【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 );
        }
    },