_jquery = window.jquery;
_$ = window.$;
这两个变量是jquery唯一使用的两个全局变量。在jquery.noconflict()函数中，会把这两个变量恢复回去。
对于浏览器检测，jquery使用的是检查useragent，而没有使用特性检测。
rwebkit = /(webkit)[ \/]([\w.]+)/,
ropera = /(opear)(?:.*version)?[ \/](\w+)/,
rmsie = /(msie) ([\w.]+)/,
rmozilla = /(mozilla)(?:.*? rv:([\w.]+))?/,
初始化函数init
jquery对js对象的处理比较绕，而最终目的就是把jquery选择器得到的结果变成和数组差不多的一个对象。有length，first，last等。因为$(...)就是从dom树从选择一些节点出来。但是，$还有很多其他功能，比如常用的$(function() { ... })用来页面加载后初始化执行，$(...)来直接得到一个节点，用来append到dom树中。
接下来从93行开始就是很长的一段init函数。init: function(selector, context, rootjquery)
步骤：
1、selector是非法参数：空字符，null和undefined则直接返回this。即有默认属性的jquery对象。
2、selector是domelement。即用原生的js，比如getelementbyid等得到的元素。那么，相当于把原生的dom对象用$包装一次。把这个元素放到内部数组的第一个位置，并把length设置为1。然后返回。
3、特殊优化处理$(body)。即document.body元素。
4、selector是以结尾的字符串。那么假定是想用字符串新建一个dom元素。比如$(%e5%8d%9a%e5%ae%a2%e5%9b%ad%22)%e3%80%82%e4%b8%ba%e4%ba%86%e5%ae%89%e5%85%a8%e8%b5%b7%e8%a7%81%ef%bc%8c%e8%bf%99%e9%87%8c%e4%bd%bf%e7%94%a8%e4%ba%86%e4%b8%80%e4%b8%aa%e6%ad%a3%e5%88%99%e8%a1%a8%e8%be%be%e5%bc%8f%e6%9d%a5%e6%a3%80%e6%9f%a5%ef%bc%8c%e5%88%b0%e5%ba%95%e6%98%af...%e5%bd%a2%e5%bc%8f%e8%bf%98%e6%98%af#id%e7%9a%84%e5%bd%a2%e5%bc%8f%e3%80%82%20
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