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

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-# Backreferences: \n and $n
-
-Capturing groups may be accessed not only in the result, but in the replacement string, and in the pattern too.
-
-[cut]
-
-## Group in replacement: $n
-
-When we are using `replace` method, we can access n-th group in the replacement string using `$n`.
-
-For instance:
-
-```js run
-let name = "John Smith";
-
-name = name.replace(/(\w+) (\w+)/i, *!*"$2, $1"*/!*);
-alert( name ); // Smith, John
-```
-
-Here `pattern:$1` in the replacement string means "substitute the content of the first group here", and `pattern:$2` means "substitute the second group here".
-
-Referencing a group in the replacement string allows us to reuse the existing text during the replacement.
-
-## Group in pattern: \n
-
-A group can be referenced in the pattern using `\n`.
-
-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:['"](.*?)['"]`. That finds strings like  `match:"..."` and `match:'...'`, but it gives incorrect matches when one quote appears inside another one, 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, let's make a group of it and use the backreference:
-
-```js run
-let str = "He said: \"She's the one!\".";
-
-let reg = /(['"])(.*?)\1/g;
-
-alert( str.match(reg) ); // "She's the one!"
-```
-
-Now everything's correct! 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".
-
-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.