sizzle是jquery的核心，它用来选择匹配的元素，其代码包含在一个匿名函数中，并以window作为其上下文环境：
(function( window, undefined ) {
//此处为sizzle代码
})( window );
匿名函数体内，首先申明了很多变量，比如下边这些正则表达式：
classCache = createCache(), /*classCache: function cache( key, value ) { // Use (key + " ") to avoid collision with native prototype properties (see Issue #157) if ( keys.push( key += " " ) > Expr.cacheLength ) { // Only keep the most recent entries delete cache[ keys.shift() ]; } return (cache[ key ] = value); } */ tokenCache = createCache(), /*tokenCache: function cache( key, value ) { // Use (key + " ") to avoid collision with native prototype properties (see Issue #157) if ( keys.push( key += " " ) > Expr.cacheLength ) { // Only keep the most recent entries delete cache[ keys.shift() ]; } return (cache[ key ] = value); } */ compilerCache = createCache(), /*compilerCache: function cache( key, value ) { // Use (key + " ") to avoid collision with native prototype properties (see Issue #157) if ( keys.push( key += " " ) > Expr.cacheLength ) { // Only keep the most recent entries delete cache[ keys.shift() ]; } return (cache[ key ] = value); } */ hasDuplicate = false, sortOrder = function( a, b ) { if ( a === b ) { hasDuplicate = true; return 0; } return 0; }, // General-purpose constants strundefined = typeof undefined, MAX_NEGATIVE = 1 << 31, // Instance methods hasOwn = ({}).hasOwnProperty, arr = [], pop = arr.pop, push_native = arr.push, push = arr.push, slice = arr.slice, // Use a stripped-down indexOf if we can't use a native one indexOf = arr.indexOf || function( elem ) { var i = 0, len = this.length; for ( ; i < len; i++ ) { if ( this[i] === elem ) { return i; } } return -1; }, booleans = "checked|selected|async|autofocus|autoplay|controls|defer|disabled|hidden|ismap|loop|multiple|open|readonly|required|scoped", // Regular expressions // Whitespace characters http://www.w3.org/TR/css3-selectors/#whitespace //匹配空白 水平制表符 回车 换行 换页 whitespace = "[\x20\t\r\n\f]", // http://www.w3.org/TR/css3-syntax/#characters //可以匹配 由\. 或 单词字符(所有字母数字下划线) 或 汉字（包括汉字标点符号），这里也可以说是全角符号组成 characterEncoding = "(?:\\.|[\w-]|[^\x00-\xa0])+", // Loosely modeled on CSS identifier characters // An unquoted value should be a CSS identifier http://www.w3.org/TR/css3-selectors/#attribute-selectors // Proper syntax: http://www.w3.org/TR/CSS21/syndata.html#value-def-identifier //识别码 identifier = characterEncoding.replace( "w", "w#" ), // Acceptable operators http://www.w3.org/TR/selectors/#attribute-selectors //attributes ="[[x20 f]*((?:\.|[w-]|[^x00-xa0])+)[x20 f]*(?:([*^$|!~]?=)[x20 f]*(?:(['"])((?:\.|[^\])*?)3|((?:\.|[w#-]|[^x00-xa0])+)|)|)[x20 f]*]" //匹配属性 例如：[a="b"] [a=b] [a|='er'] attributes = "\[" + whitespace + "*(" + characterEncoding + ")" + whitespace + "*(?:([*^$|!~]?=)" + whitespace + "*(?:(['"])((?:\\.|[^\\])*?)\3|(" + identifier + ")|)|)" + whitespace + "*\]", // Prefer arguments quoted, // then not containing pseudos/brackets, // then attribute selectors/non-parenthetical expressions, // then anything else // These preferences are here to reduce the number of selectors // needing tokenize in the PSEUDO preFilter pseudos = ":(" + characterEncoding + ")(?:\(((['"])((?:\\.|[^\\])*?)\3|((?:\\.|[^\\()[\]]|" + attributes.replace( 3, 8 ) + ")*)|.*)\)|)", // Leading and non-escaped trailing whitespace, capturing some non-whitespace characters preceding the latter //匹配开始或结束的空白字符 rtrim = new RegExp( "^" + whitespace + "+|((?:^|[^\\])(?:\\.)*)" + whitespace + "+$", "g" ), //匹配逗号 rcomma = new RegExp( "^" + whitespace + "*," + whitespace + "*" ), //匹配组合器 包括> + ~ 及"[\x20\t\r\n\f]" rcombinators = new RegExp( "^" + whitespace + "*([>+~]|" + whitespace + ")" + whitespace + "*" ), //兄弟节点？ rsibling = new RegExp( whitespace + "*[+~]" ), //属性选择器的等号后边部分，如：=aa] rattributeQuotes = new RegExp( "=" + whitespace + "*([^\]'"]*)" + whitespace + "*\]", "g" ), //rpseudo=/:((?:\.|[w-]|[^x00-xa0])+)(?:(((['"])((?:\.|[^\])*?)3|((?:\.|[^\()[]]|[[x20 f]*((?:\.|[w-]|[^x00-xa0])+)[x20 f]*(?:([*^$|!~]?=)[x20 f]*(?:(['"])((?:\.|[^\])*?)8|((?:\.|[w#-]|[^x00-xa0])+)|)|)[x20 f]*])*)|.*))|)/ //伪类选择器 如：:nth-child(3) rpseudo = new RegExp( pseudos ), //匹配识别码 ridentifier = new RegExp( "^" + identifier + "$" ), matchExpr = {//匹配各种选择器的正则表达式 "ID": new RegExp( "^#(" + characterEncoding + ")" ),//ID选择器 "CLASS": new RegExp( "^\.(" + characterEncoding + ")" ),//类 "TAG": new RegExp( "^(" + characterEncoding.replace( "w", "w*" ) + ")" ), "ATTR": new RegExp( "^" + attributes ), "PSEUDO": new RegExp( "^" + pseudos ), "CHILD": new RegExp( "^:(only|first|last|nth|nth-last)-(child|of-type)(?:\(" + whitespace + "*(even|odd|(([+-]|)(\d*)n|)" + whitespace + "*(?:([+-]|)" + whitespace + "*(\d+)|))" + whitespace + "*\)|)", "i" ), "bool": new RegExp( "^(?:" + booleans + ")$", "i" ), // For use in libraries implementing .is() // We use this for POS matching in `select` "needsContext": new RegExp( "^" + whitespace + "*[>+~]|:(even|odd|eq|gt|lt|nth|first|last)(?:\(" + whitespace + "*((?:-\d)?\d*)" + whitespace + "*\)|)(?=[^-]|$)", "i" ) }, //检查是否原生代码 rnative = /^[^{]+{s*[native w/, // Easily-parseable/retrievable ID or TAG or CLASS selectors rquickExpr = /^(?:#([w-]+)|(w+)|.([w-]+))$/, //匹配input元素，其实包含了四种：input|select|textarea|button rinputs = /^(?:input|select|textarea|button)$/i, rheader = /^hd$/i, rescape = /'|\/g, // CSS escapes http://www.w3.org/TR/CSS21/syndata.html#escaped-characters //匹配解码字符 runescape = new RegExp( "\\([\da-f]{1,6}" + whitespace + "?|(" + whitespace + ")|.)", "ig" ), funescape = function( _, escaped, escapedWhitespace ) {//_代表整个匹配项，escaped代表第一个匹配项，escapedWhitespace代表第二个分组匹配项，即空白匹配项 var high = "0x" + escaped - 0x10000; // NaN means non-codepoint // Support: Firefox // Workaround erroneous numeric interpretation of +"0x" return high !== high || escapedWhitespace ? escaped : // BMP codepoint high < 0 ? String.fromCharCode( high + 0x10000 ) : // Supplemental Plane codepoint (surrogate pair) String.fromCharCode( high >> 10 | 0xD800, high & 0x3FF | 0xDC00 ); };
接下来就是一些功能函数，下边这张图说明了其中的功能函数的作用：
最后把sizzle和jquery关联起来：
jQuery.find = Sizzle;
jQuery.expr = Sizzle.selectors;
jQuery.expr[":"] = jQuery.expr.pseudos;
jQuery.unique = Sizzle.uniqueSort;
jQuery.text = Sizzle.getText;
jQuery.isXMLDoc = Sizzle.isXML;
jQuery.contains = Sizzle.contains;