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For instance, HTML has tags and attributes, JavaScript code has functions, variables, and so on.
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例如，HTML 有标签和属性，JavaScript 代码有函数、变量等。
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Writing lexical analyzers is a special area, with its own tools and algorithms, so we don't go deep in there, but there's a common task: to read something at the given position.
We'll look for variable name using regexp `pattern:\w+`. Actually, JavaScript variable names need a bit more complex regexp for accurate matching, but here it doesn't matter.
A call to `str.match(/\w+/)`will find only the first word in the line. Or all words with the flag `pattern:g`. But we need only one word at position `4`.
To search from the given position, we can use method `regexp.exec(str)`.
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要从给定位置搜索，我们可以使用方法 `regexp.exec(str)`。
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If the `regexp`doesn't have flags `pattern:g`or`pattern:y`, then this method looks for the first match in the string `str`, exactly like `str.match(regexp)`. Such simple no-flags case doesn't interest us here.
If there's flag `pattern:g`, then it performs the search in the string `str`, starting from position stored in its `regexp.lastIndex`property. And, if it finds a match, then sets `regexp.lastIndex`to the index immediately after the match.
alert(regexp.lastIndex); // 3 (position after the match)
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alert(word1[0]); // let（第一个单词）
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alert(regexp.lastIndex); // 3（匹配后的位置）
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let word2 =regexp.exec(str);
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alert(word2[0]); // varName (2nd word)
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alert(regexp.lastIndex); // 11 (position after the match)
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alert(word2[0]); // varName (第二个单词)
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alert(regexp.lastIndex); // 11（匹配后的位置）
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let word3 =regexp.exec(str);
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alert(word3); // null (no more matches)
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alert(regexp.lastIndex); // 0 (resets at search end)
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alert(word3); // null（没有更多的匹配）
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alert(regexp.lastIndex); // 0（搜索结束时重置）
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```
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Every match is returned as an array with groups and additional properties.
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每个匹配都会以数组形式返回，包含分组和附加属性。
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We can get all matches in the loop:
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我们可以在循环中得到所有的匹配。
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```js run
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let str ='let varName';
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let result;
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while (result =regexp.exec(str)) {
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alert( `Found ${result[0]} at position ${result.index}` );
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//Found let at position 0, then
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//Found varName at position 4
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alert( `Found ${result[0]}} at position ${result.index}` );
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//在位置 0 发现 let, 然后
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//在位置 4 发现 varName
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}
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```
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Such use of `regexp.exec`is an alternative to method `str.matchAll`.
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`regexp.exec`是 `str.matchAll` 方法的替代方法。
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Unlike other methods, we can set our own `lastIndex`, to start the search from the given position.
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与其他方法不同，我们可以设置自己的 `lastIndex`，从给定位置开始搜索。
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For instance, let's find a word, starting from position `4`:
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例如，让我们从位置 `4` 开始寻找一个单词。
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```js run
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let str ='let varName = "value"';
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let regexp =/\w+/g; //without flag "g", property lastIndex is ignored
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let regexp =/\w+/g; //如果没有标志 "g"，属性 lastIndex 会被忽略
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*!*
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regexp.lastIndex=4;
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alert(word); // varName
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```
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We performed a search of`pattern:\w+`, starting from position `regexp.lastIndex = 4`.
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我们从位置 `regexp.lastIndex = 4` 开始搜索`pattern:w+`。
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Please note: the search starts at position `lastIndex`and then goes further. If there's no word at position `lastIndex`, but it's somewhere after it, then it will be found:
Imagine, we have a long text, and there are no matches in it, at all. Then searching with flag `pattern:g`will go till the end of the text, and this will take significantly more time than the search with flag `pattern:y`.
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