input 和 textarea 最大文字限定插件

/* input 和 textarea 最大文字限定插件
 * 修改版, 一个中文表示1一个字, 一个英文半个字;
 * TextLimit - jQuery plugin for counting and limiting characters for input and textarea fields
 *
 * pass '-1' as speed if you don't want the char-deletion effect. (don't just put 0)
 * Example: jQuery("Textarea").textlimit('span.counter',256)
 *
 * $Version: 2009.07.25 +r2
 * Copyright (c) 2009 Yair Even-Or
 * [email protected]
*/
;
String.prototype.getBytes = function () {
    var cArr = this .match(/[^\x00-\xff]/ig);
    return this .length + (cArr == null ? 0 : cArr.length);
};
( function (jQuery) {
    jQuery.fn.textlimit= function (counter_el, thelimit, speed) {
        var charDelSpeed = speed || 15;
        var toggleCharDel = speed != -1;
        var toggleTrim = true ;
        var that = this [0];
        var isCtrl = false ;
        updateCounter();

        function updateCounter(){
            if ( typeof that == "object" )
                jQuery(counter_el).text(thelimit - Math.ceil(that.value.getBytes()/2));
        };

        this .keydown ( function (e){
            if (e.which == 17) isCtrl = true ;
            var ctrl_a = (e.which == 65 && isCtrl == true ) ? true : false ; // detect and allow CTRL + A selects all.
            var ctrl_v = (e.which == 86 && isCtrl == true ) ? true : false ; // detect and allow CTRL + V paste.
            // 8 is 'backspace' and 46 is 'delete'
            if ( this .value.length >= thelimit && e.which != '8' && e.which != '46' && ctrl_a == false && ctrl_v == false )
                e.preventDefault();
        })
        .keyup ( function (e){
            updateCounter();
            if (e.which == 17)
                isCtrl= false ;

            if ( this .value.length >= thelimit && toggleTrim ){
                if (toggleCharDel){
                    // first, trim the text a bit so the char trimming won't take forever
                    // Also check if there are more than 10 extra chars, then trim. just in case.
                    if ( ( this .value.length - thelimit) > 10 )
                        that.value = that.value.substr(0,thelimit+100);
                    var init = setInterval
                        (
                            function (){
                                if ( that.value.length <= thelimit ){
                                    init = clearInterval(init); updateCounter()
                                }
                                else {
                                    // deleting extra chars (one by one)
                                    that.value = that.value.substring(0,that.value.length-1); jQuery(counter_el).text(Math.ceil(that.value.getBytes()/2));
                                }
                            } ,charDelSpeed
                        );
                }
                else this .value = that.value.substr(0,thelimit);
            }
        });

    };
})(jQuery);