function urldecode (str) {
    // http://kevin.vanzonneveld.net
    // +   original by: Philip Peterson
    // +   improved by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
    // +      input by: AJ
    // +   improved by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
    // +   improved by: Brett Zamir (http://brett-zamir.me)
    // +      input by: travc
    // +      input by: Brett Zamir (http://brett-zamir.me)
    // +   bugfixed by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
    // +   improved by: Lars Fischer
    // +      input by: Ratheous
    // +   improved by: Orlando
    // +   reimplemented by: Brett Zamir (http://brett-zamir.me)
    // +      bugfixed by: Rob
    // +      input by: e-mike
    // +   improved by: Brett Zamir (http://brett-zamir.me)
    // +      input by: lovio
    // +   improved by: Brett Zamir (http://brett-zamir.me)
    // %        note 1: info on what encoding functions to use from: http://xkr.us/articles/javascript/encode-compare/
    // %        note 2: Please be aware that this function expects to decode from UTF-8 encoded strings, as found on
    // %        note 2: pages served as UTF-8
    // *     example 1: urldecode('Kevin+van+Zonneveld%21');
    // *     returns 1: 'Kevin van Zonneveld!'
    // *     example 2: urldecode('http%3A%2F%2Fkevin.vanzonneveld.net%2F');
    // *     returns 2: 'http://kevin.vanzonneveld.net/'
    // *     example 3: urldecode('http%3A%2F%2Fwww.google.nl%2Fsearch%3Fq%3Dphp.js%26ie%3Dutf-8%26oe%3Dutf-8%26aq%3Dt%26rls%3Dcom.ubuntu%3Aen-US%3Aunofficial%26client%3Dfirefox-a');
    // *     returns 3: 'http://www.google.nl/search?q=php.js&ie=utf-8&oe=utf-8&aq=t&rls=com.ubuntu:en-US:unofficial&client=firefox-a'
    // *     example 4: urldecode('%E5%A5%BD%3_4');
    // *     returns 4: '\u597d%3_4'
    return decodeURIComponent((str + '').replace(/%(?![\da-f]{2})/gi, function () {
        // PHP tolerates poorly formed escape sequences
        return '%25';
    }).replace(/\+/g, '%20'));
}

urldecode('Kevin+van+Zonneveld%21');

'Kevin van Zonneveld!'

urldecode('http%3A%2F%2Fkevin.vanzonneveld.net%2F');

'http://kevin.vanzonneveld.net/'

urldecode('http%3A%2F%2Fwww.google.nl%2Fsearch%3Fq%3Dphp.js%26ie%3Dutf-8%26oe%3Dutf-8%26aq%3Dt%26rls%3Dcom.ubuntu%3Aen-US%3Aunofficial%26client%3Dfirefox-a');

'http://www.google.nl/search?q=php.js&ie=utf-8&oe=utf-8&aq=t&rls=com.ubuntu:en-US:unofficial&client=firefox-a'