【jQuery源码】preFilter

 preFilter: {
         "ATTR": function( match ) {
             //属性名解码
             match[1] = match[1].replace( runescape, funescape );

             // Move the given value to match[3] whether quoted or unquoted
             //属性解码
             match[3] = ( match[3] || match[4] || match[5] || "" ).replace( runescape, funescape );

             //若判断符为~=，则在属性值两边加上空格
             if ( match[2] === "~=" ) {
                 match[3] = " " + match[3] + " ";
             }

             //返回前4个元素
             return match.slice( 0, 4 );
         },

         "CHILD": function( match ) {
             /* matches from matchExpr["CHILD"]
                 1 type (only|nth|...)
                 2 what (child|of-type)
                 3 argument (even|odd|\d*|\d*n([+-]\d+)?|...)
                 4 xn-component of xn+y argument ([+-]?\d*n|)
                 5 sign of xn-component
                 6 x of xn-component
                 7 sign of y-component
                 8 y of y-component
             */
             //match[1]：(only|first|last|nth|nth-last)
             //match[2]: (child|last-child|of-type|last-of-type)
             //match[3]: (even|odd)
             //match[4]、match[5]: an+b中的a,b

             //match[1]变成小写
             match[1] = match[1].toLowerCase();

             //如果match[1]是"nth"
             if ( match[1].slice( 0, 3 ) === "nth" ) {
                 // nth-* requires argument
                 //若选择器括号内没有有效参数，则抛出异常
                 if ( !match[3] ) {
                     Sizzle.error( match[0] );
                 }

                 // numeric x and y parameters for Expr.filter.CHILD
                 // remember that false/true cast respectively to 0/1
                 /*
                      * 下面先以nth-child()为例介绍一下语法，以便更好的理解下面代码的作用
                      * nth-child允许的几种使用方式如下：
                      *    :nth-child(even)
                      *    :nth-child(odd)
                      *    :nth-child(3n)
                      *    :nth-child(+2n+1)
                      *    :nth-child(2n-1)
                      * 下面代码中赋值号左侧的match[4]、match[5]用于分别记录括号内n前及n后的数值，包括正负号
                      * 对于:nth-child(even)和:nth-child(odd)来说，match[4]为空，
                      *     所以返回 2 * (match[3] === "even" || match[3] === "odd")的计算结果
                      *     因为在js中true=1,false=0，所以(match[3] === "even" || match[3] === "odd")等于1
                      *     因此，2 * (match[3] === "even" || match[3] === "odd")的计算结果为2
                      *
                      * 等号右侧的“+”的作用是强制类型转换，将之后的字符串转换成数值类型
                  */
                 match[4] = +( match[4] ? match[5] + (match[6] || 1) : 2 * ( match[3] === "even" || match[3] === "odd" ) );
                 match[5] = +( ( match[7] + match[8] ) || match[3] === "odd" );

             // other types prohibit arguments
             //若非nth起头的其它CHILD类型选择器带有括号说明，则抛出异常
             } else if ( match[3] ) {
                 Sizzle.error( match[0] );
             }

             return match;
         },

         "PSEUDO": function( match ) {
             var excess,
                 unquoted = !match[6] && match[2];

             if ( matchExpr["CHILD"].test( match[0] ) ) {
                 return null;
             }

             // Accept quoted arguments as-is
             if ( match[3] ) {
                 match[2] = match[4] || match[5] || "";

             // Strip excess characters from unquoted arguments
             } else if ( unquoted && rpseudo.test( unquoted ) &&
                 // Get excess from tokenize (recursively)
                 (excess = tokenize( unquoted, true )) &&
                 // advance to the next closing parenthesis
                 (excess = unquoted.indexOf( ")", unquoted.length - excess ) - unquoted.length) ) {

                 // excess is a negative index
                 match[0] = match[0].slice( 0, excess );
                 match[2] = unquoted.slice( 0, excess );
             }

             // Return only captures needed by the pseudo filter method (type and argument)
             return match.slice( 0, 3 );
         }
     },