62 lines
2.3 KiB
Markdown
62 lines
2.3 KiB
Markdown
# Backreferences: \n and $n
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Capturing groups may be accessed not only in the result, but in the replacement string, and in the pattern too.
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[cut]
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## Group in replacement: $n
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When we are using `replace` method, we can access n-th group in the replacement string using `$n`.
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For instance:
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```js run
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let name = "John Smith";
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name = name.replace(/(\w+) (\w+)/i, *!*"$2, $1"*/!*);
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alert( name ); // Smith, John
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```
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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".
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Referencing a group in the replacement string allows us to reuse the existing text during the replacement.
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## Group in pattern: \n
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A group can be referenced in the pattern using `\n`.
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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.
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How to look for them?
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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!"`:
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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, let's make a group of it and use the backreference:
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```js run
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let str = "He said: \"She's the one!\".";
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let reg = /(['"])(.*?)\1/g;
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alert( str.match(reg) ); // "She's the one!"
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```
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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.
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Further in the pattern `pattern:\1` means "find the same text as in the first group".
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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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