components

9-regular-expressions/10-regexp-backreferences/article.md

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