer = [];
            }
            if (c != undefined) {openQuote = false;}
        } else {
            if (!sawLooseComma && c == ',') {sawLooseComma = true;}
            buffer.push(c);
        }
    }
    if (buffer.length) {
        if (openQuote && buffer.indexOf(',') != -1) {sawLooseComma = true;}
        toBeSplit.push(buffer.join(''));
    }
    if (toBeSplit.length) {
        var delimiter = ' ';
        if (sawLooseComma) {delimiter = ',';}
        for (var k in toBeSplit) {
            var chunk = toBeSplit[k];
            var rawTags = chunk.split(delimiter);
            for (var l in rawTags) {
                rawTags[l] = rawTags[l].trim();
                if (rawTags[l]) {tags.push(rawTags[l]);}
            }
        }
    }
    return eliminateDuplicates(tags).sort();
}