regexp

9-regular-expressions/12-regexp-backreferences/article.md

@@ -1,31 +1,31 @@
-# Backreferences in pattern: \n and \k
+# Backreferences in pattern: \N and \k<name>
 
-We can use the contents of capturing groups `(...)` not only in the result or in the replacement string, but also in the pattern itself.
+We can use the contents of capturing groups `pattern:(...)` not only in the result or in the replacement string, but also in the pattern itself.
 
-## Backreference by number: \n
+## Backreference by number: \N
 
-A group can be referenced in the pattern using `\n`, where `n` is the group number.
+A group can be referenced in the pattern using `pattern:\N`, where `N` is the group number.
 
-To make things clear let's consider a task.
+To make clear why that's helpful, 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.
+We need to find quoted strings: either single-quoted `subject:'...'` or a double-quoted `subject:"..."` -- both variants should match.
 
-How to look for them?
+How to find them?
 
-We can put both kinds of quotes in the square brackets: `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!"`:
+We can put both kinds of quotes in the square brackets: `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 in 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
+// The result is not what we'd like to have
 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.
+As we can see, the pattern found an opening quote `match:"`, then the text is consumed 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 wrap it into a capturing group and use the backreference.
+To make sure that the pattern looks for the closing quote exactly the same as the opening one, we can wrap it into a capturing group and backreference it: `pattern:(['"])(.*?)\1`.
 
 Here's the correct code:
 
@@ -39,20 +39,27 @@ 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.
+Now it works! The regular expression engine finds the first quote `pattern:(['"])` and memorizes its content. 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:
+Similar to that, `pattern:\2` would mean the contents of the second group, `pattern:\3` - the 3rd group, and so on.
 
-- 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.
+```smart
+If we use `?:` in the group, then we can't reference it. Groups that are excluded from capturing `(?:...)` are not memorized by the engine.
+```
+
+```warn header="Don't mess up: in the pattern `pattern:\1`, in the replacement: `pattern:$1`"
+In the replacement string we use a dollar sign: `pattern:$1`, while in the pattern - a backslash `pattern:\1`.
+```
 
 ## Backreference by name: `\k<name>`
 
-For named groups, we can backreference by `\k<name>`.
+If a regexp has many parentheses, it's convenient to give them names.
 
-The same example with the named group:
+To reference a named group we can use `pattern:\k<имя>`.
+
+In the example below the group with quotes is named `pattern:?<quote>`, so the backreference is `pattern:\k<quote>`:
 
 ```js run
 let str = `He said: "She's the one!".`;