输入框字符限制插件·

(function($) {

    $.fn.maxlength = function(settings) {

        if (typeof settings == 'string') {
            settings = { feedback: settings };
        }

        settings = $.extend({}, $.fn.maxlength.defaults, settings);

        function length(el) {
            var parts = $(el).attr("value");
            if (settings.words)
                parts = $(el).attr("value").length ? parts.split(/\s+/) : { length: 0 };
            return parts.length;
        }

        return this.each(function() {
            var field = this,
                        $field = $(field),
                        limit = $field.attr('maxlength') || settings.limit,
                          $charsLeft = $("" + settings.feedback + "");

            function limitCheck(event) {
                var len = length(this),
                            exceeded = len >= limit,
                            code = event.keyCode;
                if (!exceeded)
                    return;
                switch (code) {
                    case 8:  // allow delete
                    case 9:
                    case 17:
                    case 36: // and cursor keys
                    case 35:
                    case 37:
                    case 38:
                    case 39:
                    case 40:
                    case 46:
                    case 65:
                    case 67:
                        return;
                    default:
                        return settings.words && code != 32 && code != 13 && len == limit;
                }
            }

            var updateCount = function() {
                var len = length(field),
                        msg = "";
                diff = limit - len;
                if (len >= settings.middle) {
                    msg = settings.Msg + settings.DefaultMsg;
                }
                else {
                    msg = settings.DefaultMsg;
                }
                $charsLeft.html(msg.replace(/@/g, (diff || "0")));
                // truncation code
                if (settings.hardLimit && diff < 0) {
                    field.value = settings.words ?
                    // split by white space, capturing it in the result, then glue them back
                            field.value.split(/(\s+)/, (limit * 2) - 1).join('') :
                            field.value.substr(0, limit);
                    updateCount();
                }
            };

            $field.bind('focus keyup input paste', function() {
                setTimeout(function() {
                    updateCount();
                }, 100);
            });
            if (settings.hardLimit) {
                $field.keydown(limitCheck);
            }
            updateCount();
        });
    };
    $.fn.maxlength.defaults = {
        useInput: false,
        hardLimit: true,
        feedback: '#msg',
        words: false,
        middle: 70,
        limit: 140,
        Msg: "已经超过<span style='color:red'>70</span>个字,将分两条短信发送。",
        DefaultMsg: "还可输入<span style='color:green'>@</span>个字"
    };

})(jQuery);

对鼠标的粘贴事件也做了监视