138 lines
5.2 KiB
Markdown
138 lines
5.2 KiB
Markdown
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# Sticky flag "y", searching at position
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The flag `pattern:y` allows to perform the search at the given position in the source string.
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To grasp the use case of `pattern:y` flag, and better understand the ways of regexps, let's explore a practical example.
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One of common tasks for regexps is "lexical analysis": we get a text, e.g. in a programming language, and need to find its structural elements. For instance, HTML has tags and attributes, JavaScript code has functions, variables, and so on.
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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.
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E.g. we have a code string `subject:let varName = "value"`, and we need to read the variable name from it, that starts at position `4`.
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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.
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- A call to `str.match(/\w+/)` will find only the first word in the line (`let`). That's not it.
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- We can add the flag `pattern:g`. But then the call `str.match(/\w+/g)` will look for all words in the text, while we need one word at position `4`. Again, not what we need.
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**So, how to search for a regexp exactly at the given position?**
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Let's try using method `regexp.exec(str)`.
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For a `regexp` without flags `pattern:g` and `pattern:y`, this method looks only for the first match, it works exactly like `str.match(regexp)`.
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...But if there's flag `pattern:g`, then it performs the search in `str`, starting from position stored in the `regexp.lastIndex` property. And, if it finds a match, then sets `regexp.lastIndex` to the index immediately after the match.
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In other words, `regexp.lastIndex` serves as a starting point for the search, that each `regexp.exec(str)` call resets to the new value ("after the last match"). That's only if there's `pattern:g` flag, of course.
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So, successive calls to `regexp.exec(str)` return matches one after another.
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Here's an example of such calls:
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```js run
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let str = 'let varName'; // Let's find all words in this string
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let regexp = /\w+/g;
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alert(regexp.lastIndex); // 0 (initially lastIndex=0)
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let word1 = regexp.exec(str);
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alert(word1[0]); // let (1st word)
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alert(regexp.lastIndex); // 3 (position after the match)
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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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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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```
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We can get all matches in the loop:
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```js run
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let str = 'let varName';
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let regexp = /\w+/g;
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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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}
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```
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Such use of `regexp.exec` is an alternative to method `str.matchAll`, with a bit more control over the process.
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Let's go back to our task.
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We can manually set `lastIndex` to `4`, to start the search from the given position!
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Like this:
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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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*!*
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regexp.lastIndex = 4;
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*/!*
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let word = regexp.exec(str);
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alert(word); // varName
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```
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Hooray! Problem solved!
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We performed a search of `pattern:\w+`, starting from position `regexp.lastIndex = 4`.
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The result is correct.
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...But wait, not so fast.
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Please note: the `regexp.exec` call starts searching 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:
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```js run
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let str = 'let varName = "value"';
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let regexp = /\w+/g;
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*!*
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// start the search from position 3
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regexp.lastIndex = 3;
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*/!*
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let word = regexp.exec(str);
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// found the match at position 4
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alert(word[0]); // varName
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alert(word.index); // 4
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```
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For some tasks, including the lexical analysis, that's just wrong. We need to find a match exactly at the given position at the text, not somewhere after it. And that's what the flag `y` is for.
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**The flag `pattern:y` makes `regexp.exec` to search exactly at position `lastIndex`, not "starting from" it.**
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Here's the same search with flag `pattern:y`:
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```js run
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let str = 'let varName = "value"';
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let regexp = /\w+/y;
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regexp.lastIndex = 3;
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alert( regexp.exec(str) ); // null (there's a space at position 3, not a word)
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regexp.lastIndex = 4;
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alert( regexp.exec(str) ); // varName (word at position 4)
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```
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As we can see, regexp `pattern:/\w+/y` doesn't match at position `3` (unlike the flag `pattern:g`), but matches at position `4`.
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Not only that's what we need, there's an important performance gain when using flag `pattern:y`.
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Imagine, we have a long text, and there are no matches in it, at all. Then a search with flag `pattern:g` will go till the end of the text and find nothing, and this will take significantly more time than the search with flag `pattern:y`, that checks only the exact position.
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In tasks like lexical analysis, there are usually many searches at an exact position, to check what we have there. Using flag `pattern:y` is the key for correct implementations and a good performance.
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