String.prototype.replaceAll = function(str1, str2) {

    var str = this;

    var result = str.replace(eval("/" + str1 + "/gi"), str2);

    return result;

}

String.prototype.replaceAll = function(reallyDo, replaceWith, ignoreCase) {

    if (!RegExp.prototype.isPrototypeOf(reallyDo)) {

        return this.replace(new RegExp(reallyDo, (ignoreCase ? "gi": "g")), replaceWith);

    } else {

        return this.replace(reallyDo, replaceWith);

    }

}

String.prototype.replaceAll = function(s1, s2) {

    return this.replace(new RegExp(s1, "gm"), s2);

}

  String.prototype.replaceAll = function (oldStr, newStr) {

            var content = this;

            while (content.indexOf(oldStr) >= ) {

                content = content.replace(oldStr, newStr);

            }

            return content;

        }

   var val = "[red][b]&lt;script&gt;alert('asdfsd');&lt;/script&gt;[/b][/red]<br>";

        val = val.replaceAll('[b]', '<b>');

        val = val.replaceAll('[/b]', '</b>');

        val = val.replaceAll("[red]", "<font color='red'>");

        val = val.replaceAll("[/red]", "</font>");

        alert(val);