65 lines
2.2 KiB
Markdown
65 lines
2.2 KiB
Markdown
# Backreferences in pattern: \n and \k
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Capturing groups can be accessed not only in the result or in the replacement string, but also in the pattern itself.
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## Backreference by number: \n
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A group can be referenced in the pattern using `\n`, where `n` is the group number.
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To make things clear let's consider a task.
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We need to find a quoted string: either a single-quoted `subject:'...'` or a double-quoted `subject:"..."` -- both variants need to match.
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How to look for them?
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We can put two kinds of quotes in the pattern: `pattern:['"](.*?)['"]`, but it would find strings with mixed quotes, like `match:"...'` and `match:'..."`. That would lead to incorrect matches when one quote appears inside other ones, like the string `subject:"She's the one!"`:
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```js run
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let str = `He said: "She's the one!".`;
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let reg = /['"](.*?)['"]/g;
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// The result is not what we expect
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alert( str.match(reg) ); // "She'
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```
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As we can see, the pattern found an opening quote `match:"`, then the text is consumed lazily till the other quote `match:'`, that closes the match.
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To make sure that the pattern looks for the closing quote exactly the same as the opening one, we can make a groups of it and use the backreference.
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Here's the correct code:
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```js run
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let str = `He said: "She's the one!".`;
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*!*
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let reg = /(['"])(.*?)\1/g;
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*/!*
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alert( str.match(reg) ); // "She's the one!"
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```
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Now it works! The regular expression engine finds the first quote `pattern:(['"])` and remembers the content of `pattern:(...)`, that's the first capturing group.
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Further in the pattern `pattern:\1` means "find the same text as in the first group", exactly the same quote in our case.
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Please note:
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- To reference a group inside a replacement string -- we use `$1`, while in the pattern -- a backslash `\1`.
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- If we use `?:` in the group, then we can't reference it. Groups that are excluded from capturing `(?:...)` are not remembered by the engine.
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## Backreference by name: `\k<name>`
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For named groups, we can backreference by `\k<name>`.
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The same example with the named group:
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```js run
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let str = `He said: "She's the one!".`;
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*!*
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let reg = /(?<quote>['"])(.*?)\k<quote>/g;
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*/!*
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alert( str.match(reg) ); // "She's the one!"
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```
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