Merge branch 'regexp2'

1-js/05-data-types/10-destructuring-assignment/article.md

@@ -403,7 +403,7 @@ alert(item1);  // Cake
 alert(item2);  // Donut
 ```
 
-The whole `options` object except `extra` that was not mentioned, is assigned to corresponding variables:
+All properties of `options` object except `extra` that is absent in the left part, are assigned to corresponding variables:
 
 ![](destructuring-complex.svg)
 

1-js/09-classes/02-class-inheritance/article.md

@@ -524,7 +524,7 @@ In the example below a non-method syntax is used for comparison. `[[HomeObject]]
 
 ```js run
 let animal = {
-  eat: function() { // should be the short syntax: eat() {...}
+  eat: function() { // intentially writing like this instead of eat() {...
     // ...
   }
 };

2-ui/4-forms-controls/2-focus-blur/article.md

@@ -106,22 +106,27 @@ The best recipe is to be careful when using these events. If we want to track us
 
 By default many elements do not support focusing.
 
-The list varies between browsers, but one thing is always correct: `focus/blur` support is guaranteed for elements that a visitor can interact with: `<button>`, `<input>`, `<select>`, `<a>` and so on.
+The list varies a bit between browsers, but one thing is always correct: `focus/blur` support is guaranteed for elements that a visitor can interact with: `<button>`, `<input>`, `<select>`, `<a>` and so on.
 
-From the other hand, elements that exist to format something like `<div>`, `<span>`, `<table>` -- are unfocusable by default. The method `elem.focus()` doesn't work on them, and `focus/blur` events are never triggered.
+From the other hand, elements that exist to format something, such as `<div>`, `<span>`, `<table>` -- are unfocusable by default. The method `elem.focus()` doesn't work on them, and `focus/blur` events are never triggered.
 
 This can be changed using HTML-attribute `tabindex`.
 
-The purpose of this attribute is to specify the order number of the element when `key:Tab` is used to switch between them.
+Any element becomes focusable if it has `tabindex`. The value of the attribute is the order number of the element when `key:Tab` (or something like that) is used to switch between them.
 
-That is: if we have two elements, the first has `tabindex="1"`, and the second has `tabindex="2"`, then pressing `key:Tab` while in the first element -- moves us to the second one.
+That is: if we have two elements, the first has `tabindex="1"`, and the second has `tabindex="2"`, then pressing `key:Tab` while in the first element -- moves the focus into the second one.
+
+The switch order is: elements with `tabindex` from `1` and above go first (in the `tabindex` order), and then elements without `tabindex` (e.g. a regular `<input>`).
+
+Elements with matching `tabindex` are switched in the document source order (the default order).
 
 There are two special values:
 
-- `tabindex="0"` makes the element the last one.
+- `tabindex="0"` puts an element among those without `tabindex`. That is, when we switch elements, elements with `tabindex=0` go after elements with `tabindex ≥ 1`.
-- `tabindex="-1"` means that `key:Tab` should ignore that element.
 
-**Any element supports focusing if it has `tabindex`.**
+    Usually it's used to make an element focusable, but keep the default switching order. To make an element a part of the form on par with `<input>`.
+
+- `tabindex="-1"` allows only programmatic focusing on an element. The `key:Tab` key ignores such elements, but method `elem.focus()` works.
 
 For instance, here's a list. Click the first item and press `key:Tab`:
 
@@ -140,9 +145,9 @@ Click the first item and press Tab. Keep track of the order. Please note that ma
 </style>
 ```
 
-The order is like this: `1 - 2 - 0` (zero is always the last). Normally, `<li>` does not support focusing, but `tabindex` full enables it, along with events and styling with `:focus`.
+The order is like this: `1 - 2 - 0`. Normally, `<li>` does not support focusing, but `tabindex` full enables it, along with events and styling with `:focus`.
 
-```smart header="`elem.tabIndex` works too"
+```smart header="The property `elem.tabIndex` works too"
 We can add `tabindex` from JavaScript by using the `elem.tabIndex` property. That has the same effect.
 ```
 

9-regular-expressions/01-regexp-introduction/article.md

@@ -2,7 +2,7 @@
 
 Regular expressions is a powerful way to search and replace in text.
 
-In JavaScript, they are available as `RegExp` object, and also integrated in methods of strings.
+In JavaScript, they are available as [RegExp](mdn:js/RegExp) object, and also integrated in methods of strings.
 
 ## Regular Expressions
 
@@ -23,35 +23,43 @@ regexp = /pattern/; // no flags
 regexp = /pattern/gmi; // with flags g,m and i (to be covered soon)
 ```
 
-Slashes `"/"` tell JavaScript that we are creating a regular expression. They play the same role as quotes for strings.
+Slashes `pattern:/.../` tell JavaScript that we are creating a regular expression. They play the same role as quotes for strings.
 
-## Usage
+In both cases `regexp` becomes an object of the built-in `RegExp` class.
 
-To search inside a string, we can use method [search](mdn:js/String/search).
+The main difference between these two syntaxes is that slashes `pattern:/.../` do not allow to insert expressions (like strings with `${...}`). They are fully static.
 
-Here's an example:
+Slashes are used when we know the regular expression at the code writing time -- and that's the most common situation. While `new RegExp` is used when we need to create a regexp "on the fly", from a dynamically generated string, for instance:
 
-```js run
+```js
-let str = "I love JavaScript!"; // will search here
+let tag = prompt("What tag do you want to find?", "h2");
 
-let regexp = /love/;
+let regexp = new RegExp(`<${tag}>`); // same as /<h2>/ if answered "h2" in the prompt above
-alert( str.search(regexp) ); // 2
 ```
 
-The `str.search` method looks for the pattern `pattern:/love/` and returns the position inside the string. As we might guess, `pattern:/love/` is the simplest possible pattern. What it does is a simple substring search.
+## Flags
 
-The code above is the same as:
+Regular expressions may have flags that affect the search.
 
-```js run
+There are only 6 of them in JavaScript:
-let str = "I love JavaScript!"; // will search here
 
-let substr = 'love';
+`pattern:i`
-alert( str.search(substr) ); // 2
+: With this flag the search is case-insensitive: no difference between `A` and `a` (see the example below).
-```
 
-So searching for `pattern:/love/` is the same as searching for `"love"`.
+`pattern:g`
+: With this flag the search looks for all matches, without it -- only the first one.
 
-But that's only for now. Soon we'll create more complex regular expressions with much more searching power.
+`pattern:m`
+: Multiline mode (covered in the chapter <info:regexp-multiline-mode>).
+
+`pattern:s`
+: Enables "dotall" mode, that allows a dot `pattern:.` to match newline character `\n` (covered in the chapter <info:regexp-character-classes>).
+
+`pattern:u`
+: Enables full unicode support. The flag enables correct processing of surrogate pairs. More about that in the chapter <info:regexp-unicode>.
+
+`pattern:y`
+: "Sticky" mode: searching at the exact position in the text  (covered in the chapter <info:regexp-sticky>)
 
 ```smart header="Colors"
 From here on the color scheme is:
@@ -61,65 +69,109 @@ From here on the color scheme is:
 - result -- `match:green`
 ```
 
+## Searching: str.match
 
-````smart header="When to use `new RegExp`?"
+As it was said previously, regular expressions are integrated with string methods.
-Normally we use the short syntax `/.../`. But it does not support variable insertions `${...}`.
 
-On the other hand, `new RegExp` allows to construct a pattern dynamically from a string, so it's more flexible.
+The method `str.match(regexp)` finds all matches of `regexp` in the string `str`.
 
-Here's an example of a dynamically generated regexp:
+It has 3 working modes:
+
+1. If the regular expression has flag `pattern:g`, it returns an array of all matches:
+    ```js run
+    let str = "We will, we will rock you";
+
+    alert( str.match(/we/gi) ); // We,we (an array of 2 substrings that match)
+    ```
+    Please note that both `match:We` and `match:we` are found, because flag `pattern:i` makes the regular expression case-insensitive.
+
+2. If there's no such flag it returns only the first match in the form of an array, with the full match at index `0` and some additional details in properties:
+    ```js run
+    let str = "We will, we will rock you";
+
+    let result = str.match(/we/i); // without flag g
+
+    alert( result[0] );     // We (1st match)
+    alert( result.length ); // 1
+
+    // Details:
+    alert( result.index );  // 0 (position of the match)
+    alert( result.input );  // We will, we will rock you (source string)
+    ```
+    The array may have other indexes, besides `0` if a part of the regular expression is enclosed in parentheses. We'll cover that in the chapter  <info:regexp-groups>.
+
+3. And, finally, if there are no matches, `null` is returned (doesn't matter if there's flag `pattern:g` or not).
+
+    That's a very important nuance. If there are no matches, we get not an empty array, but `null`. Forgetting about that may lead to errors, e.g.:
+
+    ```js run
+    let matches = "JavaScript".match(/HTML/); // = null
+
+    if (!matches.length) { // Error: Cannot read property 'length' of null
+      alert("Error in the line above");
+    }
+    ```
+
+    If we'd like the result to be always an array, we can write it this way:
+
+    ```js run
+    let matches = "JavaScript".match(/HTML/)*!* || []*/!*;
+
+    if (!matches.length) {
+      alert("No matches"); // now it works
+    }
+    ```
+
+## Replacing: str.replace
+
+The method `str.replace(regexp, replacement)` replaces matches with `regexp` in string `str` with `replacement` (all matches, if there's flag `pattern:g`, otherwise only the first one).
+
+For instance:
 
 ```js run
-let tag = prompt("Which tag you want to search?", "h2");
+// no flag g
-let regexp = new RegExp(`<${tag}>`);
+alert( "We will, we will".replace(/we/i, "I") ); // I will, we will
 
-// finds <h2> by default
+// with flag g
-alert( "<h1> <h2> <h3>".search(regexp));
+alert( "We will, we will".replace(/we/ig, "I") ); // I will, I will
 ```
-````
 
+The second argument is the `replacement` string. We can use special character combinations in it to insert fragments of the match:
 
-## Flags
+| Symbols | Action in the replacement string |
+|--------|--------|
+|`$&`|inserts the whole match|
+|<code>$&#096;</code>|inserts a part of the string before the match|
+|`$'`|inserts a part of the string after the match|
+|`$n`|if `n` is a 1-2 digit number, then it inserts the contents of n-th parentheses, more about it in the chapter <info:regexp-groups>|
+|`$<name>`|inserts the contents of the parentheses with the given `name`, more about it in the chapter <info:regexp-groups>|
+|`$$`|inserts character `$` |
 
-Regular expressions may have flags that affect the search.
+An example with `pattern:$&`:
-
-There are only 6 of them in JavaScript:
-
-`i`
-: With this flag the search is case-insensitive: no difference between `A` and `a` (see the example below).
-
-`g`
-: With this flag the search looks for all matches, without it -- only the first one (we'll see uses in the next chapter).
-
-`m`
-: Multiline mode (covered in the chapter <info:regexp-multiline-mode>).
-
-`s`
-: "Dotall" mode, allows `.` to match newlines (covered in the chapter <info:regexp-character-classes>).
-
-`u`
-: Enables full unicode support. The flag enables correct processing of surrogate pairs. More about that in the chapter <info:regexp-unicode>.
-
-`y`
-: Sticky mode (covered in the chapter <info:regexp-sticky>)
-
-We'll cover all these flags further in the tutorial.
-
-For now, the simplest flag is `i`, here's an example:
 
 ```js run
-let str = "I love JavaScript!";
+alert( "I love HTML".replace(/HTML/, "$& and JavaScript") ); // I love HTML and JavaScript
-
-alert( str.search(/LOVE/i) ); // 2 (found lowercased)
-
-alert( str.search(/LOVE/) ); // -1 (nothing found without 'i' flag)
 ```
 
-So the `i` flag already makes regular expressions more powerful than a simple substring search. But there's so much more. We'll cover other flags and features in the next chapters.
+## Testing: regexp.test
 
+The method `regexp.test(str)` looks for at least one match, if found, returns `true`, otherwise `false`.
+
+```js run
+let str = "I love JavaScript";
+let regexp = /LOVE/i;
+
+alert( regexp.test(str) ); // true
+```
+
+Further in this chapter we'll study more regular expressions, come across many other examples and also meet other methods.
+
+Full information about the methods is given in the article <info:regexp-methods>.
 
 ## Summary
 
-- A regular expression consists of a pattern and optional flags: `g`, `i`, `m`, `u`, `s`, `y`.
+- A regular expression consists of a pattern and optional flags: `pattern:g`, `pattern:i`, `pattern:m`, `pattern:u`, `pattern:s`, `pattern:y`.
-- Without flags and special symbols that we'll study later, the search by a regexp is the same as a  substring search.
+- Without flags and special symbols that we'll study later, the search by a regexp is the same as a substring search.
-- The method `str.search(regexp)` returns the index where the match is found or `-1` if there's no match. In the next chapter we'll see other methods.
+- The method `str.match(regexp)` looks for matches: all of them if there's `pattern:g` flag, otherwise only the first one.
+- The method `str.replace(regexp, replacement)` replaces matches with `regexp` by `replacement`: all of them if there's `pattern:g` flag, otherwise only the first one.
+- The method `regexp.test(str)` returns `true` if there's at least one match, otherwise `false`.

9-regular-expressions/02-regexp-character-classes/article.md

@@ -0,0 +1,189 @@
+# Character classes
+
+Consider a practical task -- we have a phone number like `"+7(903)-123-45-67"`, and we need to turn it into pure numbers: `79035419441`.
+
+To do so, we can find and remove anything that's not a number. Character classes can help with that.
+
+A *character class* is a special notation that matches any symbol from a certain set.
+
+For the start, let's explore the "digit" class. It's written as `pattern:\d` and corresponds to "any single digit".
+
+For instance, the let's find the first digit in the phone number:
+
+```js run
+let str = "+7(903)-123-45-67";
+
+let regexp = /\d/;
+
+alert( str.match(regexp) ); // 7
+```
+
+Without the flag `pattern:g`, the regular expression only looks for the first match, that is the first digit `pattern:\d`.
+
+Let's add the `pattern:g` flag to find all digits:
+
+```js run
+let str = "+7(903)-123-45-67";
+
+let regexp = /\d/g;
+
+alert( str.match(regexp) ); // array of matches: 7,9,0,3,1,2,3,4,5,6,7
+
+// let's make the digits-only phone number of them:
+alert( str.match(regexp).join('') ); // 79035419441
+```
+
+That was a character class for digits. There are other character classes as well.
+
+Most used are:
+
+`pattern:\d` ("d" is from "digit")
+: A digit: a character from `0` to `9`.
+
+`pattern:\s` ("s" is from "space")
+: A space symbol: includes spaces, tabs `\t`, newlines `\n` and few other rare characters, such as `\v`, `\f` and `\r`.
+
+`pattern:\w` ("w" is from "word")
+: A "wordly" character: either a letter of Latin alphabet or a digit or an underscore `_`. Non-Latin letters (like cyrillic or hindi) do not belong to `pattern:\w`.
+
+For instance, `pattern:\d\s\w` means a "digit" followed by a "space character" followed by a "wordly character", such as `match:1 a`.
+
+**A regexp may contain both regular symbols and character classes.**
+
+For instance, `pattern:CSS\d` matches a string `match:CSS` with a digit after it:
+
+```js run
+let str = "Is there CSS4?";
+let regexp = /CSS\d/
+
+alert( str.match(regexp) ); // CSS4
+```
+
+Also we can use many character classes:
+
+```js run
+alert( "I love HTML5!".match(/\s\w\w\w\w\d/) ); // ' HTML5'
+```
+
+The match (each regexp character class has the corresponding result character):
+
+![](love-html5-classes.svg)
+
+## Inverse classes
+
+For every character class there exists an "inverse class", denoted with the same letter, but uppercased.
+
+The "inverse" means that it matches all other characters, for instance:
+
+`pattern:\D`
+: Non-digit: any character except `pattern:\d`, for instance a letter.
+
+`pattern:\S`
+: Non-space: any character except `pattern:\s`, for instance a letter.
+
+`pattern:\W`
+: Non-wordly character: anything but `pattern:\w`, e.g a non-latin letter or a space.
+
+In the beginning of the chapter we saw how to make a number-only phone number from a string like `subject:+7(903)-123-45-67`: find all digits and join them.
+
+```js run
+let str = "+7(903)-123-45-67";
+
+alert( str.match(/\d/g).join('') ); // 79031234567
+```
+
+An alternative, shorter way is to find non-digits `pattern:\D` and remove them from the string:
+
+```js run
+let str = "+7(903)-123-45-67";
+
+alert( str.replace(/\D/g, "") ); // 79031234567
+```
+
+## A dot is any character
+
+A dot `pattern:.` is a special character class that matches "any character except a newline".
+
+For instance:
+
+```js run
+alert( "Z".match(/./) ); // Z
+```
+
+Or in the middle of a regexp:
+
+```js run
+let regexp = /CS.4/;
+
+alert( "CSS4".match(regexp) ); // CSS4
+alert( "CS-4".match(regexp) ); // CS-4
+alert( "CS 4".match(regexp) ); // CS 4 (space is also a character)
+```
+
+Please note that a dot means "any character", but not the "absense of a character". There must be a character to match it:
+
+```js run
+alert( "CS4".match(/CS.4/) ); // null, no match because there's no character for the dot
+```
+
+### Dot as literally any character with "s" flag
+
+Usually a dot doesn't match a newline character `\n`.
+
+For instance, the regexp `pattern:A.B` matches `match:A`, and then `match:B` with any character between them, except a newline `\n`:
+
+```js run
+alert( "A\nB".match(/A.B/) ); // null (no match)
+```
+
+There are many situations when we'd like a dot to mean literally "any character", newline included.
+
+That's what flag `pattern:s` does. If a regexp has it, then a dot `pattern:.` matches literally any character:
+
+```js run
+alert( "A\nB".match(/A.B/s) ); // A\nB (match!)
+```
+
+````warn header="Pay attention to spaces"
+Usually we pay little attention to spaces. For us strings `subject:1-5` and `subject:1 - 5` are nearly identical.
+
+But if a regexp doesn't take spaces into account, it may fail to work.
+
+Let's try to find digits separated by a hyphen:
+
+```js run
+alert( "1 - 5".match(/\d-\d/) ); // null, no match!
+```
+
+Let's fix it adding spaces into the regexp `pattern:\d - \d`:
+
+```js run
+alert( "1 - 5".match(/\d - \d/) ); // 1 - 5, now it works
+// or we can use \s class:
+alert( "1 - 5".match(/\d\s-\s\d/) ); // 1 - 5, also works
+```
+
+**A space is a character. Equal in importance with any other character.**
+
+We can't add or remove spaces from a regular expression and expect to work the same.
+
+In other words, in a regular expression all characters matter, spaces too.
+````
+
+## Summary
+
+There exist following character classes:
+
+- `pattern:\d` -- digits.
+- `pattern:\D` -- non-digits.
+- `pattern:\s` -- space symbols, tabs, newlines.
+- `pattern:\S` -- all but `pattern:\s`.
+- `pattern:\w` -- Latin letters, digits, underscore `'_'`.
+- `pattern:\W` -- all but `pattern:\w`.
+- `pattern:.` -- any character if with the regexp `'s'` flag, otherwise any except a newline `\n`.
+
+...But that's not all!
+
+Unicode encoding, used by JavaScript for strings, provides many properties for characters, like: which language the letter belongs to (if it's a letter) it is it a punctuation sign, etc.
+
+We can search by these properties as well. That requires flag `pattern:u`, covered in the next article.

9-regular-expressions/02-regexp-methods/article.md

@@ -1,458 +0,0 @@
-# Methods of RegExp and String
-
-There are two sets of methods to deal with regular expressions.
-
-1. First, regular expressions are objects of the built-in [RegExp](mdn:js/RegExp) class, which provides many methods.
-2. Additionally, there are methods in regular strings that can work with regexps.
-
-
-## Recipes
-
-Which method to use depends on what we'd like to do.
-
-Methods become much easier to understand if we separate them by their use in real-life tasks.
-
-So, here are general recipes, the details to follow:
-
-**To search for all matches:**
-
-Use regexp `g` flag and:
-- Get a flat array of matches -- `str.match(reg)`
-- Get an array or matches with details -- `str.matchAll(reg)`.
-
-**To search for the first match only:**
-- Get the full first match -- `str.match(reg)` (without `g` flag).
-- Get the string position of the first match -- `str.search(reg)`.
-- Check if there's a match -- `regexp.test(str)`.
-- Find the match from the given position -- `regexp.exec(str)` (set `regexp.lastIndex` to position).
-
-**To replace all matches:**
-- Replace with another string or a function result -- `str.replace(reg, str|func)`
-
-**To split the string by a separator:**
-- `str.split(str|reg)`
-
-Now you can continue reading this chapter to get the details about every method... But if you're reading for the first time, then you probably want to know more about regexps. So you can move to the next chapter, and then return here if something about a method is unclear.
-
-## str.search(reg)
-
-We've seen this method already. It returns the position of the first match or `-1` if none found:
-
-```js run
-let str = "A drop of ink may make a million think";
-
-alert( str.search( *!*/a/i*/!* ) ); // 0 (first match at zero position)
-```
-
-**The important limitation: `search` only finds  the first match.**
-
-We can't find next matches using `search`, there's just no syntax for that. But there are other methods that can.
-
-## str.match(reg), no "g" flag
-
-The behavior of `str.match` varies depending on whether `reg` has `g` flag or not.
-
-First, if there's no `g` flag, then `str.match(reg)` looks for the first match only.
-
-The result is an array with that match and additional properties:
-
-- `index` -- the position of the match inside the string,
-- `input` -- the subject string.
-
-For instance:
-
-```js run
-let str = "Fame is the thirst of youth";
-
-let result = str.match( *!*/fame/i*/!* );
-
-alert( result[0] );    // Fame (the match)
-alert( result.index ); // 0 (at the zero position)
-alert( result.input ); // "Fame is the thirst of youth" (the string)
-```
-
-A match result may have more than one element.
-
-**If a part of the pattern is delimited by parentheses `(...)`, then it becomes a separate element in the array.**
-
-If parentheses have a name, designated by `(?<name>...)` at their start, then `result.groups[name]` has the content. We'll see that later in the chapter [about groups](info:regexp-groups).
-
-For instance:
-
-```js run
-let str = "JavaScript is a programming language";
-
-let result = str.match( *!*/JAVA(SCRIPT)/i*/!* );
-
-alert( result[0] ); // JavaScript (the whole match)
-alert( result[1] ); // script (the part of the match that corresponds to the parentheses)
-alert( result.index ); // 0
-alert( result.input ); // JavaScript is a programming language
-```
-
-Due to the `i` flag the search is case-insensitive, so it finds `match:JavaScript`. The part of the match that corresponds to `pattern:SCRIPT` becomes a separate array item.
-
-So, this method is used to find one full match with all details.
-
-
-## str.match(reg) with "g" flag
-
-When there's a `"g"` flag, then `str.match` returns an array of all matches. There are no additional properties in that array, and parentheses do not create any elements.
-
-For instance:
-
-```js run
-let str = "HO-Ho-ho!";
-
-let result = str.match( *!*/ho/ig*/!* );
-
-alert( result ); // HO, Ho, ho (array of 3 matches, case-insensitive)
-```
-
-Parentheses do not change anything, here we go:
-
-```js run
-let str = "HO-Ho-ho!";
-
-let result = str.match( *!*/h(o)/ig*/!* );
-
-alert( result ); // HO, Ho, ho
-```
-
-**So, with `g` flag `str.match` returns a simple array of all matches, without details.**
-
-If we want to get information about match positions and contents of parentheses then we should use `matchAll`  method that we'll cover below.
-
-````warn header="If there are no matches, `str.match` returns `null`"
-Please note, that's important. If there are no matches, the result is not an empty array, but `null`.
-
-Keep that in mind to evade pitfalls like this:
-
-```js run
-let str = "Hey-hey-hey!";
-
-alert( str.match(/Z/g).length ); // Error: Cannot read property 'length' of null
-```
-
-Here `str.match(/Z/g)` is `null`, it has no `length` property.
-````
-
-## str.matchAll(regexp)
-
-The method `str.matchAll(regexp)` is used to find all matches with all details.
-
-For instance:
-
-```js run
-let str = "Javascript or JavaScript? Should we uppercase 'S'?";
-
-let result = str.matchAll( *!*/java(script)/ig*/!* );
-
-let [match1, match2] = result;
-
-alert( match1[0] ); // Javascript (the whole match)
-alert( match1[1] ); // script (the part of the match that corresponds to the parentheses)
-alert( match1.index ); // 0
-alert( match1.input ); // = str (the whole original string)
-
-alert( match2[0] ); // JavaScript (the whole match)
-alert( match2[1] ); // Script (the part of the match that corresponds to the parentheses)
-alert( match2.index ); // 14
-alert( match2.input ); // = str (the whole original string)
-```
-
-````warn header="`matchAll` returns an iterable, not array"
-For instance, if we try to get the first match by index, it won't work:
-
-```js run
-let str = "Javascript or JavaScript??";
-
-let result = str.matchAll( /javascript/ig );
-
-*!*
-alert(result[0]); // undefined (?! there must be a match)
-*/!*
-```
-
-The reason is that the iterator is not an array. We need to run `Array.from(result)` on it, or use `for..of` loop to get matches.
-
-In practice, if we need all matches, then `for..of` works, so it's not a problem.
-
-And, to get only few matches, we can use destructuring:
-
-```js run
-let str = "Javascript or JavaScript??";
-
-*!*
-let [firstMatch] = str.matchAll( /javascript/ig );
-*/!*
-
-alert(firstMatch); // Javascript
-```
-````
-
-```warn header="`matchAll` is supernew, may need a polyfill"
-The method may not work in old browsers. A polyfill might be needed (this site uses core-js).
-
-Or you could make a loop with `regexp.exec`, explained below.
-```
-
-## str.split(regexp|substr, limit)
-
-Splits the string using the regexp (or a substring) as a delimiter.
-
-We already used `split` with strings, like this:
-
-```js run
-alert('12-34-56'.split('-')) // array of [12, 34, 56]
-```
-
-But we can split by a regular expression, the same way:
-
-```js run
-alert('12-34-56'.split(/-/)) // array of [12, 34, 56]
-```
-
-## str.replace(str|reg, str|func)
-
-This is a generic method for searching and replacing, one of most useful ones. The swiss army knife for searching and replacing.  
-
-We can use it without regexps, to search and replace a substring:
-
-```js run
-// replace a dash by a colon
-alert('12-34-56'.replace("-", ":")) // 12:34-56
-```
-
-There's a pitfall though.
-
-**When the first argument of `replace` is a string, it only looks for the first match.**
-
-You can see that in the example above: only the first `"-"` is replaced by `":"`.
-
-To find all dashes, we need to use not the string `"-"`, but a regexp `pattern:/-/g`, with an obligatory `g` flag:
-
-```js run
-// replace all dashes by a colon
-alert( '12-34-56'.replace( *!*/-/g*/!*, ":" ) )  // 12:34:56
-```
-
-The second argument is a replacement string. We can use special characters in it:
-
-| Symbol | Inserts |
-|--------|--------|
-|`$$`|`"$"` |
-|`$&`|the whole match|
-|<code>$&#096;</code>|a part of the string before the match|
-|`$'`|a part of the string after the match|
-|`$n`|if `n` is a 1-2 digit number, then it means the contents of n-th parentheses counting from left to right, otherwise it means a parentheses with the given name |
-
-
-For instance if we use `$&` in the replacement string, that means "put the whole match here".
-
-Let's use it to prepend all entries of `"John"` with `"Mr."`:
-
-```js run
-let str = "John Doe, John Smith and John Bull";
-
-// for each John - replace it with Mr. and then John
-alert(str.replace(/John/g, 'Mr.$&'));  // Mr.John Doe, Mr.John Smith and Mr.John Bull
-```
-
-Quite often we'd like to reuse parts of the source string, recombine them in the replacement or wrap into something.
-
-To do so, we should:
-1. First, mark the parts by parentheses in regexp.
-2. Use `$1`, `$2` (and so on) in the replacement string to get the content matched by 1st, 2nd and so on parentheses.
-
-For instance:
-
-```js run
-let str = "John Smith";
-
-// swap first and last name
-alert(str.replace(/(john) (smith)/i, '$2, $1')) // Smith, John
-```
-
-**For situations that require "smart" replacements, the second argument can be a function.**
-
-It will be called for each match, and its result will be inserted as a replacement.
-
-For instance:
-
-```js run
-let i = 0;
-
-// replace each "ho" by the result of the function
-alert("HO-Ho-ho".replace(/ho/gi, function() {
-  return ++i;
-})); // 1-2-3
-```
-
-In the example above the function just returns the next number every time, but usually the result is based on the match.
-
-The function is called with arguments `func(str, p1, p2, ..., pn, offset, input, groups)`:
-
-1. `str` -- the match,
-2. `p1, p2, ..., pn` -- contents of parentheses (if there are any),
-3. `offset` -- position of the match,
-4. `input` -- the source string,
-5. `groups` -- an object with named groups (see chapter [](info:regexp-groups)).
-
-If there are no parentheses in the regexp, then there are only 3 arguments: `func(str, offset, input)`.
-
-Let's use it to show full information about matches:
-
-```js run
-// show and replace all matches
-function replacer(str, offset, input) {
-  alert(`Found ${str} at position ${offset} in string ${input}`);
-  return str.toLowerCase();
-}
-
-let result = "HO-Ho-ho".replace(/ho/gi, replacer);
-alert( 'Result: ' + result ); // Result: ho-ho-ho
-
-// shows each match:
-// Found HO at position 0 in string HO-Ho-ho
-// Found Ho at position 3 in string HO-Ho-ho
-// Found ho at position 6 in string HO-Ho-ho
-```
-
-In the example below there are two parentheses, so `replacer` is called with 5 arguments: `str` is the full match, then parentheses, and then `offset` and `input`:
-
-```js run
-function replacer(str, name, surname, offset, input) {
-  // name is the first parentheses, surname is the second one
-  return surname + ", " + name;
-}
-
-let str = "John Smith";
-
-alert(str.replace(/(John) (Smith)/, replacer)) // Smith, John
-```
-
-Using a function gives us the ultimate replacement power, because it gets all the information about the match, has access to outer variables and can do everything.
-
-## regexp.exec(str)
-
-We've already seen these searching methods:
-
-- `search` -- looks for the position of the match,
-- `match` -- if there's no `g` flag, returns the first match with parentheses and all details,
-- `match` -- if there's a `g` flag -- returns all matches, without details parentheses,
-- `matchAll` -- returns all matches with details.
-
-The `regexp.exec` method is the most flexible searching method of all. Unlike previous methods, `exec` should be called on a regexp, rather than on a string.
-
-It behaves differently depending on whether the regexp has the `g` flag.
-
-If there's no `g`, then `regexp.exec(str)` returns the first match, exactly as `str.match(reg)`. Such behavior does not give us anything new.
-
-But if there's `g`, then:
-- `regexp.exec(str)` returns the first match and *remembers* the position after it in `regexp.lastIndex` property.
-- The next call starts to search from `regexp.lastIndex` and returns the next match.
-- If there are no more matches then `regexp.exec` returns `null` and `regexp.lastIndex` is set to `0`.
-
-We could use it to get all matches with their positions and parentheses groups in a loop, instead of `matchAll`:
-
-```js run
-let str = 'A lot about JavaScript at https://javascript.info';
-
-let regexp = /javascript/ig;
-
-let result;
-
-while (result = regexp.exec(str)) {
-  alert( `Found ${result[0]} at ${result.index}` );
-  // shows: Found JavaScript at 12, then:
-  // shows: Found javascript at 34
-}
-```
-
-Surely, `matchAll` does the same, at least for modern browsers. But what `matchAll` can't do -- is to search from a given position.
-
-Let's search from position `13`. What we need is to assign `regexp.lastIndex=13` and call `regexp.exec`:
-
-```js run
-let str = "A lot about JavaScript at https://javascript.info";
-
-let regexp = /javascript/ig;
-*!*
-regexp.lastIndex = 13;
-*/!*
-
-let result;
-
-while (result = regexp.exec(str)) {
-  alert( `Found ${result[0]} at ${result.index}` );
-  // shows: Found javascript at 34
-}
-```
-
-Now, starting from the given position `13`, there's only one match.
-
-
-## regexp.test(str)
-
-The method `regexp.test(str)` looks for a match and returns `true/false` whether it finds it.
-
-For instance:
-
-```js run
-let str = "I love JavaScript";
-
-// these two tests do the same
-alert( *!*/love/i*/!*.test(str) ); // true
-alert( str.search(*!*/love/i*/!*) != -1 ); // true
-```
-
-An example with the negative answer:
-
-```js run
-let str = "Bla-bla-bla";
-
-alert( *!*/love/i*/!*.test(str) ); // false
-alert( str.search(*!*/love/i*/!*) != -1 ); // false
-```
-
-If the regexp has `'g'` flag, then `regexp.test` advances `regexp.lastIndex` property, just like `regexp.exec`.
-
-So we can use it to search from a given position:
-
-```js run
-let regexp = /love/gi;
-
-let str = "I love JavaScript";
-
-// start the search from position 10:
-regexp.lastIndex = 10
-alert( regexp.test(str) ); // false (no match)
-```
-
-
-
-````warn header="Same global regexp tested repeatedly may fail to match"
-If we apply the same global regexp to different inputs, it may lead to wrong result, because `regexp.test` call advances `regexp.lastIndex` property, so the search in another string may start from non-zero position.
-
-For instance, here we call `regexp.test` twice on the same text, and the second time fails:
-
-```js run
-let regexp = /javascript/g;  // (regexp just created: regexp.lastIndex=0)
-
-alert( regexp.test("javascript") ); // true (regexp.lastIndex=10 now)
-alert( regexp.test("javascript") ); // false
-```
-
-That's exactly because `regexp.lastIndex` is non-zero on the second test.
-
-To work around that, one could use non-global regexps or re-adjust `regexp.lastIndex=0` before a new search.
-````
-
-## Summary
-
-There's a variety of many methods on both regexps and strings.
-
-Their abilities and methods overlap quite a bit, we can do the same by different calls. Sometimes that may cause confusion when starting to learn the language.
-
-Then please refer to the recipes at the beginning of this chapter, as they provide solutions for the majority of regexp-related tasks.

9-regular-expressions/03-regexp-character-classes/article.md

@@ -1,270 +0,0 @@
-# Character classes
-
-Consider a practical task -- we have a phone number `"+7(903)-123-45-67"`, and we need to turn it into pure numbers: `79035419441`.
-
-To do so, we can find and remove anything that's not a number. Character classes can help with that.
-
-A character class is a special notation that matches any symbol from a certain set.
-
-For the start, let's explore a "digit" class. It's written as `\d`. We put it in the pattern, that means "any single digit".
-
-For instance, the let's find the first digit in the phone number:
-
-```js run
-let str = "+7(903)-123-45-67";
-
-let reg = /\d/;
-
-alert( str.match(reg) ); // 7
-```
-
-Without the flag `g`, the regular expression only looks for the first match, that is the first digit `\d`.
-
-Let's add the `g` flag to find all digits:
-
-```js run
-let str = "+7(903)-123-45-67";
-
-let reg = /\d/g;
-
-alert( str.match(reg) ); // array of matches: 7,9,0,3,1,2,3,4,5,6,7
-
-alert( str.match(reg).join('') ); // 79035419441
-```
-
-That was a character class for digits. There are other character classes as well.
-
-Most used are:
-
-`\d` ("d" is from "digit")
-: A digit: a character from `0` to `9`.
-
-`\s` ("s" is from "space")
-: A space symbol: that includes spaces, tabs, newlines.
-
-`\w` ("w" is from "word")
-: A "wordly" character: either a letter of English alphabet or a digit or an underscore. Non-Latin letters (like cyrillic or hindi) do not belong to `\w`.
-
-For instance, `pattern:\d\s\w` means a "digit" followed by a "space character" followed by a "wordly character", like `"1 a"`.
-
-**A regexp may contain both regular symbols and character classes.**
-
-For instance, `pattern:CSS\d` matches a string `match:CSS` with a digit after it:
-
-```js run
-let str = "CSS4 is cool";
-let reg = /CSS\d/
-
-alert( str.match(reg) ); // CSS4
-```
-
-Also we can use many character classes:
-
-```js run
-alert( "I love HTML5!".match(/\s\w\w\w\w\d/) ); // ' HTML5'
-```
-
-The match (each character class corresponds to one result character):
-
-![](love-html5-classes.svg)
-
-## Word boundary: \b
-
-A word boundary `pattern:\b` -- is a special character class.
-
-It does not denote a character, but rather a boundary between characters.
-
-For instance, `pattern:\bJava\b` matches `match:Java` in the string `subject:Hello, Java!`, but not in the script `subject:Hello, JavaScript!`.
-
-```js run
-alert( "Hello, Java!".match(/\bJava\b/) ); // Java
-alert( "Hello, JavaScript!".match(/\bJava\b/) ); // null
-```
-
-The boundary has "zero width" in a sense that usually a character class means a character in the result (like a wordly character or a digit), but not in this case.
-
-The boundary is a test.
-
-When regular expression engine is doing the search, it's moving along the string in an attempt to find the match. At each string position it tries to find the pattern.
-
-When the pattern contains `pattern:\b`, it tests that the position in string is a word boundary, that is one of three variants:
-
-There are three different positions that qualify as word boundaries:
-
-- At string start, if the first string character is a word character `\w`.
-- Between two characters in the string, where one is a word character `\w` and the other is not.
-- At string end, if the last string character is a word character `\w`.
-
-For instance, in the string `subject:Hello, Java!` the following positions match `\b`:
-
-![](hello-java-boundaries.svg)
-
-So it matches `pattern:\bHello\b`, because:
-
-1. At the beginning of the string the first `\b` test matches.
-2. Then the word `Hello` matches.
-3. Then `\b` matches, as we're between `o` (a word character) and a space (not a word character).
-
-Pattern `pattern:\bJava\b` also matches. But not `pattern:\bHell\b` (because there's no word boundary after `l`) and not `Java!\b` (because the exclamation sign is not a wordly character, so there's no word boundary after it).
-
-```js run
-alert( "Hello, Java!".match(/\bHello\b/) ); // Hello
-alert( "Hello, Java!".match(/\bJava\b/) );  // Java
-alert( "Hello, Java!".match(/\bHell\b/) );  // null (no match)
-alert( "Hello, Java!".match(/\bJava!\b/) ); // null (no match)
-```
-
-Once again let's note that `pattern:\b` makes the searching engine to test for the boundary, so that `pattern:Java\b` finds `match:Java` only when followed by a word boundary, but it does not add a letter to the result.
-
-Usually we use `\b` to find standalone English words. So that if we want `"Java"` language then `pattern:\bJava\b` finds exactly a standalone word and ignores it when it's a part of another word, e.g. it won't match `match:Java` in `subject:JavaScript`.
-
-Another example: a regexp `pattern:\b\d\d\b` looks for standalone two-digit numbers. In other words, it requires that before and after `pattern:\d\d` must be a symbol different from `\w` (or beginning/end of the string).
-
-```js run
-alert( "1 23 456 78".match(/\b\d\d\b/g) ); // 23,78
-```
-
-```warn header="Word boundary doesn't work for non-Latin alphabets"
-The word boundary check `\b` tests for a boundary between `\w` and something else. But `\w` means an English letter (or a digit or an underscore), so the test won't work for other characters (like cyrillic or hieroglyphs).
-
-Later we'll come by Unicode character classes that allow to solve the similar task for different languages.
-```
-
-
-## Inverse classes
-
-For every character class there exists an "inverse class", denoted with the same letter, but uppercased.
-
-The "reverse" means that it matches all other characters, for instance:
-
-`\D`
-: Non-digit: any character except `\d`, for instance a letter.
-
-`\S`
-: Non-space: any character except `\s`, for instance a letter.
-
-`\W`
-: Non-wordly character: anything but `\w`.
-
-`\B`
-: Non-boundary: a test reverse to `\b`.
-
-In the beginning of the chapter we saw how to get all digits from the phone `subject:+7(903)-123-45-67`.
-
-One way was to match all digits and join them:
-
-```js run
-let str = "+7(903)-123-45-67";
-
-alert( str.match(/\d/g).join('') ); // 79031234567
-```
-
-An alternative, shorter way is to find non-digits `\D` and remove them from the string:
-
-
-```js run
-let str = "+7(903)-123-45-67";
-
-alert( str.replace(/\D/g, "") ); // 79031234567
-```
-
-## Spaces are regular characters
-
-Usually we pay little attention to spaces. For us strings `subject:1-5` and `subject:1 - 5` are nearly identical.
-
-But if a regexp doesn't take spaces into account, it may fail to work.
-
-Let's try to find digits separated by a dash:
-
-```js run
-alert( "1 - 5".match(/\d-\d/) ); // null, no match!
-```
-
-Here we fix it by adding spaces into the regexp `pattern:\d - \d`:
-
-```js run
-alert( "1 - 5".match(/\d - \d/) ); // 1 - 5, now it works
-```
-
-**A space is a character. Equal in importance with any other character.**
-
-Of course, spaces in a regexp are needed only if we look for them. Extra spaces (just like any other extra characters) may prevent a match:
-
-```js run
-alert( "1-5".match(/\d - \d/) ); // null, because the string 1-5 has no spaces
-```
-
-In other words, in a regular expression all characters matter, spaces too.
-
-## A dot is any character
-
-The dot `"."` is a special character class that matches "any character except a newline".
-
-For instance:
-
-```js run
-alert( "Z".match(/./) ); // Z
-```
-
-Or in the middle of a regexp:
-
-```js run
-let reg = /CS.4/;
-
-alert( "CSS4".match(reg) ); // CSS4
-alert( "CS-4".match(reg) ); // CS-4
-alert( "CS 4".match(reg) ); // CS 4 (space is also a character)
-```
-
-Please note that the dot means "any character", but not the "absense of a character". There must be a character to match it:
-
-```js run
-alert( "CS4".match(/CS.4/) ); // null, no match because there's no character for the dot
-```
-
-### The dotall "s" flag
-
-Usually a dot doesn't match a newline character.
-
-For instance, `pattern:A.B` matches `match:A`, and then `match:B` with any character between them, except a newline.
-
-This doesn't match:
-
-```js run
-alert( "A\nB".match(/A.B/) ); // null (no match)
-
-// a space character would match, or a letter, but not \n
-```
-
-Sometimes it's inconvenient, we really want "any character", newline included.
-
-That's what `s` flag does. If a regexp has it, then the dot `"."` match literally any character:
-
-```js run
-alert( "A\nB".match(/A.B/s) ); // A\nB (match!)
-```
-
-## Summary
-
-There exist following character classes:
-
-- `pattern:\d` -- digits.
-- `pattern:\D` -- non-digits.
-- `pattern:\s` -- space symbols, tabs, newlines.
-- `pattern:\S` -- all but `pattern:\s`.
-- `pattern:\w` -- English letters, digits, underscore `'_'`.
-- `pattern:\W` -- all but `pattern:\w`.
-- `pattern:.` -- any character if with the regexp `'s'` flag, otherwise any except a newline.
-
-...But that's not all!
-
-The Unicode encoding, used by JavaScript for strings, provides many properties for characters, like: which language the letter belongs to (if a letter) it is it a punctuation sign, etc.
-
-Modern JavaScript allows to use these properties in regexps to look for characters, for instance:
-
-- A cyrillic letter is: `pattern:\p{Script=Cyrillic}` or `pattern:\p{sc=Cyrillic}`.
-- A dash (be it a small hyphen `-` or a long dash `—`): `pattern:\p{Dash_Punctuation}` or `pattern:\p{pd}`.
-- A currency symbol, such as `$`, `€` or another: `pattern:\p{Currency_Symbol}` or `pattern:\p{sc}`.
-- ...And much more. Unicode has a lot of character categories that we can select from.
-
-These patterns require `'u'` regexp flag to work. More about that in the chapter [](info:regexp-unicode).

9-regular-expressions/03-regexp-unicode/article.md

@@ -0,0 +1,167 @@
+# Unicode: flag "u" and class \p{...}
+
+JavaScript uses [Unicode encoding](https://en.wikipedia.org/wiki/Unicode) for strings. Most characters are encoding with 2 bytes, but that allows to represent at most 65536 characters.
+
+That range is not big enough to encode all possible characters, that's why some rare characters are encoded with 4 bytes, for instance like `𝒳` (mathematical X) or `😄` (a smile), some hieroglyphs and so on.
+
+Here are the unicode values of some characters:
+
+| Character  | Unicode | Bytes count in unicode  |
+|------------|---------|--------|
+| a | `0x0061` |  2 |
+| ≈ | `0x2248` |  2 |
+|𝒳| `0x1d4b3` | 4 |
+|𝒴| `0x1d4b4` | 4 |
+|😄| `0x1f604` | 4 |
+
+So characters like `a` and `≈` occupy 2 bytes, while codes for `𝒳`, `𝒴` and `😄` are longer, they have 4 bytes.
+
+Long time ago, when JavaScript language was created, Unicode encoding was simpler: there were no 4-byte characters. So, some language features still handle them incorrectly.
+
+For instance, `length` thinks that here are two characters:
+
+```js run
+alert('😄'.length); // 2
+alert('𝒳'.length); // 2
+```
+
+...But we can see that there's only one, right? The point is that `length` treats 4 bytes as two 2-byte characters. That's incorrect, because they must be considered only together (so-called "surrogate pair", you can read about them in the article <info:string>).
+
+By default, regular expressions also treat 4-byte "long characters" as a pair of 2-byte ones. And, as it happens with strings, that may lead to odd results. We'll see that a bit later, in the article <info:regexp-character-sets-and-ranges>.
+
+Unlike strings, regular expressions have flag `pattern:u` that fixes such problems. With such flag, a regexp handles 4-byte characters correctly. And also Unicode property search becomes available, we'll get to it next.
+
+## Unicode properties \p{...}
+
+```warn header="Not supported in Firefox and Edge"
+Despite being a part of the standard since 2018, unicode proeprties are not supported in Firefox ([bug](https://bugzilla.mozilla.org/show_bug.cgi?id=1361876)) and Edge ([bug](https://github.com/Microsoft/ChakraCore/issues/2969)).
+
+There's [XRegExp](http://xregexp.com) library that provides "extended" regular expressions with cross-browser support for unicode properties.
+```
+
+Every character in Unicode has a lot of properties. They describe what "category" the character belongs to, contain miscellaneous information about it.
+
+For instance, if a character has `Letter` property, it means that the character belongs to an alphabet (of any language). And `Number` property means that it's a digit: maybe Arabic or Chinese, and so on.
+
+We can search for characters with a property, written as `pattern:\p{…}`. To use `pattern:\p{…}`, a regular expression must have flag `pattern:u`.
+
+For instance, `\p{Letter}` denotes a letter in any of language. We can also use `\p{L}`, as `L` is an alias of `Letter`. There are shorter aliases for almost every property.
+
+In the example below three kinds of letters will be found: English, Georgean and Korean.
+
+```js run
+let str = "A ბ ㄱ";
+
+alert( str.match(/\p{L}/gu) ); // A,ბ,ㄱ
+alert( str.match(/\p{L}/g) ); // null (no matches, as there's no flag "u")
+```
+
+Here's the main character categories and their subcategories:
+
+- Letter `L`:
+  - lowercase `Ll`
+  - modifier `Lm`,
+  - titlecase `Lt`,
+  - uppercase `Lu`,
+  - other `Lo`.
+- Number `N`:
+  - decimal digit `Nd`,
+  - letter number `Nl`,
+  - other `No`.
+- Punctuation `P`:
+  - connector `Pc`,
+  - dash `Pd`,
+  - initial quote `Pi`,
+  - final quote `Pf`,
+  - open `Ps`,
+  - close `Pe`,
+  - other `Po`.
+- Mark `M` (accents etc):
+  - spacing combining `Mc`,
+  - enclosing `Me`,
+  - non-spacing `Mn`.
+- Symbol `S`:
+  - currency `Sc`,
+  - modifier `Sk`,
+  - math `Sm`,
+  - other `So`.
+- Separator `Z`:
+  - line `Zl`,
+  - paragraph `Zp`,
+  - space `Zs`.
+- Other `C`:
+  - control `Cc`,
+  - format `Cf`,
+  - not assigned `Cn`,
+  -- private use `Co`,
+  - surrogate `Cs`.
+
+
+So, e.g. if we need letters in lower case, we can write `pattern:\p{Ll}`, punctuation signs: `pattern:\p{P}` and so on.
+
+There are also other derived categories, like:
+- `Alphabetic` (`Alpha`), includes Letters `L`, plus letter numbers `Nl` (e.g. Ⅻ - a character for the roman number 12), plus some other symbols `Other_Alphabetic` (`OAlpha`).
+- `Hex_Digit` includes hexadecimal digits: `0-9`, `a-f`.
+- ...And so on.
+
+Unicode supports many different properties, their full list would require a lot of space, so here are the references:
+
+- List all properties by a character: <https://unicode.org/cldr/utility/character.jsp>.
+- List all characters by a property: <https://unicode.org/cldr/utility/list-unicodeset.jsp>.
+- Short aliases for properties: <https://www.unicode.org/Public/UCD/latest/ucd/PropertyValueAliases.txt>.
+- A full base of Unicode characters in text format, with all properties, is here: <https://www.unicode.org/Public/UCD/latest/ucd/>.
+
+### Example: hexadecimal numbers
+
+For instance, let's look for hexadecimal numbers, written as `xFF`, where `F` is a hex digit (0..1 or A..F).
+
+A hex digit can be denoted as `pattern:\p{Hex_Digit}`:
+
+```js run
+let regexp = /x\p{Hex_Digit}\p{Hex_Digit}/u;
+
+alert("number: xAF".match(regexp)); // xAF
+```
+
+### Example: Chinese hieroglyphs
+
+Let's look for Chinese hieroglyphs.
+
+There's a unicode property `Script` (a writing system), that may have a value: `Cyrillic`, `Greek`, `Arabic`, `Han` (Chinese) and so on, [here's the full list]("https://en.wikipedia.org/wiki/Script_(Unicode)").
+
+To look for characters in a given writing system we should use `pattern:Script=<value>`, e.g. for Cyrillic letters: `pattern:\p{sc=Cyrillic}`, for Chinese hieroglyphs: `pattern:\p{sc=Han}`, and so on:
+
+```js run
+let regexp = /\p{sc=Han}/gu; // returns Chinese hieroglyphs
+
+let str = `Hello Привет 你好 123_456`;
+
+alert( str.match(regexp) ); // 你,好
+```
+
+### Example: currency
+
+Characters that denote a currency, such as `$`, `€`, `¥`, have unicode property  `pattern:\p{Currency_Symbol}`, the short alias: `pattern:\p{Sc}`.
+
+Let's use it to look for prices in the format "currency, followed by a digit":
+
+```js run
+let regexp = /\p{Sc}\d/gu;
+
+let  str = `Prices: $2, €1, ¥9`;
+
+alert( str.match(regexp) ); // $2,€1,¥9
+```
+
+Later, in the article <info:regexp-quantifiers> we'll see how to look for numbers that contain many digits.
+
+## Summary
+
+Flag `pattern:u` enables the support of Unicode in regular expressions.
+
+That means two things:
+
+1. Characters of 4 bytes are handled correctly: as a single character, not two 2-byte characters.
+2. Unicode properties can be used in the search: `\p{…}`.
+
+With Unicode properties we can look for words in given languages, special characters (quotes, currencies) and so on.

9-regular-expressions/04-regexp-anchors/1-start-end/solution.md

@@ -1,5 +1,4 @@
-
+An empty string is the only match: it starts and immediately finishes.
-The empty string is the only match: it starts and immediately finishes.
 
 The task once again demonstrates that anchors are not characters, but tests.
 

9-regular-expressions/04-regexp-anchors/article.md

@@ -0,0 +1,52 @@
+# Anchors: string start ^ and end $
+
+The caret `pattern:^` and dollar `pattern:$` characters have special meaning in a regexp. They are called "anchors".
+
+The caret `pattern:^` matches at the beginning of the text, and the dollar `pattern:$` -- at the end.
+
+For instance, let's test if the text starts with `Mary`:
+
+```js run
+let str1 = "Mary had a little lamb";
+alert( /^Mary/.test(str1) ); // true
+```
+
+The pattern `pattern:^Mary` means: "string start and then Mary".
+
+Similar to this, we can test if the string ends with `snow` using `pattern:snow$`:
+
+```js run
+let str1 = "it's fleece was white as snow";
+alert( /snow$/.test(str1) ); // true
+```
+
+In these particular cases we could use string methods `startsWith/endsWith` instead. Regular expressions should be used for more complex tests.
+
+## Testing for a full match
+
+Both anchors together `pattern:^...$` are often used to test whether or not a string fully matches the pattern. For instance, to check if the user input is in the right format.
+
+Let's check whether or not a string is a time in `12:34` format. That is: two digits, then a colon, and then another two digits.
+
+In regular expressions language that's `pattern:\d\d:\d\d`:
+
+```js run
+let goodInput = "12:34";
+let badInput = "12:345";
+
+let regexp = /^\d\d:\d\d$/;
+alert( regexp.test(goodInput) ); // true
+alert( regexp.test(badInput) ); // false
+```
+
+Here the match for `pattern:\d\d:\d\d` must start exactly after the beginning of the text `pattern:^`, and the end `pattern:$` must immediately follow.
+
+The whole string must be exactly in this format. If there's any deviation or an extra character, the result is `false`.
+
+Anchors behave differently if flag `pattern:m` is present. We'll see that in the next article.
+
+```smart header="Anchors have \"zero width\""
+Anchors `pattern:^` and `pattern:$` are tests. They have zero width.
+
+In other words, they do not match a character, but rather force the regexp engine to check the condition (text start/end).
+```

9-regular-expressions/05-regexp-character-sets-and-ranges/article.md

@@ -1,114 +0,0 @@
-# Sets and ranges [...]
-
-Several characters or character classes inside square brackets `[…]` mean to "search for any character among given".
-
-## Sets
-
-For instance, `pattern:[eao]` means any of the 3 characters: `'a'`, `'e'`, or `'o'`.
-
-That's called a *set*. Sets can be used in a regexp along with regular characters:
-
-```js run
-// find [t or m], and then "op"
-alert( "Mop top".match(/[tm]op/gi) ); // "Mop", "top"
-```
-
-Please note that although there are multiple characters in the set, they correspond to exactly one character in the match.
-
-So the example below gives no matches:
-
-```js run
-// find "V", then [o or i], then "la"
-alert( "Voila".match(/V[oi]la/) ); // null, no matches
-```
-
-The pattern assumes:
-
-- `pattern:V`,
-- then *one* of the letters `pattern:[oi]`,
-- then `pattern:la`.
-
-So there would be a match for `match:Vola` or `match:Vila`.
-
-## Ranges
-
-Square brackets may also contain *character ranges*.
-
-For instance, `pattern:[a-z]` is a character in range from `a` to `z`, and `pattern:[0-5]` is a digit from `0` to `5`.
-
-In the example below we're searching for `"x"` followed by two digits or letters from `A` to `F`:
-
-```js run
-alert( "Exception 0xAF".match(/x[0-9A-F][0-9A-F]/g) ); // xAF
-```
-
-Please note that in the word `subject:Exception` there's a substring `subject:xce`. It didn't match the pattern, because the letters are lowercase, while in the set `pattern:[0-9A-F]` they are uppercase.
-
-If we want to find it too, then we can add a range `a-f`: `pattern:[0-9A-Fa-f]`. The `i` flag would allow lowercase too.
-
-**Character classes are shorthands for certain character sets.**
-
-For instance:
-
-- **\d** -- is the same as `pattern:[0-9]`,
-- **\w** -- is the same as `pattern:[a-zA-Z0-9_]`,
-- **\s** -- is the same as `pattern:[\t\n\v\f\r ]` plus few other unicode space characters.
-
-We can use character classes inside `[…]` as well.
-
-For instance, we want to match all wordly characters or a dash, for words like "twenty-third". We can't do it with `pattern:\w+`, because `pattern:\w` class does not include a dash. But we can use `pattern:[\w-]`.
-
-We also can use several classes, for example `pattern:[\s\S]` matches spaces or non-spaces -- any character. That's wider than a dot `"."`, because the dot matches any character except a newline (unless `s` flag is set).
-
-## Excluding ranges
-
-Besides normal ranges, there are "excluding" ranges that look like `pattern:[^…]`.
-
-They are denoted by a caret character `^` at the start and match any character *except the given ones*.
-
-For instance:
-
-- `pattern:[^aeyo]` -- any character except  `'a'`, `'e'`, `'y'` or `'o'`.
-- `pattern:[^0-9]` -- any character except a digit, the same as `\D`.
-- `pattern:[^\s]` -- any non-space character, same as `\S`.
-
-The example below looks for any characters except letters, digits and spaces:
-
-```js run
-alert( "alice15@gmail.com".match(/[^\d\sA-Z]/gi) ); // @ and .
-```
-
-## No escaping in […]
-
-Usually when we want to find exactly the dot character, we need to escape it like `pattern:\.`. And if we need a backslash, then we use `pattern:\\`.
-
-In square brackets the vast majority of special characters can be used without escaping:
-
-- A dot `pattern:'.'`.
-- A plus `pattern:'+'`.
-- Parentheses `pattern:'( )'`.
-- Dash `pattern:'-'` in the beginning or the end (where it does not define a range).
-- A caret `pattern:'^'` if not in the beginning (where it means exclusion).
-- And the opening square bracket `pattern:'['`.
-
-In other words, all special characters are allowed except where they mean something for square brackets.
-
-A dot `"."` inside square brackets means just a dot. The pattern `pattern:[.,]` would look for one of characters: either a dot or a comma.
-
-In the example below the regexp `pattern:[-().^+]` looks for one of the characters `-().^+`:
-
-```js run
-// No need to escape
-let reg = /[-().^+]/g;
-
-alert( "1 + 2 - 3".match(reg) ); // Matches +, -
-```
-
-...But if you decide to escape them "just in case", then there would be no harm:
-
-```js run
-// Escaped everything
-let reg = /[\-\(\)\.\^\+]/g;
-
-alert( "1 + 2 - 3".match(reg) ); // also works: +, -
-```

9-regular-expressions/05-regexp-multiline-mode/article.md

@@ -0,0 +1,87 @@
+# Multiline mode of anchors ^ $, flag "m"
+
+The multiline mode is enabled by the flag `pattern:m`.
+
+It only affects the behavior of `pattern:^` and `pattern:$`.
+
+In the multiline mode they match not only at the beginning and the end of the string, but also at start/end of line.
+
+## Searching at line start ^
+
+In the example below the text has multiple lines. The pattern `pattern:/^\d/gm` takes a digit from the beginning of each line:
+
+```js run
+let str = `1st place: Winnie
+2nd place: Piglet
+3rd place: Eeyore`;
+
+*!*
+alert( str.match(/^\d/gm) ); // 1, 2, 3
+*/!*
+```
+
+Without the flag `pattern:m` only the first digit is matched:
+
+```js run
+let str = `1st place: Winnie
+2nd place: Piglet
+3rd place: Eeyore`;
+
+*!*
+alert( str.match(/^\d/g) ); // 1
+*/!*
+```
+
+That's because by default a caret `pattern:^` only matches at the beginning of the text, and in the multiline mode -- at the start of any line.
+
+```smart
+"Start of a line" formally means "immediately after a line break": the test  `pattern:^` in multiline mode matches at all positions preceeded by a newline character `\n`.
+
+And at the text start.
+```
+
+## Searching at line end $
+
+The dollar sign `pattern:$` behaves similarly.
+
+The regular expression `pattern:\d$` finds the last digit in every line
+
+```js run
+let str = `Winnie: 1
+Piglet: 2
+Eeyore: 3`;
+
+alert( str.match(/\d$/gm) ); // 1,2,3
+```
+
+Without the flag `m`, the dollar `pattern:$` would only match the end of the whole text, so only the very last digit would be found.
+
+```smart
+"End of a line" formally means "immediately before a line break": the test  `pattern:^` in multiline mode matches at all positions succeeded by a newline character `\n`.
+
+And at the text end.
+```
+
+## Searching for \n instead of ^ $
+
+To find a newline, we can use not only anchors `pattern:^` and `pattern:$`, but also the newline character `\n`.
+
+What's the difference? Let's see an example.
+
+Here we search for `pattern:\d\n` instead of `pattern:\d$`:
+
+```js run
+let str = `Winnie: 1
+Piglet: 2
+Eeyore: 3`;
+
+alert( str.match(/\d\n/gm) ); // 1\n,2\n
+```
+
+As we can see, there are 2 matches instead of 3.
+
+That's because there's no newline after `subject:3` (there's text end though, so it matches `pattern:$`).
+
+Another difference: now every match includes a newline character `match:\n`. Unlike the anchors `pattern:^` `pattern:$`, that only test the condition (start/end of a line), `\n` is a character, so it becomes a part of the result.
+
+So, a `\n` in the pattern is used when we need newline characters in the result, while anchors are used to find something at the beginning/end of a line.

9-regular-expressions/06-regexp-boundary/1-find-time-hh-mm/task.md

@@ -5,4 +5,5 @@ The time has a format: `hours:minutes`. Both hours and minutes has two digits, l
 Make a regexp to find time in the string: `subject:Breakfast at 09:00 in the room 123:456.`
 
 P.S. In this task there's no need to check time correctness yet, so `25:99` can also be a valid result.
+
 P.P.S. The regexp shouldn't match `123:456`.

9-regular-expressions/06-regexp-boundary/article.md

@@ -0,0 +1,52 @@
+# Word boundary: \b
+
+A word boundary `pattern:\b` is a test, just like `pattern:^` and `pattern:$`.
+
+When the regexp engine (program module that implements searching for regexps) comes across `pattern:\b`, it checks that the position in the string is a word boundary.
+
+There are three different positions that qualify as word boundaries:
+
+- At string start, if the first string character is a word character `pattern:\w`.
+- Between two characters in the string, where one is a word character `pattern:\w` and the other is not.
+- At string end, if the last string character is a word character `pattern:\w`.
+
+For instance, regexp `pattern:\bJava\b` will be found in `subject:Hello, Java!`, where `subject:Java` is a standalone word, but not in `subject:Hello, JavaScript!`.
+
+```js run
+alert( "Hello, Java!".match(/\bJava\b/) ); // Java
+alert( "Hello, JavaScript!".match(/\bJava\b/) ); // null
+```
+
+In the string `subject:Hello, Java!` following positions correspond to `pattern:\b`:
+
+![](hello-java-boundaries.svg)
+
+So, it matches the pattern `pattern:\bHello\b`, because:
+
+1. At the beginning of the string matches the first test `pattern:\b`.
+2. Then matches the word `pattern:Hello`.
+3. Then the test `pattern:\b` matches again, as we're between `subject:o` and a space.
+
+The pattern `pattern:\bJava\b` would also match. But not `pattern:\bHell\b` (because there's no word boundary after `l`) and not `Java!\b` (because the exclamation sign is not a wordly character `pattern:\w`, so there's no word boundary after it).
+
+```js run
+alert( "Hello, Java!".match(/\bHello\b/) ); // Hello
+alert( "Hello, Java!".match(/\bJava\b/) );  // Java
+alert( "Hello, Java!".match(/\bHell\b/) );  // null (no match)
+alert( "Hello, Java!".match(/\bJava!\b/) ); // null (no match)
+```
+
+We can use `pattern:\b` not only with words, but with digits as well.
+
+For example, the pattern `pattern:\b\d\d\b` looks for standalone 2-digit numbers. In other words, it looks for 2-digit numbers that are surrounded by characters different from `pattern:\w`, such as spaces or punctuation (or text start/end).
+
+```js run
+alert( "1 23 456 78".match(/\b\d\d\b/g) ); // 23,78
+alert( "12,34,56".match(/\b\d\d\b/g) ); // 12,34,56
+```
+
+```warn header="Word boundary `pattern:\b` doesn't work for non-latin alphabets"
+The word boundary test `pattern:\b` checks that there should be `pattern:\w` on the one side from the position and "not `pattern:\w`" - on the other side.
+
+But `pattern:\w` means a latin letter `a-z` (or a digit or an underscore), so the test doesn't work for other characters, e.g. cyrillic letters or hieroglyphs.
+```

9-regular-expressions/07-regexp-escaping/article.md

@@ -1,7 +1,7 @@
 
 # Escaping, special characters
 
-As we've seen, a backslash `"\"` is used to denote character classes. So it's a special character in regexps (just like in a regular string).
+As we've seen, a backslash `pattern:\` is used to denote character classes, e.g. `pattern:\d`. So it's a special character in regexps (just like in regular strings).
 
 There are other special characters as well, that have special meaning in a regexp. They are used to do more powerful searches. Here's a full list of them: `pattern:[ \ ^ $ . | ? * + ( )`.
 
@@ -9,7 +9,7 @@ Don't try to remember the list -- soon we'll deal with each of them separately a
 
 ## Escaping
 
-Let's say we want to find a dot literally. Not "any character", but just a dot.
+Let's say we want to find literally a dot. Not "any character", but just a dot.
 
 To use a special character as a regular one, prepend it with a backslash: `pattern:\.`.
 
@@ -43,11 +43,11 @@ Here's what a search for a slash `'/'` looks like:
 alert( "/".match(/\//) ); // '/'
 ```
 
-On the other hand, if we're not using `/.../`, but create a regexp using `new RegExp`, then we don't need to escape it:
+On the other hand, if we're not using `pattern:/.../`, but create a regexp using `new RegExp`, then we don't need to escape it:
 
 ```js run
-alert( "/".match(new RegExp("/")) ); // '/'
+alert( "/".match(new RegExp("/")) ); // finds /
-```                                                                                                                                                                                   
+```
 
 ## new RegExp
 
@@ -56,30 +56,30 @@ If we are creating a regular expression with `new RegExp`, then we don't have to
 For instance, consider this:
 
 ```js run
-let reg = new RegExp("\d\.\d");
+let regexp = new RegExp("\d\.\d");
 
-alert( "Chapter 5.1".match(reg) ); // null
+alert( "Chapter 5.1".match(regexp) ); // null
 ```
 
-The search worked with `pattern:/\d\.\d/`, but with `new RegExp("\d\.\d")` it doesn't work, why?
+The similar search in one of previous examples worked with `pattern:/\d\.\d/`, but `new RegExp("\d\.\d")` doesn't work, why?
 
-The reason is that backslashes are "consumed" by a string. Remember, regular strings have their own special characters like `\n`, and a backslash is used for escaping.
+The reason is that backslashes are "consumed" by a string. As we may recall, regular strings have their own special characters, such as `\n`, and a backslash is used for escaping.
 
-Please, take a look, what "\d\.\d" really is:
+Here's how "\d\.\d" is preceived:
 
 ```js run
 alert("\d\.\d"); // d.d
 ```
 
-The quotes "consume" backslashes and interpret them, for instance:
+String quotes "consume" backslashes and interpret them on their own, for instance:
 
 - `\n` -- becomes a newline character,
 - `\u1234` -- becomes the Unicode character with such code,
-- ...And when there's no special meaning: like `\d` or `\z`, then the backslash is simply removed.
+- ...And when there's no special meaning: like `pattern:\d` or `\z`, then the backslash is simply removed.
 
-So the call to `new RegExp` gets a string without backslashes. That's why the search doesn't work!
+So `new RegExp` gets a string without backslashes. That's why the search doesn't work!
 
-To fix it, we need to double backslashes, because quotes turn `\\` into `\`:
+To fix it, we need to double backslashes, because string quotes turn `\\` into `\`:
 
 ```js run
 *!*
@@ -87,13 +87,13 @@ let regStr = "\\d\\.\\d";
 */!*
 alert(regStr); // \d\.\d (correct now)
 
-let reg = new RegExp(regStr);
+let regexp = new RegExp(regStr);
 
-alert( "Chapter 5.1".match(reg) ); // 5.1
+alert( "Chapter 5.1".match(regexp) ); // 5.1
 ```
 
 ## Summary
 
-- To search special characters `pattern:[ \ ^ $ . | ? * + ( )` literally, we need to prepend them with `\` ("escape them").
+- To search for special characters `pattern:[ \ ^ $ . | ? * + ( )` literally, we need to prepend them with a backslash `\` ("escape them").
 - We also need to escape `/` if we're inside `pattern:/.../` (but not inside `new RegExp`).
-- When passing a string `new RegExp`, we need to double backslashes `\\`, cause strings consume one of them.
+- When passing a string `new RegExp`, we need to double backslashes `\\`, cause string quotes consume one of them.

9-regular-expressions/08-regexp-character-sets-and-ranges/2-find-time-2-formats/solution.md

@@ -1,8 +1,8 @@
 Answer: `pattern:\d\d[-:]\d\d`.
 
 ```js run
-let reg = /\d\d[-:]\d\d/g;
+let regexp = /\d\d[-:]\d\d/g;
-alert( "Breakfast at 09:00. Dinner at 21-30".match(reg) ); // 09:00, 21-30
+alert( "Breakfast at 09:00. Dinner at 21-30".match(regexp) ); // 09:00, 21-30
 ```
 
 Please note that the dash `pattern:'-'` has a special meaning in square brackets, but only between other characters, not when it's in the beginning or at the end, so we don't need to escape it.

9-regular-expressions/08-regexp-character-sets-and-ranges/2-find-time-2-formats/task.md

@@ -5,8 +5,8 @@ The time can be in the format `hours:minutes` or `hours-minutes`. Both hours and
 Write a regexp to find time:
 
 ```js
-let reg = /your regexp/g;
+let regexp = /your regexp/g;
-alert( "Breakfast at 09:00. Dinner at 21-30".match(reg) ); // 09:00, 21-30
+alert( "Breakfast at 09:00. Dinner at 21-30".match(regexp) ); // 09:00, 21-30
 ```
 
 P.S. In this task we assume that the time is always correct, there's no need to filter out bad strings like "45:67". Later we'll deal with that too.

9-regular-expressions/08-regexp-character-sets-and-ranges/article.md

@@ -0,0 +1,197 @@
+# Sets and ranges [...]
+
+Several characters or character classes inside square brackets `[…]` mean to "search for any character among given".
+
+## Sets
+
+For instance, `pattern:[eao]` means any of the 3 characters: `'a'`, `'e'`, or `'o'`.
+
+That's called a *set*. Sets can be used in a regexp along with regular characters:
+
+```js run
+// find [t or m], and then "op"
+alert( "Mop top".match(/[tm]op/gi) ); // "Mop", "top"
+```
+
+Please note that although there are multiple characters in the set, they correspond to exactly one character in the match.
+
+So the example below gives no matches:
+
+```js run
+// find "V", then [o or i], then "la"
+alert( "Voila".match(/V[oi]la/) ); // null, no matches
+```
+
+The pattern searches for:
+
+- `pattern:V`,
+- then *one* of the letters `pattern:[oi]`,
+- then `pattern:la`.
+
+So there would be a match for `match:Vola` or `match:Vila`.
+
+## Ranges
+
+Square brackets may also contain *character ranges*.
+
+For instance, `pattern:[a-z]` is a character in range from `a` to `z`, and `pattern:[0-5]` is a digit from `0` to `5`.
+
+In the example below we're searching for `"x"` followed by two digits or letters from `A` to `F`:
+
+```js run
+alert( "Exception 0xAF".match(/x[0-9A-F][0-9A-F]/g) ); // xAF
+```
+
+Here `pattern:[0-9A-F]` has two ranges: it searches for a character that is either a digit from `0` to `9` or a letter from `A` to `F`.
+
+If we'd like to look for lowercase letters as well, we can add the range `a-f`: `pattern:[0-9A-Fa-f]`. Or add the flag `pattern:i`.
+
+We can also use character classes inside `[…]`.
+
+For instance, if we'd like to look for a wordly character `pattern:\w` or a hyphen `pattern:-`, then the set is `pattern:[\w-]`.
+
+Combining multiple classes is also possible, e.g. `pattern:[\s\d]` means "a space character or a digit".
+
+```smart header="Character classes are shorthands for certain character sets"
+For instance:
+
+- **\d** -- is the same as `pattern:[0-9]`,
+- **\w** -- is the same as `pattern:[a-zA-Z0-9_]`,
+- **\s** -- is the same as `pattern:[\t\n\v\f\r ]`, plus few other rare unicode space characters.
+```
+
+### Example: multi-language \w
+
+As the character class `pattern:\w` is a shorthand for `pattern:[a-zA-Z0-9_]`, it can't find Chinese hieroglyphs, Cyrillic letters, etc.
+
+We can write a more universal pattern, that looks for wordly characters in any language. That's easy with unicode properties: `pattern:[\p{Alpha}\p{M}\p{Nd}\p{Pc}\p{Join_C}]`.
+
+Let's decipher it. Similar to `pattern:\w`, we're making a set of our own that includes characters with following unicode properties:
+
+- `Alphabetic` (`Alpha`) - for letters,
+- `Mark` (`M`) - for accents,
+- `Decimal_Number` (`Nd`) - for digits,
+- `Connector_Punctuation` (`Pc`) - for the underscore `'_'` and similar characters,
+- `Join_Control` (`Join_C`) - two special codes `200c` and `200d`, used in ligatures, e.g. in Arabic.
+
+An example of use:
+
+```js run
+let regexp = /[\p{Alpha}\p{M}\p{Nd}\p{Pc}\p{Join_C}]/gu;
+
+let str = `Hi 你好 12`;
+
+// finds all letters and digits:
+alert( str.match(regexp) ); // H,i,你,好,1,2
+```
+
+Of course, we can edit this pattern: add unicode properties or remove them. Unicode properties are covered in more details in the article <info:regexp-unicode>.
+
+```warn header="Unicode properties aren't supported in Edge and Firefox"
+Unicode properties `pattern:p{…}` are not yet implemented in Edge and Firefox. If we really need them, we can use library [XRegExp](http://xregexp.com/).
+
+Or just use ranges of characters in a language that interests us, e.g.  `pattern:[а-я]` for Cyrillic letters.
+```
+
+## Excluding ranges
+
+Besides normal ranges, there are "excluding" ranges that look like `pattern:[^…]`.
+
+They are denoted by a caret character `^` at the start and match any character *except the given ones*.
+
+For instance:
+
+- `pattern:[^aeyo]` -- any character except  `'a'`, `'e'`, `'y'` or `'o'`.
+- `pattern:[^0-9]` -- any character except a digit, the same as `pattern:\D`.
+- `pattern:[^\s]` -- any non-space character, same as `\S`.
+
+The example below looks for any characters except letters, digits and spaces:
+
+```js run
+alert( "alice15@gmail.com".match(/[^\d\sA-Z]/gi) ); // @ and .
+```
+
+## Escaping in […]
+
+Usually when we want to find exactly a special character, we need to escape it like `pattern:\.`. And if we need a backslash, then we use `pattern:\\`, and so on.
+
+In square brackets we can use the vast majority of special characters without escaping:
+
+- Symbols `pattern:. + ( )` never need escaping.
+- A hyphen `pattern:-` is not escaped in the beginning or the end (where it does not define a range).
+- A caret `pattern:^` is only escaped in the beginning (where it means exclusion).
+- The closing square bracket `pattern:]` is always escaped (if we need to look for that symbol).
+
+In other words, all special characters are allowed without escaping, except when they mean something for square brackets.
+
+A dot `.` inside square brackets means just a dot. The pattern `pattern:[.,]` would look for one of characters: either a dot or a comma.
+
+In the example below the regexp `pattern:[-().^+]` looks for one of the characters `-().^+`:
+
+```js run
+// No need to escape
+let regexp = /[-().^+]/g;
+
+alert( "1 + 2 - 3".match(regexp) ); // Matches +, -
+```
+
+...But if you decide to escape them "just in case", then there would be no harm:
+
+```js run
+// Escaped everything
+let regexp = /[\-\(\)\.\^\+]/g;
+
+alert( "1 + 2 - 3".match(regexp) ); // also works: +, -
+```
+
+## Ranges and flag "u"
+
+If there are surrogate pairs in the set, flag `pattern:u` is required for them to work correctly.
+
+For instance, let's look for `pattern:[𝒳𝒴]` in the string `subject:𝒳`:
+
+```js run
+alert( '𝒳'.match(/[𝒳𝒴]/) ); // shows a strange character, like [?]
+// (the search was performed incorrectly, half-character returned)
+```
+
+The result is incorrect, because by default regular expressions "don't know" about surrogate pairs.
+
+The regular expression engine thinks that `[𝒳𝒴]` -- are not two, but four characters:
+1. left half of `𝒳` `(1)`,
+2. right half of `𝒳` `(2)`,
+3. left half of `𝒴` `(3)`,
+4. right half of `𝒴` `(4)`.
+
+We can see their codes like this:
+
+```js run
+for(let i=0; i<'𝒳𝒴'.length; i++) {
+  alert('𝒳𝒴'.charCodeAt(i)); // 55349, 56499, 55349, 56500
+};
+```
+
+So, the example above finds and shows the left half of `𝒳`.
+
+If we add flag `pattern:u`, then the behavior will be correct:
+
+```js run
+alert( '𝒳'.match(/[𝒳𝒴]/u) ); // 𝒳
+```
+
+The similar situation occurs when looking for a range, such as `[𝒳-𝒴]`.
+
+If we forget to add flag `pattern:u`, there will be an error:
+
+```js run
+'𝒳'.match(/[𝒳-𝒴]/); // Error: Invalid regular expression
+```
+
+The reason is that without flag `pattern:u` surrogate pairs are perceived as two characters, so `[𝒳-𝒴]` is interpreted as `[<55349><56499>-<55349><56500>]` (every surrogate pair is replaced with its codes). Now it's easy to see that the range `56499-55349` is invalid: its starting code `56499` is greater than the end `55349`. That's the formal reason for the error.
+
+With the flag `pattern:u` the pattern works correctly:
+
+```js run
+// look for characters from 𝒳 to 𝒵
+alert( '𝒴'.match(/[𝒳-𝒵]/u) ); // 𝒴
+```

9-regular-expressions/08-regexp-greedy-and-lazy/3-find-html-comments/solution.md

@@ -1,17 +0,0 @@
-We need to find the beginning of the comment `match:<!--`, then everything till the end of `match:-->`.
-
-The first idea could be `pattern:<!--.*?-->` -- the lazy quantifier makes the dot stop right before  `match:-->`.
-
-But a dot in JavaScript means "any symbol except the newline". So multiline comments won't be found.
-
-We can use `pattern:[\s\S]` instead of the dot to match "anything":
-
-```js run
-let reg = /<!--[\s\S]*?-->/g;
-
-let str = `... <!-- My -- comment
- test --> ..  <!----> ..
-`;
-
-alert( str.match(reg) ); // '<!-- My -- comment \n test -->', '<!---->'
-```

9-regular-expressions/09-regexp-groups/1-find-webcolor-3-or-6/solution.md

@@ -1,29 +0,0 @@
-A regexp to search 3-digit color `#abc`: `pattern:/#[a-f0-9]{3}/i`.
-
-We can add exactly 3 more optional hex digits. We don't need more or less. Either we have them or we don't.
-
-The simplest way to add them -- is to append to the regexp: `pattern:/#[a-f0-9]{3}([a-f0-9]{3})?/i`
-
-We can do it in a smarter way though: `pattern:/#([a-f0-9]{3}){1,2}/i`.
-
-Here the regexp `pattern:[a-f0-9]{3}` is in parentheses to apply the quantifier `pattern:{1,2}` to it as a whole.
-
-In action:
-
-```js run
-let reg = /#([a-f0-9]{3}){1,2}/gi;
-
-let str = "color: #3f3; background-color: #AA00ef; and: #abcd";
-
-alert( str.match(reg) ); // #3f3 #AA00ef #abc
-```
-
-There's a minor problem here: the pattern found `match:#abc` in `subject:#abcd`. To prevent that we can add `pattern:\b` to the end:
-
-```js run
-let reg = /#([a-f0-9]{3}){1,2}\b/gi;
-
-let str = "color: #3f3; background-color: #AA00ef; and: #abcd";
-
-alert( str.match(reg) ); // #3f3 #AA00ef
-```

9-regular-expressions/09-regexp-groups/article.md

@@ -1,242 +0,0 @@
-# Capturing groups
-
-A part of a pattern can be enclosed in parentheses `pattern:(...)`. This is called a "capturing group".
-
-That has two effects:
-
-1. It allows to place a part of the match into a separate array.
-2. If we put a quantifier after the parentheses, it applies to the parentheses as a whole, not the last character.
-
-## Example
-
-In the example below the pattern `pattern:(go)+` finds one or more `match:'go'`:
-
-```js run
-alert( 'Gogogo now!'.match(/(go)+/i) ); // "Gogogo"
-```
-
-Without parentheses, the pattern `pattern:/go+/` means `subject:g`, followed by `subject:o` repeated one or more times. For instance, `match:goooo` or `match:gooooooooo`.
-
-Parentheses group the word `pattern:(go)` together.
-
-Let's make something more complex -- a regexp to match an email.
-
-Examples of emails:
-
-```
-my@mail.com
-john.smith@site.com.uk
-```
-
-The pattern: `pattern:[-.\w]+@([\w-]+\.)+[\w-]{2,20}`.
-
-1. The first part `pattern:[-.\w]+` (before `@`) may include any alphanumeric word characters, a dot and a dash, to match `match:john.smith`.
-2. Then `pattern:@`, and the domain. It may be a subdomain like `host.site.com.uk`, so we match it as "a word followed by a dot `pattern:([\w-]+\.)` (repeated), and then the last part must be a word: `match:com` or `match:uk` (but not very long: 2-20 characters).
-
-That regexp is not perfect, but good enough to fix errors or occasional mistypes.
-
-For instance, we can find all emails in the string:
-
-```js run
-let reg = /[-.\w]+@([\w-]+\.)+[\w-]{2,20}/g;
-
-alert("my@mail.com @ his@site.com.uk".match(reg)); // my@mail.com, his@site.com.uk
-```
-
-In this example parentheses were used to make a group for repetitions `pattern:([\w-]+\.)+`. But there are other uses too, let's see them.
-
-## Contents of parentheses  
-
-Parentheses are numbered from left to right. The search engine remembers the content matched by each of them and allows to reference it in the pattern or in the replacement string.
-
-For instance, we'd like to find HTML tags `pattern:<.*?>`, and process them.
-
-Let's wrap the inner content into parentheses, like this: `pattern:<(.*?)>`.
-
-Then we'll get both the tag as a whole and its content:
-
-```js run
-let str = '<h1>Hello, world!</h1>';
-let reg = /<(.*?)>/;
-
-alert( str.match(reg) ); // Array: ["<h1>", "h1"]
-```
-
-The call to [String#match](mdn:js/String/match) returns groups only if the regexp only looks for the first match, that is: has no `pattern:/.../g` flag.
-
-If we need all matches with their groups then we can use `.matchAll` or `regexp.exec` as described in <info:regexp-methods>:
-
-```js run
-let str = '<h1>Hello, world!</h1>';
-
-// two matches: opening <h1> and closing </h1> tags
-let reg = /<(.*?)>/g;
-
-let matches = Array.from( str.matchAll(reg) );
-
-alert(matches[0]); //  Array: ["<h1>", "h1"]
-alert(matches[1]); //  Array: ["</h1>", "/h1"]
-```
-
-Here we have two matches for `pattern:<(.*?)>`, each of them is an array with the full match and groups.
-
-## Nested groups
-
-Parentheses can be nested. In this case the numbering also goes from left to right.
-
-For instance, when searching a tag in `subject:<span class="my">` we may be interested in:
-
-1. The tag content as a whole: `match:span class="my"`.
-2. The tag name: `match:span`.
-3. The tag attributes: `match:class="my"`.
-
-Let's add parentheses for them:
-
-```js run
-let str = '<span class="my">';
-
-let reg = /<(([a-z]+)\s*([^>]*))>/;
-
-let result = str.match(reg);
-alert(result); // <span class="my">, span class="my", span, class="my"
-```
-
-Here's how groups look:
-
-![](regexp-nested-groups.svg)
-
-At the zero index of the `result` is always the full match.
-
-Then groups, numbered from left to right. Whichever opens first gives the first group `result[1]`. Here it encloses the whole tag content.
-
-Then in `result[2]` goes the group from the second opening `pattern:(` till the corresponding `pattern:)` -- tag name, then we don't group spaces, but group attributes for `result[3]`.
-
-**Even if a group is optional and doesn't exist in the match, the corresponding `result` array item is present (and equals `undefined`).**
-
-For instance, let's consider the regexp `pattern:a(z)?(c)?`. It looks for `"a"` optionally followed by `"z"` optionally followed by `"c"`.
-
-If we run it on the string with a single letter `subject:a`, then the result is:
-
-```js run
-let match = 'a'.match(/a(z)?(c)?/);
-
-alert( match.length ); // 3
-alert( match[0] ); // a (whole match)
-alert( match[1] ); // undefined
-alert( match[2] ); // undefined
-```
-
-The array has the length of `3`, but all groups are empty.
-
-And here's a more complex match for the string `subject:ack`:
-
-```js run
-let match = 'ack'.match(/a(z)?(c)?/)
-
-alert( match.length ); // 3
-alert( match[0] ); // ac (whole match)
-alert( match[1] ); // undefined, because there's nothing for (z)?
-alert( match[2] ); // c
-```
-
-The array length is permanent: `3`. But there's nothing for the group `pattern:(z)?`, so the result is `["ac", undefined, "c"]`.
-
-## Named groups
-
-Remembering groups by their numbers is hard. For simple patterns it's doable, but for more complex ones we can give names to parentheses.
-
-That's done by putting `pattern:?<name>` immediately after the opening paren, like this:
-
-```js run
-*!*
-let dateRegexp = /(?<year>[0-9]{4})-(?<month>[0-9]{2})-(?<day>[0-9]{2})/;
-*/!*
-let str = "2019-04-30";
-
-let groups = str.match(dateRegexp).groups;
-
-alert(groups.year); // 2019
-alert(groups.month); // 04
-alert(groups.day); // 30
-```
-
-As you can see, the groups reside in the `.groups` property of the match.
-
-We can also use them in the replacement string, as `pattern:$<name>` (like `$1..9`, but a name instead of a digit).
-
-For instance, let's reformat the date into `day.month.year`:
-
-```js run
-let dateRegexp = /(?<year>[0-9]{4})-(?<month>[0-9]{2})-(?<day>[0-9]{2})/;
-
-let str = "2019-04-30";
-
-let rearranged = str.replace(dateRegexp, '$<day>.$<month>.$<year>');
-
-alert(rearranged); // 30.04.2019
-```
-
-If we use a function for the replacement, then named `groups` object is always the last argument:
-
-```js run
-let dateRegexp = /(?<year>[0-9]{4})-(?<month>[0-9]{2})-(?<day>[0-9]{2})/;
-
-let str = "2019-04-30";
-
-let rearranged = str.replace(dateRegexp,
-  (str, year, month, day, offset, input, groups) =>
-   `${groups.day}.${groups.month}.${groups.year}`
-);
-
-alert(rearranged); // 30.04.2019
-```
-
-Usually, when we intend to use named groups, we don't need positional arguments of the function. For the majority of real-life cases we only need `str` and `groups`.
-
-So we can write it a little bit shorter:
-
-```js
-let rearranged = str.replace(dateRegexp, (str, ...args) => {
-  let {year, month, day} = args.pop();
-  alert(str); // 2019-04-30
-  alert(year); // 2019
-  alert(month); // 04
-  alert(day); // 30
-});
-```
-
-
-## Non-capturing groups with ?:
-
-Sometimes we need parentheses to correctly apply a quantifier, but we don't want their contents in results.
-
-A group may be excluded by adding `pattern:?:` in the beginning.
-
-For instance, if we want to find `pattern:(go)+`, but don't want to remember the contents (`go`) in a separate array item, we can write: `pattern:(?:go)+`.
-
-In the example below we only get the name "John" as a separate member of the `results` array:
-
-```js run
-let str = "Gogo John!";
-*!*
-// exclude Gogo from capturing
-let reg = /(?:go)+ (\w+)/i;
-*/!*
-
-let result = str.match(reg);
-
-alert( result.length ); // 2
-alert( result[1] ); // John
-```
-
-## Summary
-
-Parentheses group together a part of the regular expression, so that the quantifier applies to it as a whole.
-
-Parentheses groups are numbered left-to-right, and can optionally be named with  `(?<name>...)`.
-
-The content, matched by a group, can be referenced both in the replacement string as `$1`, `$2` etc, or by the name `$name` if named.
-
-So, parentheses groups are called "capturing groups", as they "capture" a part of the match. We get that part separately from the result as a member of the array or in `.groups` if it's named.
-
-We can exclude the group from remembering (make in "non-capturing") by putting `?:` at the start: `(?:...)`, that's used if we'd like to apply a quantifier to the whole group, but don't need it in the result.

9-regular-expressions/09-regexp-quantifiers/1-find-text-manydots/solution.md

@@ -2,8 +2,8 @@
 Solution:
 
 ```js run
-let reg = /\.{3,}/g;
+let regexp = /\.{3,}/g;
-alert( "Hello!... How goes?.....".match(reg) ); // ..., .....
+alert( "Hello!... How goes?.....".match(regexp) ); // ..., .....
 ```
 
 Please note that the dot is a special character, so we have to escape it and insert as `\.`.

9-regular-expressions/09-regexp-quantifiers/1-find-text-manydots/task.md

@@ -9,6 +9,6 @@ Create a regexp to find ellipsis: 3 (or more?) dots in a row.
 Check it:
 
 ```js
-let reg = /your regexp/g;
+let regexp = /your regexp/g;
-alert( "Hello!... How goes?.....".match(reg) ); // ..., .....
+alert( "Hello!... How goes?.....".match(regexp) ); // ..., .....
 ```

9-regular-expressions/09-regexp-quantifiers/2-find-html-colors-6hex/solution.md

@@ -1,17 +1,17 @@
 We need to look for `#` followed by 6 hexadecimal characters.
 
-A hexadecimal character can be described as `pattern:[0-9a-fA-F]`. Or if we use the `i` flag, then just  `pattern:[0-9a-f]`.
+A hexadecimal character can be described as `pattern:[0-9a-fA-F]`. Or if we use the `pattern:i` flag, then just  `pattern:[0-9a-f]`.
 
 Then we can look for 6 of them using the quantifier `pattern:{6}`.
 
 As a result, we have the regexp: `pattern:/#[a-f0-9]{6}/gi`.
 
 ```js run
-let reg = /#[a-f0-9]{6}/gi;
+let regexp = /#[a-f0-9]{6}/gi;
 
 let str = "color:#121212; background-color:#AA00ef bad-colors:f#fddee #fd2"
 
-alert( str.match(reg) );  // #121212,#AA00ef
+alert( str.match(regexp) );  // #121212,#AA00ef
 ```
 
 The problem is that it finds the color in longer sequences:

9-regular-expressions/09-regexp-quantifiers/2-find-html-colors-6hex/task.md

@@ -5,11 +5,11 @@ Create a regexp to search HTML-colors written as `#ABCDEF`: first `#` and then 6
 An example of use:
 
 ```js
-let reg = /...your regexp.../
+let regexp = /...your regexp.../
 
 let str = "color:#121212; background-color:#AA00ef bad-colors:f#fddee #fd2 #12345678";
 
-alert( str.match(reg) )  // #121212,#AA00ef
+alert( str.match(regexp) )  // #121212,#AA00ef
 ```
 
 P.S. In this task we do not need other color formats like `#123` or `rgb(1,2,3)` etc.

9-regular-expressions/09-regexp-quantifiers/article.md

@@ -2,7 +2,7 @@
 
 Let's say we have a string like `+7(903)-123-45-67` and want to find all numbers in it. But unlike before, we are interested not in single digits, but full numbers: `7, 903, 123, 45, 67`.
 
-A number is a sequence of 1 or more digits `\d`. To mark how many we need, we need to append a *quantifier*.
+A number is a sequence of 1 or more digits `pattern:\d`. To mark how many we need, we can append a *quantifier*.
 
 ## Quantity {n}
 
@@ -12,7 +12,7 @@ A quantifier is appended to a character (or a character class, or a `[...]` set
 
 It has a few advanced forms, let's see examples:
 
-The exact count: `{5}`
+The exact count: `pattern:{5}`
 : `pattern:\d{5}` denotes exactly 5 digits, the same as `pattern:\d\d\d\d\d`.
 
     The example below looks for a 5-digit number:
@@ -23,7 +23,7 @@ The exact count: `{5}`
 
     We can add `\b` to exclude longer numbers: `pattern:\b\d{5}\b`.
 
-The range: `{3,5}`, match 3-5 times
+The range: `pattern:{3,5}`, match 3-5 times
 : To find numbers from 3 to 5 digits we can put the limits into curly braces: `pattern:\d{3,5}`
 
     ```js run
@@ -54,8 +54,8 @@ alert(numbers); // 7,903,123,45,67
 
 There are shorthands for most used quantifiers:
 
-`+`
+`pattern:+`
-: Means "one or more", the same as `{1,}`.
+: Means "one or more", the same as `pattern:{1,}`.
 
     For instance, `pattern:\d+` looks for numbers:
 
@@ -65,8 +65,8 @@ There are shorthands for most used quantifiers:
     alert( str.match(/\d+/g) ); // 7,903,123,45,67
     ```
 
-`?`
+`pattern:?`
-: Means "zero or one", the same as `{0,1}`. In other words, it makes the symbol optional.
+: Means "zero or one", the same as `pattern:{0,1}`. In other words, it makes the symbol optional.
 
     For instance, the pattern `pattern:ou?r` looks for `match:o` followed by zero or one `match:u`, and then `match:r`.
 
@@ -78,16 +78,16 @@ There are shorthands for most used quantifiers:
     alert( str.match(/colou?r/g) ); // color, colour
     ```
 
-`*`
+`pattern:*`
-: Means "zero or more", the same as `{0,}`. That is, the character may repeat any times or be absent.
+: Means "zero or more", the same as `pattern:{0,}`. That is, the character may repeat any times or be absent.
 
-    For example, `pattern:\d0*` looks for a digit followed by any number of zeroes:
+    For example, `pattern:\d0*` looks for a digit followed by any number of zeroes (may be many or none):
 
     ```js run
     alert( "100 10 1".match(/\d0*/g) ); // 100, 10, 1
     ```
 
-    Compare it with `'+'` (one or more):
+    Compare it with `pattern:+` (one or more):
 
     ```js run
     alert( "100 10 1".match(/\d0+/g) ); // 100, 10
@@ -98,43 +98,45 @@ There are shorthands for most used quantifiers:
 
 Quantifiers are used very often. They serve as the main "building block" of complex regular expressions, so let's see more examples.
 
-Regexp "decimal fraction" (a number with a floating point): `pattern:\d+\.\d+`
+**Regexp for decimal fractions (a number with a floating point): `pattern:\d+\.\d+`**
-: In action:
-    ```js run
-    alert( "0 1 12.345 7890".match(/\d+\.\d+/g) ); // 12.345
-    ```
 
-Regexp "open HTML-tag without attributes", like `<span>` or `<p>`: `pattern:/<[a-z]+>/i`
+In action:
-: In action:
+```js run
+alert( "0 1 12.345 7890".match(/\d+\.\d+/g) ); // 12.345
+```
+
+**Regexp for an "opening HTML-tag without attributes", such as `<span>` or `<p>`.**
+
+1. The simplest one: `pattern:/<[a-z]+>/i`
 
     ```js run
     alert( "<body> ... </body>".match(/<[a-z]+>/gi) ); // <body>
     ```
 
-    We look for character `pattern:'<'` followed by one or more Latin letters, and then  `pattern:'>'`.
+    The regexp looks for character `pattern:'<'` followed by one or more Latin letters, and then  `pattern:'>'`.
 
-Regexp "open HTML-tag without attributes" (improved): `pattern:/<[a-z][a-z0-9]*>/i`
+2. Improved: `pattern:/<[a-z][a-z0-9]*>/i`
-: Better regexp: according to the standard, HTML tag name may have a digit at any position except the first one, like `<h1>`.
+
+    According to the standard, HTML tag name may have a digit at any position except the first one, like `<h1>`.
 
     ```js run
     alert( "<h1>Hi!</h1>".match(/<[a-z][a-z0-9]*>/gi) ); // <h1>
     ```
 
-Regexp "opening or closing HTML-tag without attributes": `pattern:/<\/?[a-z][a-z0-9]*>/i`
+**Regexp "opening or closing HTML-tag without attributes": `pattern:/<\/?[a-z][a-z0-9]*>/i`**
-: We added an optional slash `pattern:/?` before the tag. Had to escape it with a backslash, otherwise JavaScript would think it is the pattern end.
 
-    ```js run
+We added an optional slash `pattern:/?` near the beginning of the pattern. Had to escape it with a backslash, otherwise JavaScript would think it is the pattern end.
-    alert( "<h1>Hi!</h1>".match(/<\/?[a-z][a-z0-9]*>/gi) ); // <h1>, </h1>
+
-    ```
+```js run
+alert( "<h1>Hi!</h1>".match(/<\/?[a-z][a-z0-9]*>/gi) ); // <h1>, </h1>
+```
 
 ```smart header="To make a regexp more precise, we often need make it more complex"
 We can see one common rule in these examples: the more precise is the regular expression -- the longer and more complex it is.
 
-For instance, for HTML tags we could use a simpler regexp: `pattern:<\w+>`.
+For instance, for HTML tags we could use a simpler regexp: `pattern:<\w+>`. But as HTML has stricter restrictions for a tag name, `pattern:<[a-z][a-z0-9]*>` is more reliable.
 
-...But because `pattern:\w` means any Latin letter or a digit or `'_'`, the regexp also matches non-tags, for instance `match:<_>`. So it's much simpler than `pattern:<[a-z][a-z0-9]*>`, but less reliable.
+Can we use `pattern:<\w+>` or we need `pattern:<[a-z][a-z0-9]*>`?
 
-Are we ok with `pattern:<\w+>` or we need `pattern:<[a-z][a-z0-9]*>`?
+In real life both variants are acceptable. Depends on how tolerant we can be to "extra" matches and whether it's difficult or not to remove them from the result by other means.
-
-In real life both variants are acceptable. Depends on how tolerant we can be to "extra" matches and whether it's difficult or not to filter them out by other means.
 ```

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

@@ -1,65 +0,0 @@
-# Backreferences in pattern: \n and \k
-
-We can use the contents of capturing groups `(...)` 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 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!"`:
-
-```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 wrap it into a capturing group 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!"
-```

9-regular-expressions/10-regexp-greedy-and-lazy/3-find-html-comments/solution.md

@@ -0,0 +1,15 @@
+We need to find the beginning of the comment `match:<!--`, then everything till the end of `match:-->`.
+
+An acceptable variant is `pattern:<!--.*?-->` -- the lazy quantifier makes the dot stop right before `match:-->`. We also need to add flag `pattern:s` for the dot to include newlines.
+
+Otherwise multiline comments won't be found:
+
+```js run
+let regexp = /<!--.*?-->/gs;
+
+let str = `... <!-- My -- comment
+ test --> ..  <!----> ..
+`;
+
+alert( str.match(regexp) ); // '<!-- My -- comment \n test -->', '<!---->'
+```

9-regular-expressions/10-regexp-greedy-and-lazy/3-find-html-comments/task.md

@@ -3,11 +3,11 @@
 Find all HTML comments in the text:
 
 ```js
-let reg = /your regexp/g;
+let regexp = /your regexp/g;
 
 let str = `... <!-- My -- comment
  test --> ..  <!----> .. 
 `;
 
-alert( str.match(reg) ); // '<!-- My -- comment \n test -->', '<!---->'
+alert( str.match(regexp) ); // '<!-- My -- comment \n test -->', '<!---->'
 ```

9-regular-expressions/10-regexp-greedy-and-lazy/4-find-html-tags-greedy-lazy/solution.md

@@ -2,9 +2,9 @@
 The solution is `pattern:<[^<>]+>`.
 
 ```js run
-let reg = /<[^<>]+>/g;
+let regexp = /<[^<>]+>/g;
 
 let str = '<> <a href="/"> <input type="radio" checked> <b>';
 
-alert( str.match(reg) ); // '<a href="/">', '<input type="radio" checked>', '<b>'
+alert( str.match(regexp) ); // '<a href="/">', '<input type="radio" checked>', '<b>'
 ```

9-regular-expressions/10-regexp-greedy-and-lazy/4-find-html-tags-greedy-lazy/task.md

@@ -5,11 +5,11 @@ Create a regular expression to find all (opening and closing) HTML tags with the
 An example of use:
 
 ```js run
-let reg = /your regexp/g;
+let regexp = /your regexp/g;
 
 let str = '<> <a href="/"> <input type="radio" checked> <b>';
 
-alert( str.match(reg) ); // '<a href="/">', '<input type="radio" checked>', '<b>'
+alert( str.match(regexp) ); // '<a href="/">', '<input type="radio" checked>', '<b>'
 ```
 
 Here we assume that tag attributes may not contain `<` and `>` (inside squotes too), that simplifies things a bit. 

9-regular-expressions/10-regexp-greedy-and-lazy/article.md

@@ -8,7 +8,7 @@ Let's take the following task as an example.
 
 We have a text and need to replace all quotes `"..."` with guillemet marks: `«...»`. They are preferred for typography in many countries.
 
-For instance: `"Hello, world"` should become `«Hello, world»`. Some countries prefer other quotes, like `„Witam, świat!”` (Polish) or `「你好，世界」` (Chinese), but for our task let's choose `«...»`.
+For instance: `"Hello, world"` should become `«Hello, world»`. There exist other quotes, such as `„Witam, świat!”` (Polish) or `「你好，世界」` (Chinese), but for our task let's choose `«...»`.
 
 The first thing to do is to locate quoted strings, and then we can replace them.
 
@@ -17,11 +17,11 @@ A regular expression like `pattern:/".+"/g` (a quote, then something, then the o
 Let's try it:
 
 ```js run
-let reg = /".+"/g;
+let regexp = /".+"/g;
 
 let str = 'a "witch" and her "broom" is one';
 
-alert( str.match(reg) ); // "witch" and her "broom"
+alert( str.match(regexp) ); // "witch" and her "broom"
 ```
 
 ...We can see that it works not as intended!
@@ -35,7 +35,7 @@ That can be described as "greediness is the cause of all evil".
 To find a match, the regular expression engine uses the following algorithm:
 
 - For every position in the string
-    - Match the pattern at that position.
+    - Try to match the pattern at that position.
     - If there's no match, go to the next position.
 
 These common words do not make it obvious why the regexp fails, so let's elaborate how the search works for the pattern `pattern:".+"`.
@@ -44,7 +44,7 @@ These common words do not make it obvious why the regexp fails, so let's elabora
 
     The regular expression engine tries to find it at the zero position of the source string `subject:a "witch" and her "broom" is one`, but there's `subject:a` there, so there's immediately no match.
 
-    Then it advances: goes to the next positions in the source string and tries to find the first character of the pattern there, and finally finds the quote at the 3rd position:
+    Then it advances: goes to the next positions in the source string and tries to find the first character of the pattern there, fails again, and finally finds the quote at the 3rd position:
 
     ![](witch_greedy1.svg)
 
@@ -54,13 +54,13 @@ These common words do not make it obvious why the regexp fails, so let's elabora
 
     ![](witch_greedy2.svg)
 
-3. Then the dot repeats because of the quantifier `pattern:.+`. The regular expression engine builds the match by taking characters one by one while it is possible.
+3. Then the dot repeats because of the quantifier `pattern:.+`. The regular expression engine adds to the match one character after another.
 
-    ...When does it become impossible? All characters match the dot, so it only stops when it reaches the end of the string:
+    ...Until when? All characters match the dot, so it only stops when it reaches the end of the string:
 
     ![](witch_greedy3.svg)
 
-4. Now the engine finished repeating for `pattern:.+` and tries to find the next character of the pattern. It's the quote `pattern:"`. But there's a problem: the string has finished, there are no more characters!
+4. Now the engine finished repeating `pattern:.+` and tries to find the next character of the pattern. It's the quote `pattern:"`. But there's a problem: the string has finished, there are no more characters!
 
     The regular expression engine understands that it took too many `pattern:.+` and starts to *backtrack*.
 
@@ -68,9 +68,9 @@ These common words do not make it obvious why the regexp fails, so let's elabora
 
     ![](witch_greedy4.svg)
 
-    Now it assumes that `pattern:.+` ends one character before the end and tries to match the rest of the pattern from that position.
+    Now it assumes that `pattern:.+` ends one character before the string end and tries to match the rest of the pattern from that position.
 
-    If there were a quote there, then that would be the end, but the last character is `subject:'e'`, so there's no match.
+    If there were a quote there, then the search would end, but the last character is `subject:'e'`, so there's no match.
 
 5. ...So the engine decreases the number of repetitions of `pattern:.+` by one more character:
 
@@ -84,19 +84,19 @@ These common words do not make it obvious why the regexp fails, so let's elabora
 
 7. The match is complete.
 
-8. So the first match is `match:"witch" and her "broom"`. The further search starts where the first match ends, but there are no more quotes in the rest of the string `subject:is one`, so no more results.
+8. So the first match is `match:"witch" and her "broom"`. If the regular expression has flag `pattern:g`, then the search will continue from where the first match ends. There are no more quotes in the rest of the string `subject:is one`, so no more results.
 
 That's probably not what we expected, but that's how it works.
 
-**In the greedy mode (by default) the quantifier is repeated as many times as possible.**
+**In the greedy mode (by default) a quantifier is repeated as many times as possible.**
 
-The regexp engine tries to fetch as many characters as it can by `pattern:.+`, and then shortens that one by one.
+The regexp engine adds to the match as many characters as it can for `pattern:.+`, and then shortens that one by one, if the rest of the pattern doesn't match.
 
-For our task we want another thing. That's what the lazy quantifier mode is for.
+For our task we want another thing. That's where a lazy mode can help.
 
 ## Lazy mode
 
-The lazy mode of quantifier is an opposite to the greedy mode. It means: "repeat minimal number of times".
+The lazy mode of quantifiers is an opposite to the greedy mode. It means: "repeat minimal number of times".
 
 We can enable it by putting a question mark `pattern:'?'` after the quantifier, so that it becomes  `pattern:*?` or `pattern:+?` or even `pattern:??` for `pattern:'?'`.
 
@@ -105,11 +105,11 @@ To make things clear: usually a question mark `pattern:?` is a quantifier by its
 The regexp `pattern:/".+?"/g` works as intended: it finds `match:"witch"` and `match:"broom"`:
 
 ```js run
-let reg = /".+?"/g;
+let regexp = /".+?"/g;
 
 let str = 'a "witch" and her "broom" is one';
 
-alert( str.match(reg) ); // witch, broom
+alert( str.match(regexp) ); // witch, broom
 ```
 
 To clearly understand the change, let's trace the search step by step.
@@ -149,20 +149,19 @@ Other quantifiers remain greedy.
 For instance:
 
 ```js run
-alert( "123 456".match(/\d+ \d+?/g) ); // 123 4
+alert( "123 456".match(/\d+ \d+?/) ); // 123 4
 ```
 
-1. The pattern `pattern:\d+` tries to match as many numbers as it can (greedy mode), so it finds  `match:123` and stops, because the next character is a space `pattern:' '`.
+1. The pattern `pattern:\d+` tries to match as many digits as it can (greedy mode), so it finds  `match:123` and stops, because the next character is a space `pattern:' '`.
-2. Then there's a space in pattern, it matches.
+2. Then there's a space in the pattern, it matches.
 3. Then there's `pattern:\d+?`. The quantifier is in lazy mode, so it finds one digit `match:4` and tries to check if the rest of the pattern matches from there.
 
     ...But there's nothing in the pattern after `pattern:\d+?`.
 
     The lazy mode doesn't repeat anything without a need. The pattern finished, so we're done. We have a match `match:123 4`.
-4. The next search starts from the character `5`.
 
 ```smart header="Optimizations"
-Modern regular expression engines can optimize internal algorithms to work faster. So they may work a bit different from the described algorithm.
+Modern regular expression engines can optimize internal algorithms to work faster. So they may work a bit differently from the described algorithm.
 
 But to understand how regular expressions work and to build regular expressions, we don't need to know about that. They are only used internally to optimize things.
 
@@ -176,11 +175,11 @@ With regexps, there's often more than one way to do the same thing.
 In our case we can find quoted strings without lazy mode using the regexp `pattern:"[^"]+"`:
 
 ```js run
-let reg = /"[^"]+"/g;
+let regexp = /"[^"]+"/g;
 
 let str = 'a "witch" and her "broom" is one';
 
-alert( str.match(reg) ); // witch, broom
+alert( str.match(regexp) ); // witch, broom
 ```
 
 The regexp `pattern:"[^"]+"` gives correct results, because it looks for a quote `pattern:'"'` followed by one or more non-quotes `pattern:[^"]`, and then the closing quote.
@@ -202,20 +201,20 @@ The first idea might be: `pattern:/<a href=".*" class="doc">/g`.
 Let's check it:
 ```js run
 let str = '...<a href="link" class="doc">...';
-let reg = /<a href=".*" class="doc">/g;
+let regexp = /<a href=".*" class="doc">/g;
 
 // Works!
-alert( str.match(reg) ); // <a href="link" class="doc">
+alert( str.match(regexp) ); // <a href="link" class="doc">
 ```
 
 It worked. But let's see what happens if there are many links in the text?
 
 ```js run
 let str = '...<a href="link1" class="doc">... <a href="link2" class="doc">...';
-let reg = /<a href=".*" class="doc">/g;
+let regexp = /<a href=".*" class="doc">/g;
 
 // Whoops! Two links in one match!
-alert( str.match(reg) ); // <a href="link1" class="doc">... <a href="link2" class="doc">
+alert( str.match(regexp) ); // <a href="link1" class="doc">... <a href="link2" class="doc">
 ```
 
 Now the result is wrong for the same reason as our "witches" example. The quantifier `pattern:.*` took too many characters.
@@ -231,10 +230,10 @@ Let's modify the pattern by making the quantifier `pattern:.*?` lazy:
 
 ```js run
 let str = '...<a href="link1" class="doc">... <a href="link2" class="doc">...';
-let reg = /<a href=".*?" class="doc">/g;
+let regexp = /<a href=".*?" class="doc">/g;
 
 // Works!
-alert( str.match(reg) ); // <a href="link1" class="doc">, <a href="link2" class="doc">
+alert( str.match(regexp) ); // <a href="link1" class="doc">, <a href="link2" class="doc">
 ```
 
 Now it seems to work, there are two matches:
@@ -248,10 +247,10 @@ Now it seems to work, there are two matches:
 
 ```js run
 let str = '...<a href="link1" class="wrong">... <p style="" class="doc">...';
-let reg = /<a href=".*?" class="doc">/g;
+let regexp = /<a href=".*?" class="doc">/g;
 
 // Wrong match!
-alert( str.match(reg) ); // <a href="link1" class="wrong">... <p style="" class="doc">
+alert( str.match(regexp) ); // <a href="link1" class="wrong">... <p style="" class="doc">
 ```
 
 Now it fails. The match includes not just a link, but also a lot of text after it, including `<p...>`.
@@ -264,7 +263,7 @@ That's what's going on:
 2. Then it looks for `pattern:.*?`: takes one character (lazily!), check if there's a match for `pattern:" class="doc">` (none).
 3. Then takes another character into `pattern:.*?`, and so on... until it finally reaches `match:" class="doc">`.
 
-But the problem is: that's already beyond the link, in another tag `<p>`. Not what we want.
+But the problem is: that's already beyond the link `<a...>`, in another tag `<p>`. Not what we want.
 
 Here's the picture of the match aligned with the text:
 
@@ -273,22 +272,20 @@ Here's the picture of the match aligned with the text:
 <a href="link1" class="wrong">... <p style="" class="doc">
 ```
 
-So the laziness did not work for us here.
+So, we need the pattern to look for `<a href="...something..." class="doc">`, but both greedy and lazy variants have problems.
 
-We need the pattern to look for `<a href="...something..." class="doc">`, but both greedy and lazy variants have problems.
+The correct variant can be: `pattern:href="[^"]*"`. It will take all characters inside the `href` attribute till the nearest quote, just what we need.
-
-The correct variant would be: `pattern:href="[^"]*"`. It will take all characters inside the `href` attribute till the nearest quote, just what we need.
 
 A working example:
 
 ```js run
 let str1 = '...<a href="link1" class="wrong">... <p style="" class="doc">...';
 let str2 = '...<a href="link1" class="doc">... <a href="link2" class="doc">...';
-let reg = /<a href="[^"]*" class="doc">/g;
+let regexp = /<a href="[^"]*" class="doc">/g;
 
 // Works!
-alert( str1.match(reg) ); // null, no matches, that's correct
+alert( str1.match(regexp) ); // null, no matches, that's correct
-alert( str2.match(reg) ); // <a href="link1" class="doc">, <a href="link2" class="doc">
+alert( str2.match(regexp) ); // <a href="link1" class="doc">, <a href="link2" class="doc">
 ```
 
 ## Summary
@@ -301,4 +298,4 @@ Greedy
 Lazy
 : Enabled by the question mark `pattern:?` after the quantifier. The regexp engine tries to match the rest of the pattern before each repetition of the quantifier.
 
-As we've seen, the lazy mode is not a "panacea" from the greedy search. An alternative is a "fine-tuned" greedy search, with exclusions. Soon we'll see more examples of it.
+As we've seen, the lazy mode is not a "panacea" from the greedy search. An alternative is a "fine-tuned" greedy search, with exclusions, as in the pattern `pattern:"[^"]+"`.

9-regular-expressions/11-regexp-alternation/article.md

@@ -1,59 +0,0 @@
-# Alternation (OR) |
-
-Alternation is the term in regular expression that is actually a simple "OR".
-
-In a regular expression it is denoted with a vertical line character `pattern:|`.
-
-For instance, we need to find programming languages: HTML, PHP, Java or JavaScript.
-
-The corresponding regexp: `pattern:html|php|java(script)?`.
-
-A usage example:
-
-```js run
-let reg = /html|php|css|java(script)?/gi;
-
-let str = "First HTML appeared, then CSS, then JavaScript";
-
-alert( str.match(reg) ); // 'HTML', 'CSS', 'JavaScript'
-```
-
-We already know a similar thing -- square brackets. They allow to choose between multiple character, for instance `pattern:gr[ae]y` matches `match:gray` or `match:grey`.
-
-Square brackets allow only characters or character sets. Alternation allows any expressions. A regexp `pattern:A|B|C` means one of expressions `A`, `B` or `C`.
-
-For instance:
-
-- `pattern:gr(a|e)y` means exactly the same as `pattern:gr[ae]y`.
-- `pattern:gra|ey` means `match:gra` or `match:ey`.
-
-To separate a part of the pattern for alternation we usually enclose it in parentheses, like this: `pattern:before(XXX|YYY)after`.
-
-## Regexp for time
-
-In previous chapters there was a task to build a regexp for searching time in the form `hh:mm`, for instance `12:00`. But a simple `pattern:\d\d:\d\d` is too vague. It accepts `25:99` as the time (as 99 seconds match the pattern).
-
-How can we make a better one?
-
-We can apply more careful matching. First, the hours:
-
-- If the first digit is `0` or `1`, then the next digit can by anything.
-- Or, if the first digit is `2`, then the next must be `pattern:[0-3]`.
-
-As a regexp: `pattern:[01]\d|2[0-3]`.
-
-Next, the minutes must be from `0` to `59`. In the regexp language that means `pattern:[0-5]\d`: the first digit `0-5`, and then any digit.
-
-Let's glue them together into the pattern: `pattern:[01]\d|2[0-3]:[0-5]\d`.
-
-We're almost done, but there's a problem. The alternation `pattern:|` now happens to be between `pattern:[01]\d` and `pattern:2[0-3]:[0-5]\d`.
-
-That's wrong, as it should be applied only to hours `[01]\d` OR `2[0-3]`. That's a common mistake when starting to work with regular expressions.
-
-The correct variant:
-
-```js run
-let reg = /([01]\d|2[0-3]):[0-5]\d/g;
-
-alert("00:00 10:10 23:59 25:99 1:2".match(reg)); // 00:00,10:10,23:59
-```

9-regular-expressions/11-regexp-groups/01-test-mac/solution.md

@@ -0,0 +1,21 @@
+A two-digit hex number is `pattern:[0-9a-f]{2}` (assuming the flag `pattern:i` is set).
+
+We need that number `NN`, and then `:NN` repeated 5 times (more numbers);
+
+The regexp is: `pattern:[0-9a-f]{2}(:[0-9a-f]{2}){5}`
+
+Now let's show that the match should capture all the text: start at the beginning and end at the end. That's done by wrapping the pattern in `pattern:^...$`.
+
+Finally:
+
+```js run
+let regexp = /^[0-9a-fA-F]{2}(:[0-9a-fA-F]{2}){5}$/i;
+
+alert( regexp.test('01:32:54:67:89:AB') ); // true
+
+alert( regexp.test('0132546789AB') ); // false (no colons)
+
+alert( regexp.test('01:32:54:67:89') ); // false (5 numbers, need 6)
+
+alert( regexp.test('01:32:54:67:89:ZZ') ) // false (ZZ in the end)
+```

9-regular-expressions/11-regexp-groups/01-test-mac/task.md

@@ -8,13 +8,13 @@ Write a regexp that checks whether a string is MAC-address.
 
 Usage:
 ```js
-let reg = /your regexp/;
+let regexp = /your regexp/;
 
-alert( reg.test('01:32:54:67:89:AB') ); // true
+alert( regexp.test('01:32:54:67:89:AB') ); // true
 
-alert( reg.test('0132546789AB') ); // false (no colons)
+alert( regexp.test('0132546789AB') ); // false (no colons)
 
-alert( reg.test('01:32:54:67:89') ); // false (5 numbers, must be 6)
+alert( regexp.test('01:32:54:67:89') ); // false (5 numbers, must be 6)
 
-alert( reg.test('01:32:54:67:89:ZZ') ) // false (ZZ ad the end)
+alert( regexp.test('01:32:54:67:89:ZZ') ) // false (ZZ ad the end)
 ```

9-regular-expressions/11-regexp-groups/02-find-webcolor-3-or-6/solution.md

@@ -0,0 +1,27 @@
+A regexp to search 3-digit color `#abc`: `pattern:/#[a-f0-9]{3}/i`.
+
+We can add exactly 3 more optional hex digits. We don't need more or less. The color has either 3 or 6 digits.
+
+Let's use the quantifier `pattern:{1,2}` for that: we'll have `pattern:/#([a-f0-9]{3}){1,2}/i`.
+
+Here the pattern `pattern:[a-f0-9]{3}` is enclosed in parentheses to apply the quantifier `pattern:{1,2}`.
+
+In action:
+
+```js run
+let regexp = /#([a-f0-9]{3}){1,2}/gi;
+
+let str = "color: #3f3; background-color: #AA00ef; and: #abcd";
+
+alert( str.match(regexp) ); // #3f3 #AA00ef #abc
+```
+
+There's a minor problem here: the pattern found `match:#abc` in `subject:#abcd`. To prevent that we can add `pattern:\b` to the end:
+
+```js run
+let regexp = /#([a-f0-9]{3}){1,2}\b/gi;
+
+let str = "color: #3f3; background-color: #AA00ef; and: #abcd";
+
+alert( str.match(regexp) ); // #3f3 #AA00ef
+```

9-regular-expressions/11-regexp-groups/02-find-webcolor-3-or-6/task.md

@@ -4,11 +4,11 @@ Write a RegExp that matches colors in the format `#abc` or `#abcdef`. That is: `
 
 Usage example:
 ```js
-let reg = /your regexp/g;
+let regexp = /your regexp/g;
 
 let str = "color: #3f3; background-color: #AA00ef; and: #abcd";
 
-alert( str.match(reg) ); // #3f3 #AA00ef
+alert( str.match(regexp) ); // #3f3 #AA00ef
 ```
 
-P.S. This should be exactly 3 or 6 hex digits: values like `#abcd` should not match.
+P.S. This should be exactly 3 or 6 hex digits. Values with 4 digits, such as `#abcd`, should not match.

9-regular-expressions/11-regexp-groups/03-find-decimal-numbers/solution.md

@@ -1,11 +1,11 @@
 A positive number with an optional decimal part is (per previous task): `pattern:\d+(\.\d+)?`.
 
-Let's add an optional `-` in the beginning:
+Let's add the optional `pattern:-` in the beginning:
 
 ```js run
-let reg = /-?\d+(\.\d+)?/g;
+let regexp = /-?\d+(\.\d+)?/g;
 
 let str = "-1.5 0 2 -123.4.";
 
-alert( str.match(reg) );   // -1.5, 0, 2, -123.4
+alert( str.match(regexp) );   // -1.5, 0, 2, -123.4
 ```

9-regular-expressions/11-regexp-groups/03-find-decimal-numbers/task.md

@@ -5,9 +5,9 @@ Write a regexp that looks for all decimal numbers including integer ones, with t
 An example of use:
 
 ```js
-let reg = /your regexp/g;
+let regexp = /your regexp/g;
 
 let str = "-1.5 0 2 -123.4.";
 
-alert( str.match(reg) ); // -1.5, 0, 2, -123.4
+alert( str.match(regexp) ); // -1.5, 0, 2, -123.4
 ```

9-regular-expressions/11-regexp-groups/04-parse-expression/solution.md

@@ -1,23 +1,26 @@
 A regexp for a number is: `pattern:-?\d+(\.\d+)?`. We created it in previous tasks.
 
-An operator is `pattern:[-+*/]`.
+An operator is `pattern:[-+*/]`. The hyphen `pattern:-` goes first in the square brackets, because in the middle it would mean a character range, while we just want a character `-`.
 
-Please note:
+The slash `/` should be escaped inside a JavaScript regexp `pattern:/.../`, we'll do that later.
-- Here the dash `pattern:-` goes first in the brackets, because in the middle it would mean a character range, while we just want a character `-`.
-- A slash `/` should be escaped inside a JavaScript regexp `pattern:/.../`, we'll do that later.
 
 We need a number, an operator, and then another number. And optional spaces between them.
 
 The full regular expression: `pattern:-?\d+(\.\d+)?\s*[-+*/]\s*-?\d+(\.\d+)?`.
 
-To get a result as an array let's put parentheses around the data that we need: numbers and the operator: `pattern:(-?\d+(\.\d+)?)\s*([-+*/])\s*(-?\d+(\.\d+)?)`.
+It has 3 parts, with `pattern:\s*` between them:
+1. `pattern:-?\d+(\.\d+)?` - the first number,
+1. `pattern:[-+*/]` - the operator,
+1. `pattern:-?\d+(\.\d+)?` - the second number.
+
+To make each of these parts a separate element of the result array, let's enclose them in parentheses: `pattern:(-?\d+(\.\d+)?)\s*([-+*/])\s*(-?\d+(\.\d+)?)`.
 
 In action:
 
 ```js run
-let reg = /(-?\d+(\.\d+)?)\s*([-+*\/])\s*(-?\d+(\.\d+)?)/;
+let regexp = /(-?\d+(\.\d+)?)\s*([-+*\/])\s*(-?\d+(\.\d+)?)/;
 
-alert( "1.2 + 12".match(reg) );
+alert( "1.2 + 12".match(regexp) );
 ```
 
 The result includes:
@@ -29,19 +32,19 @@ The result includes:
 - `result[4] == "12"` (forth group `(-?\d+(\.\d+)?)` -- the second number)
 - `result[5] == undefined` (fifth group `(\.\d+)?` -- the last decimal part is absent, so it's undefined)
 
-We only want the numbers and the operator, without the full match or the decimal parts.
+We only want the numbers and the operator, without the full match or the decimal parts, so let's "clean" the result a bit.
 
-The full match (the arrays first item) can be removed by shifting the array `pattern:result.shift()`.
+The full match (the arrays first item) can be removed by shifting the array `result.shift()`.
 
-The decimal groups can be removed by making them into non-capturing groups, by adding `pattern:?:` to the beginning: `pattern:(?:\.\d+)?`.
+Groups that contain decimal parts (number 2 and 4) `pattern:(.\d+)` can be excluded by adding  `pattern:?:` to the beginning: `pattern:(?:\.\d+)?`.
 
 The final solution:
 
 ```js run
 function parse(expr) {
-  let reg = /(-?\d+(?:\.\d+)?)\s*([-+*\/])\s*(-?\d+(?:\.\d+)?)/;
+  let regexp = /(-?\d+(?:\.\d+)?)\s*([-+*\/])\s*(-?\d+(?:\.\d+)?)/;
 
-  let result = expr.match(reg);
+  let result = expr.match(regexp);
 
   if (!result) return [];
   result.shift();

9-regular-expressions/11-regexp-groups/article.md

@@ -0,0 +1,364 @@
+# Capturing groups
+
+A part of a pattern can be enclosed in parentheses `pattern:(...)`. This is called a "capturing group".
+
+That has two effects:
+
+1. It allows to get a part of the match as a separate item in the result array.
+2. If we put a quantifier after the parentheses, it applies to the parentheses as a whole.
+
+## Examples
+
+Let's see how parentheses work in examples.
+
+### Example: gogogo
+
+Without parentheses, the pattern `pattern:go+` means `subject:g` character, followed by `subject:o` repeated one or more times. For instance, `match:goooo` or `match:gooooooooo`.
+
+Parentheses group characters together, so `pattern:(go)+` means `match:go`, `match:gogo`, `match:gogogo` and so on.
+
+```js run
+alert( 'Gogogo now!'.match(/(go)+/i) ); // "Gogogo"
+```
+
+### Example: domain
+
+Let's make something more complex -- a regular expression to search for a website domain.
+
+For example:
+
+```
+mail.com
+users.mail.com
+smith.users.mail.com
+```
+
+As we can see, a domain consists of repeated words, a dot after each one except the last one.
+
+In regular expressions that's `pattern:(\w+\.)+\w+`:
+
+```js run
+let regexp = /(\w+\.)+\w+/g;
+
+alert( "site.com my.site.com".match(regexp) ); // site.com,my.site.com
+```
+
+The search works, but the pattern can't match a domain with a hyphen, e.g. `my-site.com`, because the hyphen does not belong to class `pattern:\w`.
+
+We can fix it by replacing `pattern:\w` with `pattern:[\w-]` in every word except the last one: `pattern:([\w-]+\.)+\w+`.
+
+### Example: email
+
+The previous example can be extended. We can create a regular expression for emails based on it.
+
+The email format is: `name@domain`. Any word can be the name, hyphens and dots are allowed. In regular expressions that's `pattern:[-.\w]+`.
+
+The pattern:
+
+```js run
+let regexp = /[-.\w]+@([\w-]+\.)+[\w-]+/g;
+
+alert("my@mail.com @ his@site.com.uk".match(regexp)); // my@mail.com, his@site.com.uk
+```
+
+That regexp is not perfect, but mostly works and helps to fix accidental mistypes. The only truly reliable check for an email can only be done by sending a letter.
+
+## Parentheses contents in the match
+
+Parentheses are numbered from left to right. The search engine memorizes the content matched by each of them and allows to get it in the result.
+
+The method `str.match(regexp)`, if `regexp` has no flag `g`, looks for the first match and returns it as an array:
+
+1. At index `0`: the full match.
+2. At index `1`: the contents of the first parentheses.
+3. На позиции `2`: the contents of the second parentheses.
+4. ...and so on...
+
+For instance, we'd like to find HTML tags `pattern:<.*?>`, and process them. It would be convenient to have tag content (what's inside the angles), in a separate variable.
+
+Let's wrap the inner content into parentheses, like this: `pattern:<(.*?)>`.
+
+Now we'll get both the tag as a whole `match:<h1>` and its contents `match:h1` in the resulting array:
+
+```js run
+let str = '<h1>Hello, world!</h1>';
+
+let tag = str.match(/<(.*?)>/);
+
+alert( tag[0] ); // <h1>
+alert( tag[1] ); // h1
+```
+
+### Nested groups
+
+Parentheses can be nested. In this case the numbering also goes from left to right.
+
+For instance, when searching a tag in `subject:<span class="my">` we may be interested in:
+
+1. The tag content as a whole: `match:span class="my"`.
+2. The tag name: `match:span`.
+3. The tag attributes: `match:class="my"`.
+
+Let's add parentheses for them: `pattern:<(([a-z]+)\s*([^>]*))>`.
+
+Here's how they are numbered (left to right, by the opening paren):
+
+![](regexp-nested-groups-pattern.svg)
+
+In action:
+
+```js run
+let str = '<span class="my">';
+
+let regexp = /<(([a-z]+)\s*([^>]*))>/;
+
+let result = str.match(regexp);
+alert(result[0]); // <span class="my">
+alert(result[1]); // span class="my"
+alert(result[2]); // span
+alert(result[3]); // class="my"
+```
+
+The zero index of `result` always holds the full match.
+
+Then groups, numbered from left to right by an opening paren. The first group is returned as `result[1]`. Here it encloses the whole tag content.
+
+Then in `result[2]` goes the group from the second opening paren `pattern:([a-z]+)` - tag name, then in `result[3]` the tag: `pattern:([^>]*)`.
+
+The contents of every group in the string:
+
+![](regexp-nested-groups-matches.svg)
+
+### Optional groups
+
+Even if a group is optional and doesn't exist in the match (e.g. has the quantifier `pattern:(...)?`), the corresponding `result` array item is present and equals `undefined`.
+
+For instance, let's consider the regexp `pattern:a(z)?(c)?`. It looks for `"a"` optionally followed by `"z"` optionally followed by `"c"`.
+
+If we run it on the string with a single letter `subject:a`, then the result is:
+
+```js run
+let match = 'a'.match(/a(z)?(c)?/);
+
+alert( match.length ); // 3
+alert( match[0] ); // a (whole match)
+alert( match[1] ); // undefined
+alert( match[2] ); // undefined
+```
+
+The array has the length of `3`, but all groups are empty.
+
+And here's a more complex match for the string `subject:ac`:
+
+```js run
+let match = 'ac'.match(/a(z)?(c)?/)
+
+alert( match.length ); // 3
+alert( match[0] ); // ac (whole match)
+alert( match[1] ); // undefined, because there's nothing for (z)?
+alert( match[2] ); // c
+```
+
+The array length is permanent: `3`. But there's nothing for the group `pattern:(z)?`, so the result is `["ac", undefined, "c"]`.
+
+## Searching for all matches with groups: matchAll
+
+```warn header="`matchAll` is a new method, polyfill may be needed"
+The method `matchAll` is not supported in old browsers.
+
+A polyfill may be required, such as <https://github.com/ljharb/String.prototype.matchAll>.
+```
+
+When we search for all matches (flag `pattern:g`), the `match` method does not return contents for groups.
+
+For example, let's find all tags in a string:
+
+```js run
+let str = '<h1> <h2>';
+
+let tags = str.match(/<(.*?)>/g);
+
+alert( tags ); // <h1>,<h2>
+```
+
+The result is an array of matches, but without details about each of them. But in practice we usually need contents of capturing groups in the result.
+
+To get them, we should search using the method `str.matchAll(regexp)`.
+
+It was added to JavaScript language long after `match`, as its "new and improved version".
+
+Just like `match`, it looks for matches, but there are 3 differences:
+
+1. It returns not an array, but an iterable object.
+2. When the flag `pattern:g` is present, it returns every match as an array with groups.
+3. If there are no matches, it returns not `null`, but an empty iterable object.
+
+For instance:
+
+```js run
+let results = '<h1> <h2>'.matchAll(/<(.*?)>/gi);
+
+// results - is not an array, but an iterable object
+alert(results); // [object RegExp String Iterator]
+
+alert(results[0]); // undefined (*)
+
+results = Array.from(results); // let's turn it into array
+
+alert(results[0]); // <h1>,h1 (1st tag)
+alert(results[1]); // <h2>,h2 (2nd tag)
+```
+
+As we can see, the first difference is very important, as demonstrated in the line `(*)`. We can't get the match as `results[0]`, because that object isn't pseudoarray. We can turn it into a real `Array` using `Array.from`. There are more details about pseudoarrays and iterables in the article <info:iterable>.
+
+There's no need in `Array.from` if we're looping over results:
+
+```js run
+let results = '<h1> <h2>'.matchAll(/<(.*?)>/gi);
+
+for(let result of results) {
+  alert(result);
+  // первый вывод: <h1>,h1
+  // второй: <h2>,h2
+}
+```
+
+...Or using destructuring:
+
+```js
+let [tag1, tag2] = '<h1> <h2>'.matchAll(/<(.*?)>/gi);
+```
+
+Every match, returned by `matchAll`, has the same format as returned by `match` without flag `pattern:g`: it's an array with additional properties `index` (match index in the string) and `input` (source string):
+
+```js run
+let results = '<h1> <h2>'.matchAll(/<(.*?)>/gi);
+
+let [tag1, tag2] = results;
+
+alert( tag1[0] ); // <h1>
+alert( tag1[1] ); // h1
+alert( tag1.index ); // 0
+alert( tag1.input ); // <h1> <h2>
+```
+
+```smart header="Why is a result of `matchAll` an iterable object, not an array?"
+Why is the method designed like that? The reason is simple - for the optimization.
+
+The call to `matchAll` does not perform the search. Instead, it returns an iterable object, without the results initially. The search is performed each time we iterate over it, e.g. in the loop.
+
+So, there will be found as many results as needed, not more.
+
+E.g. there are potentially 100 matches in the text, but in a `for..of` loop we found 5 of them, then decided it's enough and make a `break`. Then the engine won't spend time finding other 95 mathces.
+```
+
+## Named groups
+
+Remembering groups by their numbers is hard. For simple patterns it's doable, but for more complex ones counting parentheses is inconvenient. We have a much better option: give names to parentheses.
+
+That's done by putting `pattern:?<name>` immediately after the opening paren.
+
+For example, let's look for a date in the format "year-month-day":
+
+```js run
+*!*
+let dateRegexp = /(?<year>[0-9]{4})-(?<month>[0-9]{2})-(?<day>[0-9]{2})/;
+*/!*
+let str = "2019-04-30";
+
+let groups = str.match(dateRegexp).groups;
+
+alert(groups.year); // 2019
+alert(groups.month); // 04
+alert(groups.day); // 30
+```
+
+As you can see, the groups reside in the `.groups` property of the match.
+
+To look for all dates, we can add flag `pattern:g`.
+
+We'll also need `matchAll` to obtain full matches, together with groups:
+
+```js run
+let dateRegexp = /(?<year>[0-9]{4})-(?<month>[0-9]{2})-(?<day>[0-9]{2})/g;
+
+let str = "2019-10-30 2020-01-01";
+
+let results = str.matchAll(dateRegexp);
+
+for(let result of results) {
+  let {year, month, day} = result.groups;
+
+  alert(`${day}.${month}.${year}`);
+  // first alert: 30.10.2019
+  // second: 01.01.2020
+}
+```
+
+## Capturing groups in replacement
+
+Method `str.replace(regexp, replacement)` that replaces all matches with `regexp` in `str` allows to use parentheses contents in the `replacement` string. That's done using `pattern:$n`, where `pattern:n` is the group number.
+
+For example,
+
+```js run
+let str = "John Bull";
+let regexp = /(\w+) (\w+)/;
+
+alert( str.replace(regexp, '$2, $1') ); // Bull, John
+```
+
+For named parentheses the reference will be `pattern:$<name>`.
+
+For example, let's reformat dates from "year-month-day" to "day.month.year":
+
+```js run
+let regexp = /(?<year>[0-9]{4})-(?<month>[0-9]{2})-(?<day>[0-9]{2})/g;
+
+let str = "2019-10-30, 2020-01-01";
+
+alert( str.replace(regexp, '$<day>.$<month>.$<year>') );
+// 30.10.2019, 01.01.2020
+```
+
+## Non-capturing groups with ?:
+
+Sometimes we need parentheses to correctly apply a quantifier, but we don't want their contents in results.
+
+A group may be excluded by adding `pattern:?:` in the beginning.
+
+For instance, if we want to find `pattern:(go)+`, but don't want the parentheses contents (`go`) as a separate array item, we can write: `pattern:(?:go)+`.
+
+In the example below we only get the name `match:John` as a separate member of the match:
+
+```js run
+let str = "Gogogo John!";
+
+*!*
+// ?: exludes 'go' from capturing
+let regexp = /(?:go)+ (\w+)/i;
+*/!*
+
+let result = str.match(regexp);
+
+alert( result[0] ); // Gogogo John (full match)
+alert( result[1] ); // John
+alert( result.length ); // 2 (no more items in the array)
+```
+
+## Summary
+
+Parentheses group together a part of the regular expression, so that the quantifier applies to it as a whole.
+
+Parentheses groups are numbered left-to-right, and can optionally be named with  `(?<name>...)`.
+
+The content, matched by a group, can be obtained in the results:
+
+- The method `str.match` returns capturing groups only without flag `pattern:g`.
+- The method `str.matchAll` always returns capturing groups.
+
+If the parentheses have no name, then their contents is available in the match array by its number. Named parentheses are also available in the property `groups`.
+
+We can also use parentheses contents in the replacement string in `str.replace`: by the number `$n` or the name `$<name>`.
+
+A group may be excluded from numbering by adding `pattern:?:` in its start. That's used when we need to apply a quantifier to the whole group, but don't want it as a separate item in the results array. We also can't reference such parentheses in the replacement string.

9-regular-expressions/11-regexp-groups/regexp-nested-groups-pattern.svg

@@ -0,0 +1 @@
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9-regular-expressions/12-regexp-anchors/2-test-mac/solution.md

@@ -1,21 +0,0 @@
-A two-digit hex number is `pattern:[0-9a-f]{2}` (assuming the `pattern:i` flag is enabled).
-
-We need that number `NN`, and then `:NN` repeated 5 times (more numbers);
-
-The regexp is: `pattern:[0-9a-f]{2}(:[0-9a-f]{2}){5}`
-
-Now let's show that the match should capture all the text: start at the beginning and end at the end. That's done by wrapping the pattern in `pattern:^...$`.
-
-Finally:
-
-```js run
-let reg = /^[0-9a-fA-F]{2}(:[0-9a-fA-F]{2}){5}$/i;
-
-alert( reg.test('01:32:54:67:89:AB') ); // true
-
-alert( reg.test('0132546789AB') ); // false (no colons)
-
-alert( reg.test('01:32:54:67:89') ); // false (5 numbers, need 6)
-
-alert( reg.test('01:32:54:67:89:ZZ') ) // false (ZZ in the end)
-```

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

@@ -1,55 +0,0 @@
-# String start ^ and finish $
-
-The caret `pattern:'^'` and dollar `pattern:'$'` characters have special meaning in a regexp. They are called "anchors".
-
-The caret `pattern:^` matches at the beginning of the text, and the dollar `pattern:$` -- in the end.
-
-For instance, let's test if the text starts with `Mary`:
-
-```js run
-let str1 = "Mary had a little lamb, it's fleece was white as snow";
-let str2 = 'Everywhere Mary went, the lamp was sure to go';
-
-alert( /^Mary/.test(str1) ); // true
-alert( /^Mary/.test(str2) ); // false
-```
-
-The pattern `pattern:^Mary` means: "the string start and then Mary".
-
-Now let's test whether the text ends with an email.
-
-To match an email, we can use a regexp `pattern:[-.\w]+@([\w-]+\.)+[\w-]{2,20}`.
-
-To test whether the string ends with the email, let's add `pattern:$` to the pattern:
-
-```js run
-let reg = /[-.\w]+@([\w-]+\.)+[\w-]{2,20}$/g;
-
-let str1 = 'My email is mail@site.com';
-let str2 = 'Everywhere Mary went, the lamp was sure to go';
-
-alert( reg.test(str1) ); // true
-alert( reg.test(str2) ); // false
-```
-
-We can use both anchors together to check whether the string exactly follows the pattern. That's often used for validation.
-
-For instance we want to check that `str` is exactly a color in the form `#` plus 6 hex digits. The pattern for the color is `pattern:#[0-9a-f]{6}`.
-
-To check that the *whole string* exactly matches it, we add `pattern:^...$`:
-
-```js run
-let str = "#abcdef";
-
-alert( /^#[0-9a-f]{6}$/i.test(str) ); // true
-```
-
-The regexp engine looks for the text start, then the color, and then immediately the text end. Just what we need.
-
-```smart header="Anchors have zero length"
-Anchors just like `\b` are tests. They have zero-width.
-
-In other words, they do not match a character, but rather force the regexp engine to check the condition (text start/end).
-```
-
-The behavior of anchors changes if there's a flag `pattern:m` (multiline mode). We'll explore it in the next chapter.

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

@@ -0,0 +1,72 @@
+# Backreferences in pattern: \N and \k<name>
+
+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
+
+A group can be referenced in the pattern using `pattern:\N`, where `N` is the group number.
+
+To make clear why that's helpful, let's consider a task.
+
+We need to find quoted strings: either single-quoted `subject:'...'` or a double-quoted `subject:"..."` -- both variants should match.
+
+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 in the string `subject:"She's the one!"`:
+
+```js run
+let str = `He said: "She's the one!".`;
+
+let regexp = /['"](.*?)['"]/g;
+
+// The result is not what we'd like to have
+alert( str.match(regexp) ); // "She'
+```
+
+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 backreference it: `pattern:(['"])(.*?)\1`.
+
+Here's the correct code:
+
+```js run
+let str = `He said: "She's the one!".`;
+
+*!*
+let regexp = /(['"])(.*?)\1/g;
+*/!*
+
+alert( str.match(regexp) ); // "She's the one!"
+```
+
+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.
+
+Similar to that, `pattern:\2` would mean the contents of the second group, `pattern:\3` - the 3rd group, and so on.
+
+```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>`
+
+If a regexp has many parentheses, it's convenient to give them names.
+
+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!".`;
+
+*!*
+let regexp = /(?<quote>['"])(.*?)\k<quote>/g;
+*/!*
+
+alert( str.match(regexp) ); // "She's the one!"
+```

9-regular-expressions/13-regexp-alternation/01-find-programming-language/solution.md

@@ -4,11 +4,11 @@ The first idea can be to list the languages with `|` in-between.
 But that doesn't work right:
 
 ```js run
-let reg = /Java|JavaScript|PHP|C|C\+\+/g;
+let regexp = /Java|JavaScript|PHP|C|C\+\+/g;
 
 let str = "Java, JavaScript, PHP, C, C++";
 
-alert( str.match(reg) ); // Java,Java,PHP,C,C
+alert( str.match(regexp) ); // Java,Java,PHP,C,C
 ```
 
 The regular expression engine looks for alternations one-by-one. That is: first it checks if we have  `match:Java`, otherwise -- looks for `match:JavaScript` and so on.
@@ -25,9 +25,9 @@ There are two solutions for that problem:
 In action:
 
 ```js run
-let reg = /Java(Script)?|C(\+\+)?|PHP/g;
+let regexp = /Java(Script)?|C(\+\+)?|PHP/g;
 
 let str = "Java, JavaScript, PHP, C, C++";
 
-alert( str.match(reg) ); // Java,JavaScript,PHP,C,C++
+alert( str.match(regexp) ); // Java,JavaScript,PHP,C,C++
 ```

9-regular-expressions/13-regexp-alternation/01-find-programming-language/task.md

@@ -5,7 +5,7 @@ There are many programming languages, for instance Java, JavaScript, PHP, C, C++
 Create a regexp that finds them in the string `subject:Java JavaScript PHP C++ C`:
 
 ```js
-let reg = /your regexp/g;
+let regexp = /your regexp/g;
 
-alert("Java JavaScript PHP C++ C".match(reg)); // Java JavaScript PHP C++ C
+alert("Java JavaScript PHP C++ C".match(regexp)); // Java JavaScript PHP C++ C
 ```

9-regular-expressions/13-regexp-alternation/02-find-matching-bbtags/solution.md

@@ -1,14 +1,14 @@
 
 Opening tag is `pattern:\[(b|url|quote)\]`.
 
-Then to find everything till the closing tag -- let's use the pattern `pattern:.*?` with flag `s` to match any character including the newline and then add a backreference to the closing tag.
+Then to find everything till the closing tag -- let's use the pattern `pattern:.*?` with flag `pattern:s` to match any character including the newline and then add a backreference to the closing tag.
 
 The full pattern: `pattern:\[(b|url|quote)\].*?\[/\1\]`.
 
 In action:
 
 ```js run
-let reg = /\[(b|url|quote)\].*?\[\/\1\]/gs;
+let regexp = /\[(b|url|quote)\].*?\[\/\1\]/gs;
 
 let str = `
   [b]hello![/b]
@@ -17,7 +17,7 @@ let str = `
   [/quote]
 `;
 
-alert( str.match(reg) ); // [b]hello![/b],[quote][url]http://google.com[/url][/quote]
+alert( str.match(regexp) ); // [b]hello![/b],[quote][url]http://google.com[/url][/quote]
 ```
 
 Please note that we had to escape a slash for the closing tag `pattern:[/\1]`, because normally the slash closes the pattern.

9-regular-expressions/13-regexp-alternation/02-find-matching-bbtags/task.md

@@ -32,17 +32,17 @@ Create a regexp to find all BB-tags with their contents.
 For instance:
 
 ```js
-let reg = /your regexp/flags;
+let regexp = /your regexp/flags;
 
 let str = "..[url]http://google.com[/url]..";
-alert( str.match(reg) ); // [url]http://google.com[/url]
+alert( str.match(regexp) ); // [url]http://google.com[/url]
 ```
 
 If tags are nested, then we need the outer tag (if we want we can continue the search in its content):
 
 ```js
-let reg = /your regexp/flags;
+let regexp = /your regexp/flags;
 
 let str = "..[url][b]http://google.com[/b][/url]..";
-alert( str.match(reg) ); // [url][b]http://google.com[/b][/url]
+alert( str.match(regexp) ); // [url][b]http://google.com[/b][/url]
 ```

9-regular-expressions/13-regexp-alternation/03-match-quoted-string/solution.md

@@ -10,8 +10,8 @@ Step by step:
 In action:
 
 ```js run
-let reg = /"(\\.|[^"\\])*"/g;
+let regexp = /"(\\.|[^"\\])*"/g;
 let str = ' .. "test me" .. "Say \\"Hello\\"!" .. "\\\\ \\"" .. ';
 
-alert( str.match(reg) ); // "test me","Say \"Hello\"!","\\ \""
+alert( str.match(regexp) ); // "test me","Say \"Hello\"!","\\ \""
 ```

9-regular-expressions/13-regexp-alternation/04-match-exact-tag/solution.md

@@ -10,7 +10,7 @@ In the regexp language: `pattern:<style(>|\s.*?>)`.
 In action:
 
 ```js run
-let reg = /<style(>|\s.*?>)/g;
+let regexp = /<style(>|\s.*?>)/g;
 
-alert( '<style> <styler> <style test="...">'.match(reg) ); // <style>, <style test="...">
+alert( '<style> <styler> <style test="...">'.match(regexp) ); // <style>, <style test="...">
 ```

9-regular-expressions/13-regexp-alternation/04-match-exact-tag/task.md

@@ -7,7 +7,7 @@ Write a regexp to find the tag `<style...>`. It should match the full tag: it ma
 For instance:
 
 ```js
-let reg = /your regexp/g;
+let regexp = /your regexp/g;
 
-alert( '<style> <styler> <style test="...">'.match(reg) ); // <style>, <style test="...">
+alert( '<style> <styler> <style test="...">'.match(regexp) ); // <style>, <style test="...">
 ```

9-regular-expressions/13-regexp-alternation/article.md

@@ -0,0 +1,70 @@
+# Alternation (OR) |
+
+Alternation is the term in regular expression that is actually a simple "OR".
+
+In a regular expression it is denoted with a vertical line character `pattern:|`.
+
+For instance, we need to find programming languages: HTML, PHP, Java or JavaScript.
+
+The corresponding regexp: `pattern:html|php|java(script)?`.
+
+A usage example:
+
+```js run
+let regexp = /html|php|css|java(script)?/gi;
+
+let str = "First HTML appeared, then CSS, then JavaScript";
+
+alert( str.match(regexp) ); // 'HTML', 'CSS', 'JavaScript'
+```
+
+We already saw a similar thing -- square brackets. They allow to choose between multiple characters, for instance `pattern:gr[ae]y` matches `match:gray` or `match:grey`.
+
+Square brackets allow only characters or character sets. Alternation allows any expressions. A regexp `pattern:A|B|C` means one of expressions `A`, `B` or `C`.
+
+For instance:
+
+- `pattern:gr(a|e)y` means exactly the same as `pattern:gr[ae]y`.
+- `pattern:gra|ey` means `match:gra` or `match:ey`.
+
+To apply alternation to a chosen part of the pattern, we can enclose it in parentheses:
+- `pattern:I love HTML|CSS` matches `match:I love HTML` or `match:CSS`.
+- `pattern:I love (HTML|CSS)` matches `match:I love HTML` or `match:I love CSS`.
+
+## Example: regexp for time
+
+In previous articles there was a task to build a regexp for searching time in the form `hh:mm`, for instance `12:00`. But a simple `pattern:\d\d:\d\d` is too vague. It accepts `25:99` as the time (as 99 seconds match the pattern, but that time is invalid).
+
+How can we make a better pattern?
+
+We can use more careful matching. First, the hours:
+
+- If the first digit is `0` or `1`, then the next digit can be any: `pattern:[01]\d`.
+- Otherwise, if the first digit is `2`, then the next must be `pattern:[0-3]`.
+- (no other first digit is allowed)
+
+We can write both variants in a regexp using alternation: `pattern:[01]\d|2[0-3]`.
+
+Next, minutes must be from `00` to `59`. In the regular expression language that can be written as `pattern:[0-5]\d`: the first digit `0-5`, and then any digit.
+
+If we glue minutes and seconds together, we get the pattern: `pattern:[01]\d|2[0-3]:[0-5]\d`.
+
+We're almost done, but there's a problem. The alternation `pattern:|` now happens to be between `pattern:[01]\d` and `pattern:2[0-3]:[0-5]\d`.
+
+That is: minutes are added to the second alternation variant, here's a clear picture:
+
+```
+[01]\d  |  2[0-3]:[0-5]\d
+```
+
+That pattern looks for `pattern:[01]\d` or `pattern:2[0-3]:[0-5]\d`.
+
+But that's wrong, the alternation should only be used in the "hours" part of the regular expression, to allow `pattern:[01]\d` OR `pattern:2[0-3]`. Let's correct that by enclosing "hours" into parentheses: `pattern:([01]\d|2[0-3]):[0-5]\d`.
+
+The final solution:
+
+```js run
+let regexp = /([01]\d|2[0-3]):[0-5]\d/g;
+
+alert("00:00 10:10 23:59 25:99 1:2".match(regexp)); // 00:00,10:10,23:59
+```

9-regular-expressions/13-regexp-multiline-mode/article.md

@@ -1,75 +0,0 @@
-# Multiline mode, flag "m"
-
-The multiline mode is enabled by the flag `pattern:/.../m`.
-
-It only affects the behavior of `pattern:^` and `pattern:$`.
-
-In the multiline mode they match not only at the beginning and end of the string, but also at start/end of line.
-
-## Line start ^
-
-In the example below the text has multiple lines. The pattern `pattern:/^\d+/gm` takes a number from the beginning of each one:
-
-```js run
-let str = `1st place: Winnie
-2nd place: Piglet
-33rd place: Eeyore`;
-
-*!*
-alert( str.match(/^\d+/gm) ); // 1, 2, 33
-*/!*
-```
-
-The regexp engine moves along the text and looks for a line start `pattern:^`, when finds -- continues to match the rest of the pattern `pattern:\d+`.
-
-Without the flag  `pattern:/.../m` only the first number is matched:
-
-```js run
-let str = `1st place: Winnie
-2nd place: Piglet
-33rd place: Eeyore`;
-
-*!*
-alert( str.match(/^\d+/g) ); // 1
-*/!*
-```
-
-That's because by default a caret `pattern:^` only matches at the beginning of the text, and in the multiline mode -- at the start of any line.
-
-## Line end $
-
-The dollar sign `pattern:$` behaves similarly.
-
-The regular expression `pattern:\w+$` finds the last word in every line
-
-```js run
-let str = `1st place: Winnie
-2nd place: Piglet
-33rd place: Eeyore`;
-
-alert( str.match(/\w+$/gim) ); // Winnie,Piglet,Eeyore
-```
-
-Without the `pattern:/.../m` flag the dollar `pattern:$` would only match the end of the whole string, so only the very last word would be found.
-
-## Anchors ^$ versus \n
-
-To find a newline, we can use not only `pattern:^` and `pattern:$`, but also the newline character `\n`.
-
-The first difference is that unlike anchors, the character `\n` "consumes" the newline character and adds it to the result.
-
-For instance, here we use it instead of `pattern:$`:
-
-```js run
-let str = `1st place: Winnie
-2nd place: Piglet
-33rd place: Eeyore`;
-
-alert( str.match(/\w+\n/gim) ); // Winnie\n,Piglet\n
-```
-
-Here every match is a word plus a newline character.
-
-And one more difference -- the newline `\n` does not match at the string end. That's why `Eeyore` is not found in the example above.
-
-So, anchors are usually better, they are closer to what we want to get.

9-regular-expressions/14-regexp-lookahead-lookbehind/1-find-non-negative-integers/solution.md

@@ -6,11 +6,11 @@ We can exclude negatives by prepending it with the negative lookahead: `pattern:
 Although, if we try it now, we may notice one more "extra" result:
 
 ```js run
-let reg = /(?<!-)\d+/g;
+let regexp = /(?<!-)\d+/g;
 
 let str = "0 12 -5 123 -18";
 
-console.log( str.match(reg) ); // 0, 12, 123, *!*8*/!*
+console.log( str.match(regexp) ); // 0, 12, 123, *!*8*/!*
 ```
 
 As you can see, it matches `match:8`, from `subject:-18`. To exclude it, we need to ensure that the regexp starts matching a number not from the middle of another (non-matching) number.
@@ -20,9 +20,9 @@ We can do it by specifying another negative lookbehind: `pattern:(?<!-)(?<!\d)\d
 We can also join them into a single lookbehind here:
 
 ```js run
-let reg = /(?<![-\d])\d+/g;
+let regexp = /(?<![-\d])\d+/g;
 
 let str = "0 12 -5 123 -18";
 
-alert( str.match(reg) ); // 0, 12, 123
+alert( str.match(regexp) ); // 0, 12, 123
 ```

9-regular-expressions/14-regexp-lookahead-lookbehind/1-find-non-negative-integers/task.md

@@ -6,9 +6,9 @@ Create a regexp that looks for only non-negative ones (zero is allowed).
 
 An example of use:
 ```js
-let reg = /your regexp/g;
+let regexp = /your regexp/g;
 
 let str = "0 12 -5 123 -18";
 
-alert( str.match(reg) ); // 0, 12, 123
+alert( str.match(regexp) ); // 0, 12, 123
 ```

9-regular-expressions/14-regexp-lookahead-lookbehind/2-insert-after-head/solution.md

@@ -0,0 +1,29 @@
+
+Для того, чтобы вставить после тега `<body>`, нужно вначале его найти. Будем использовать регулярное выражение `pattern:<body.*>`.
+
+Далее, нам нужно оставить сам тег `<body>` на месте и добавить текст после него.
+
+Это можно сделать вот так:
+```js run
+let str = '...<body style="...">...';
+str = str.replace(/<body.*>/, '$&<h1>Hello</h1>');
+
+alert(str); // ...<body style="..."><h1>Hello</h1>...
+```
+
+В строке замены `$&` означает само совпадение, то есть мы заменяем `pattern:<body.*>` заменяется на самого себя плюс `<h1>Hello</h1>`.
+
+Альтернативный вариант - использовать ретроспективную проверку:
+
+```js run
+let str = '...<body style="...">...';
+str = str.replace(/(?<=<body.*>)/, `<h1>Hello</h1>`);
+
+alert(str); // ...<body style="..."><h1>Hello</h1>...
+```
+
+Такое регулярное выражение на каждой позиции будет проверять, не идёт ли прямо перед ней `pattern:<body.*>`. Если да - совпадение найдено. Но сам тег `pattern:<body.*>` в совпадение не входит, он только участвует в проверке. А других символов после проверки в нём нет, так что текст совпадения будет пустым.
+
+Происходит замена "пустой строки", перед которой идёт `pattern:<body.*>` на `<h1>Hello</h1>`. Что, как раз, и есть вставка этой строки после `<body>`.
+
+P.S. Этому регулярному выражению не помешают флаги: `pattern:/<body.*>/si`, чтобы в "точку" входил перевод строки (тег может занимать несколько строк), а также чтобы теги в другом регистре типа `match:<BODY>` тоже находились.

9-regular-expressions/14-regexp-lookahead-lookbehind/2-insert-after-head/task.md

@@ -0,0 +1,30 @@
+# Вставьте после фрагмента
+
+Есть строка с HTML-документом.
+
+Вставьте после тега `<body>` (у него могут быть атрибуты) строку `<h1>Hello</h1>`.
+
+Например:
+
+```js
+let regexp = /ваше регулярное выражение/;
+
+let str = `
+<html>
+  <body style="height: 200px">
+  ...
+  </body>
+</html>
+`;
+
+str = str.replace(regexp, `<h1>Hello</h1>`);
+```
+
+После этого значение `str`:
+```html
+<html>
+  <body style="height: 200px"><h1>Hello</h1>
+  ...
+  </body>
+</html>
+```

9-regular-expressions/14-regexp-lookahead-lookbehind/article.md

@@ -1,54 +1,82 @@
 # Lookahead and lookbehind
 
-Sometimes we need to match a pattern only if followed by another pattern. For instance, we'd like to get the price from a string like `subject:1 turkey costs 30€`.
+Sometimes we need to find only those matches for a pattern that are followed or preceeded by another pattern.
 
-We need a number (let's say a price has no decimal point) followed by `subject:€` sign.
+There's a special syntax for that, called "lookahead" and "lookbehind", together referred to as "lookaround".
 
-That's what lookahead is for.
+For the start, let's find the price from the string like `subject:1 turkey costs 30€`. That is: a number, followed by `subject:€` sign.
 
 ## Lookahead
 
-The syntax is: `pattern:x(?=y)`, it means "look for `pattern:x`, but match only if followed by `pattern:y`".
+The syntax is: `pattern:X(?=Y)`, it means "look for `pattern:X`, but match only if followed by `pattern:Y`". There may be any pattern instead of `pattern:X` and `pattern:Y`.
 
-For an integer amount followed by `subject:€`, the regexp will be `pattern:\d+(?=€)`:
+For an integer number followed by `subject:€`, the regexp will be `pattern:\d+(?=€)`:
 
 ```js run
 let str = "1 turkey costs 30€";
 
-alert( str.match(/\d+(?=€)/) ); // 30 (correctly skipped the sole number 1)
+alert( str.match(/\d+(?=€)/) ); // 30, the number 1 is ignored, as it's not followed by €
 ```
 
-Let's say we want a quantity instead, that is a number, NOT followed by `subject:€`.
+Please note: the lookahead is merely a test, the contents of the parentheses `pattern:(?=...)` is not included in the result `match:30`.
 
-Here a negative lookahead can be applied.
+When we look for `pattern:X(?=Y)`, the regular expression engine finds `pattern:X` and then checks if there's `pattern:Y` immediately after it. If it's not so, then the potential match is skipped, and the search continues.
 
-The syntax is: `pattern:x(?!y)`, it means "search `pattern:x`, but only if not followed by `pattern:y`".
+More complex tests are possible, e.g. `pattern:X(?=Y)(?=Z)` means:
+
+1. Find `pattern:X`.
+2. Check if `pattern:Y` is immediately after `pattern:X` (skip if isn't).
+3. Check if `pattern:Z` is immediately after `pattern:X` (skip if isn't).
+4. If both tests passed, then it's the match.
+
+In other words, such pattern means that we're looking for `pattern:X` followed by   `pattern:Y` and `pattern:Z` at the same time.
+
+That's only possible if patterns `pattern:Y` and `pattern:Z` aren't mutually exclusive.
+
+For example, `pattern:\d+(?=\s)(?=.*30)` looks for `pattern:\d+` only if it's followed by a space, and there's `30` somewhere after it:
+
+```js run
+let str = "1 turkey costs 30€";
+
+alert( str.match(/\d+(?=\s)(?=.*30)/) ); // 1
+```
+
+In our string that exactly matches the number `1`.
+
+## Negative lookahead
+
+Let's say that we want a quantity instead, not a price from the same string. That's a number `pattern:\d+`, NOT followed by `subject:€`.
+
+For that, a negative lookahead can be applied.
+
+The syntax is: `pattern:X(?!Y)`, it means "search `pattern:X`, but only if not followed by `pattern:Y`".
 
 ```js run
 let str = "2 turkeys cost 60€";
 
-alert( str.match(/\d+(?!€)/) ); // 2 (correctly skipped the price)
+alert( str.match(/\d+(?!€)/) ); // 2 (the price is skipped)
 ```
 
 ## Lookbehind
 
-Lookahead allows to add a condition for "what goes after".
+Lookahead allows to add a condition for "what follows".
 
-Lookbehind is similar, but it looks behind. That is, it allows to match a pattern only if there's something before.
+Lookbehind is similar, but it looks behind. That is, it allows to match a pattern only if there's something before it.
 
 The syntax is:
-- Positive lookbehind: `pattern:(?<=y)x`, matches `pattern:x`, but only if it follows after `pattern:y`.
+- Positive lookbehind: `pattern:(?<=Y)X`, matches `pattern:X`, but only if there's  `pattern:Y` before it.
-- Negative lookbehind: `pattern:(?<!y)x`, matches `pattern:x`, but only if there's no `pattern:y` before.
+- Negative lookbehind: `pattern:(?<!Y)X`, matches `pattern:X`, but only if there's no `pattern:Y` before it.
 
 For example, let's change the price to US dollars. The dollar sign is usually before the number, so to look for `$30` we'll use `pattern:(?<=\$)\d+` -- an amount preceded by `subject:$`:
 
 ```js run
 let str = "1 turkey costs $30";
 
+// the dollar sign is escaped \$
 alert( str.match(/(?<=\$)\d+/) ); // 30 (skipped the sole number)
 ```
 
-And, to find the quantity -- a number, not preceded by `subject:$`, we can use a negative lookbehind `pattern:(?<!\$)\d+`:
+And, if we need the quantity -- a number, not preceded by `subject:$`, then we can use a negative lookbehind `pattern:(?<!\$)\d+`:
 
 ```js run
 let str = "2 turkeys cost $60";
@@ -56,54 +84,47 @@ let str = "2 turkeys cost $60";
 alert( str.match(/(?<!\$)\d+/) ); // 2 (skipped the price)
 ```
 
-## Capture groups
+## Capturing groups
 
-Generally, what's inside the lookaround (a common name for both lookahead and lookbehind) parentheses does not become a part of the match.
+Generally, the contents inside lookaround parentheses does not become a part of the result.
 
 E.g. in the pattern `pattern:\d+(?=€)`, the `pattern:€` sign doesn't get captured as a part of the match. That's natural: we look for a number `pattern:\d+`, while `pattern:(?=€)` is just a test that it should be followed by `subject:€`.
 
-But in some situations we might want to capture the lookaround expression as well, or a part of it. That's possible. Just  wrap that into additional parentheses.
+But in some situations we might want to capture the lookaround expression as well, or a part of it. That's possible. Just wrap that part into additional parentheses.
 
-For instance, here the currency `pattern:(€|kr)` is captured, along with the amount:
+In the example below the currency sign `pattern:(€|kr)` is captured, along with the amount:
 
 ```js run
 let str = "1 turkey costs 30€";
-let reg = /\d+(?=(€|kr))/; // extra parentheses around €|kr
+let regexp = /\d+(?=(€|kr))/; // extra parentheses around €|kr
 
-alert( str.match(reg) ); // 30, €
+alert( str.match(regexp) ); // 30, €
 ```
 
 And here's the same for lookbehind:
 
 ```js run
 let str = "1 turkey costs $30";
-let reg = /(?<=(\$|£))\d+/;
+let regexp = /(?<=(\$|£))\d+/;
 
-alert( str.match(reg) ); // 30, $
+alert( str.match(regexp) ); // 30, $
 ```
 
-Please note that for lookbehind the order stays be same, even though lookahead parentheses are before the main pattern.
-
-Usually parentheses are numbered left-to-right, but lookbehind is an exception, it is always captured after the main pattern. So the match for `pattern:\d+` goes in the result first, and then for `pattern:(\$|£)`.
-
-
 ## Summary
 
-Lookahead and lookbehind (commonly referred to as "lookaround") are useful when we'd like to take something into the match depending on the context before/after it.
+Lookahead and lookbehind (commonly referred to as "lookaround") are useful when we'd like to match something depending on the context before/after it.
 
 For simple regexps we can do the similar thing manually. That is: match everything, in any context, and then filter by context in the loop.
 
-Remember, `str.matchAll` and `reg.exec` return matches with `.index` property, so we know where exactly in the text it is, and can check the context.
+Remember, `str.match` (without flag `pattern:g`) and `str.matchAll` (always) return matches as arrays with `index` property, so we know where exactly in the text it is, and can check the context.
 
-But generally regular expressions are more convenient.
+But generally lookaround is more convenient.
 
 Lookaround types:
 
 | Pattern            | type             | matches |
 |--------------------|------------------|---------|
-| `pattern:x(?=y)`   | Positive lookahead | `x` if followed by `y` |
+| `X(?=Y)`   | Positive lookahead | `pattern:X` if followed by `pattern:Y` |
-| `pattern:x(?!y)`   | Negative lookahead | `x` if not followed by `y` |
+| `X(?!Y)`   | Negative lookahead | `pattern:X` if not followed by `pattern:Y` |
-| `pattern:(?<=y)x` |  Positive lookbehind | `x` if after `y` |
+| `(?<=Y)X` |  Positive lookbehind | `pattern:X` if after `pattern:Y` |
-| `pattern:(?<!y)x` | Negative lookbehind | `x` if not after `y` |
+| `(?<!Y)X` | Negative lookbehind | `pattern:X` if not after `pattern:Y` |
-
-Lookahead can also used to disable backtracking. Why that may be needed and other details  -- see in the next chapter.

9-regular-expressions/15-regexp-catastrophic-backtracking/article.md

@@ -0,0 +1,301 @@
+# Catastrophic backtracking
+
+Some regular expressions are looking simple, but can execute veeeeeery long time, and even "hang" the JavaScript engine.
+
+Sooner or later most developers occasionally face such behavior, because it's quite easy to create such a regexp.
+
+The typical symptom -- a regular expression works fine sometimes, but for certain strings it "hangs", consuming 100% of CPU.
+
+In such case a web-browser suggests to kill the script and reload the page. Not a good thing for sure.
+
+For server-side JavaScript it may become a vulnerability if regular expressions process user data.
+
+## Example
+
+Let's say we have a string, and we'd like to check if it consists of words  `pattern:\w+` with an optional space `pattern:\s?` after each.
+
+We'll use a regexp `pattern:^(\w+\s?)*$`, it specifies 0 or more such words.
+
+In action:
+
+```js run
+let regexp = /^(\w+\s?)*$/;
+
+alert( regexp.test("A good string") ); // true
+alert( regexp.test("Bad characters: $@#") ); // false
+```
+
+It seems to work. The result is correct. Although, on certain strings it takes a lot of time. So long that JavaScript engine "hangs" with 100% CPU consumption.
+
+If you run the example below, you probably won't see anything, as JavaScript will just "hang". A web-browser will stop reacting on events, the UI will stop working. After some time it will suggest to reloaad the page. So be careful with this:
+
+```js run
+let regexp = /^(\w+\s?)*$/;
+let str = "An input string that takes a long time or even makes this regexp to hang!";
+
+// will take a very long time
+alert( regexp.test(str) );
+```
+
+Some regular expression engines can handle such search, but most of them can't.
+
+## Simplified example
+
+What's the matter? Why the regular expression "hangs"?
+
+To understand that, let's simplify the example: remove spaces `pattern:\s?`. Then it becomes `pattern:^(\w+)*$`.
+
+And, to make things more obvious, let's replace `pattern:\w` with `pattern:\d`. The resulting regular expression still hangs, for instance:
+
+<!-- let str = `AnInputStringThatMakesItHang!`; -->
+
+```js run
+let regexp = /^(\d+)*$/;
+
+let str = "012345678901234567890123456789!";
+
+// will take a very long time
+alert( regexp.test(str) );
+```
+
+So what's wrong with the regexp?
+
+First, one may notice that the regexp `pattern:(\d+)*` is a little bit strange. The quantifier `pattern:*` looks extraneous. If we want a number, we can use `pattern:\d+`.
+
+Indeed, the regexp is artificial. But the reason why it is slow is the same as those we saw above. So let's understand it, and then the previous example will become obvious.
+
+What happens during the search of `pattern:^(\d+)*$` in the line `subject:123456789!` (shortened a bit for clarity), why does it take so long?
+
+1. First, the regexp engine tries to find a number `pattern:\d+`. The plus `pattern:+` is greedy by default, so it consumes all digits:
+
+    ```
+    \d+.......
+    (123456789)z
+    ```
+
+    Then it tries to apply the star quantifier, but there are no more digits, so it the star doesn't give anything.
+
+    The next in the pattern is the string end `pattern:$`, but in the text we have `subject:!`, so there's no match:
+
+    ```
+               X
+    \d+........$
+    (123456789)!
+    ```
+
+2. As there's no match, the greedy quantifier `pattern:+` decreases the count of repetitions, backtracks one character back.
+
+    Now `pattern:\d+` takes all digits except the last one:
+    ```
+    \d+.......
+    (12345678)9!
+    ```
+3. Then the engine tries to continue the search from the new position (`9`).
+
+    The star `pattern:(\d+)*` can be applied -- it gives the number `match:9`:
+
+    ```
+
+    \d+.......\d+
+    (12345678)(9)!
+    ```
+
+    The engine tries to match `pattern:$` again, but fails, because meets `subject:!`:
+
+    ```
+                 X
+    \d+.......\d+
+    (12345678)(9)z
+    ```
+
+
+4. There's no match, so the engine will continue backtracking, decreasing the number of repetitions. Backtracking generally works like this: the last greedy quantifier decreases the number of repetitions until it can. Then the previous greedy quantifier decreases, and so on.
+
+    All possible combinations are attempted. Here are their examples.
+
+    The first number `pattern:\d+` has 7 digits, and then a number of 2 digits:
+
+    ```
+                 X
+    \d+......\d+
+    (1234567)(89)!
+    ```
+
+    The first number has 7 digits, and then two numbers of 1 digit each:
+
+    ```
+                   X
+    \d+......\d+\d+
+    (1234567)(8)(9)!
+    ```
+
+    The first number has 6 digits, and then a number of 3 digits:
+
+    ```
+                 X
+    \d+.......\d+
+    (123456)(789)!
+    ```
+
+    The first number has 6 digits, and then 2 numbers:
+
+    ```
+                   X
+    \d+.....\d+ \d+
+    (123456)(78)(9)!
+    ```
+
+    ...And so on.
+
+
+There are many ways to split a set of digits `123456789` into numbers. To be precise, there are <code>2<sup>n</sup>-1</code>, where `n` is the length of the set.
+
+For `n=20` there are about 1 million combinations, for `n=30` - a thousand times more. Trying each of them is exactly the reason why the search takes so long.
+
+What to do?
+
+Should we turn on the lazy mode?
+
+Unfortunately, that won't help: if we replace `pattern:\d+` with `pattern:\d+?`, the regexp will still hang. The order of combinations will change, but not their total count.
+
+Some regular expression engines have tricky tests and finite automations that allow to avoid going through all combinations or make it much faster, but not all engines, and not in all cases.
+
+## Back to words and strings
+
+The similar thing happens in our first example, when we look words by pattern `pattern:^(\w+\s?)*$` in the string `subject:An input that hangs!`.
+
+The reason is that a word can be represented as one `pattern:\w+` or many:
+
+```
+(input)
+(inpu)(t)
+(inp)(u)(t)
+(in)(p)(ut)
+...
+```
+
+For a human, it's obvious that there may be no match, because the string ends with an exclamation sign `!`, but the regular expression expects a wordly character `pattern:\w` or a space `pattern:\s` at the end. But the engine doesn't know that.
+
+It tries all combinations of how the regexp `pattern:(\w+\s?)*` can "consume" the string, including variants with spaces `pattern:(\w+\s)*` and without them `pattern:(\w+)*` (because spaces `pattern:\s?` are optional). As there are many such combinations, the search takes a lot of time.
+
+## How to fix?
+
+There are two main approaches to fixing the problem.
+
+The first is to lower the number of possible combinations.
+
+Let's rewrite the regular expression as `pattern:^(\w+\s)*\w*` - we'll look for any number of words followed by a space `pattern:(\w+\s)*`, and then (optionally) a word `pattern:\w*`.
+
+This regexp is equivalent to the previous one (matches the same) and works well:
+
+```js run
+let regexp = /^(\w+\s)*\w*$/;
+let str = "An input string that takes a long time or even makes this regex to hang!";
+
+alert( regexp.test(str) ); // false
+```
+
+Why did the problem disappear?
+
+Now the star `pattern:*` goes after `pattern:\w+\s` instead of `pattern:\w+\s?`. It became impossible to represent one word of the string with multiple successive `pattern:\w+`. The time needed to try such combinations is now saved.
+
+For example, the previous pattern `pattern:(\w+\s?)*` could match the word `subject:string` as two `pattern:\w+`:
+
+```js run
+\w+\w+
+string
+```
+
+The previous pattern, due to the optional `pattern:\s` allowed variants `pattern:\w+`, `pattern:\w+\s`, `pattern:\w+\w+` and so on.
+
+With the rewritten pattern `pattern:(\w+\s)*`, that's impossible: there may be  `pattern:\w+\s` or `pattern:\w+\s\w+\s`, but not `pattern:\w+\w+`. So the overall combinations count is greatly decreased.
+
+## Preventing backtracking
+
+It's not always convenient to rewrite a regexp. And it's not always obvious how to do it.
+
+The alternative approach is to forbid backtracking for the quantifier.
+
+The regular expressions engine tries many combinations that are obviously wrong for a human.
+
+E.g. in the regexp `pattern:(\d+)*$` it's obvious for a human, that `pattern:+` shouldn't backtrack. If we replace one `pattern:\d+` with two separate `pattern:\d+\d+`, nothing changes:
+
+```
+\d+........
+(123456789)!
+
+\d+...\d+....
+(1234)(56789)!
+```
+
+And in the original example `pattern:^(\w+\s?)*$` we may want to forbid backtracking in `pattern:\w+`. That is: `pattern:\w+` should match a whole word, with the maximal possible length. There's no need to lower the repetitions count in `pattern:\w+`, try to split it into two words `pattern:\w+\w+` and so on.
+
+Modern regular expression engines support possessive quantifiers for that. They are like greedy ones, but don't backtrack (so they are actually simpler than regular quantifiers).
+
+There are also so-called "atomic capturing groups" - a way to disable backtracking inside parentheses.
+
+Unfortunately, in JavaScript they are not supported. But there's another way.
+
+### Lookahead to the rescue!
+
+We can prevent backtracking using lookahead.
+
+The pattern to take as much repetitions of `pattern:\w` as possible without backtracking is: `pattern:(?=(\w+))\1`.
+
+Let's decipher it:
+- Lookahead `pattern:?=` looks forward for the longest word `pattern:\w+` starting at the current position.
+- The contents of parentheses with `pattern:?=...` isn't memorized by the engine, so wrap `pattern:\w+` into parentheses. Then the engine will memorize their contents
+- ...And allow us to reference it in the pattern as `pattern:\1`.
+
+That is: we look ahead - and if there's a word `pattern:\w+`, then match it as `pattern:\1`.
+
+Why? That's because the lookahead finds a word `pattern:\w+` as a whole and we capture it into the pattern with `pattern:\1`. So we essentially implemented a possessive plus `pattern:+` quantifier. It captures only the whole word `pattern:\w+`, not a part of it.
+
+For instance, in the word `subject:JavaScript` it may not only match `match:Java`, but leave out `match:Script` to match the rest of the pattern.
+
+Here's the comparison of two patterns:
+
+```js run
+alert( "JavaScript".match(/\w+Script/)); // JavaScript
+alert( "JavaScript".match(/(?=(\w+))\1Script/)); // null
+```
+
+1. In the first variant `pattern:\w+` first captures the whole word `subject:JavaScript` but then `pattern:+` backtracks character by character, to try to match the rest of the pattern, until it finally succeeds (when `pattern:\w+` matches `match:Java`).
+2. In the second variant `pattern:(?=(\w+))` looks ahead and finds the word  `subject:JavaScript`, that is included into the pattern as a whole by `pattern:\1`, so there remains no way to find `subject:Script` after it.
+
+We can put a more complex regular expression into `pattern:(?=(\w+))\1` instead of `pattern:\w`, when we need to forbid backtracking for `pattern:+` after it.
+
+```smart
+There's more about the relation between possessive quantifiers and lookahead in articles [Regex: Emulate Atomic Grouping (and Possessive Quantifiers) with LookAhead](http://instanceof.me/post/52245507631/regex-emulate-atomic-grouping-with-lookahead) and [Mimicking Atomic Groups](http://blog.stevenlevithan.com/archives/mimic-atomic-groups).
+```
+
+Let's rewrite the first example using lookahead to prevent backtracking:
+
+```js run
+let regexp = /^((?=(\w+))\2\s?)*$/;
+
+alert( regexp.test("A good string") ); // true
+
+let str = "An input string that takes a long time or even makes this regex to hang!";
+
+alert( regexp.test(str) ); // false, works and fast!
+```
+
+Here `pattern:\2` is used instead of `pattern:\1`, because there are additional outer parentheses. To avoid messing up with the numbers, we can give the parentheses a name, e.g. `pattern:(?<word>\w+)`.
+
+```js run
+// parentheses are named ?<word>, referenced as \k<word>
+let regexp = /^((?=(?<word>\w+))\k<word>\s?)*$/;
+
+let str = "An input string that takes a long time or even makes this regex to hang!";
+
+alert( regexp.test(str) ); // false
+
+alert( regexp.test("A correct string") ); // true
+```
+
+The problem described in this article is called "catastrophic backtracking".
+
+We covered two ways how to solve it:
+- Rewrite the regexp to lower the possible combinations count.
+- Prevent backtracking.

9-regular-expressions/15-regexp-infinite-backtracking-problem/article.md

@@ -1,297 +0,0 @@
-# Infinite backtracking problem
-
-Some regular expressions are looking simple, but can execute veeeeeery long time, and even "hang" the JavaScript engine.
-
-Sooner or later most developers occasionally face such behavior.
-
-The typical situation -- a regular expression works fine sometimes, but for certain strings it "hangs"  consuming 100% of CPU.
-
-In a web-browser it kills the page. Not a good thing for sure.
-
-For server-side JavaScript it may become a vulnerability, and it uses regular expressions to process user data. Bad input will make the process hang, causing denial of service. The author personally saw and reported such vulnerabilities even for very well-known and widely used programs.
-
-So the problem is definitely worth to deal with.
-
-## Introduction
-
-The plan will be like this:
-
-1. First we see the problem how it may occur.
-2. Then we simplify the situation and see why it occurs.
-3. Then we fix it.
-
-For instance let's consider searching tags in HTML.
-
-We want to find all tags, with or without attributes -- like `subject:<a href="..." class="doc" ...>`. We need the regexp to work reliably, because HTML comes from the internet and can be messy.
-
-In particular, we need it to match tags like `<a test="<>" href="#">` -- with `<` and `>` in attributes. That's allowed by [HTML standard](https://html.spec.whatwg.org/multipage/syntax.html#syntax-attributes).
-
-A simple regexp like `pattern:<[^>]+>` doesn't work, because it stops at the first `>`, and we need to ignore `<>` if inside an attribute:
-
-```js run
-// the match doesn't reach the end of the tag - wrong!
-alert( '<a test="<>" href="#">'.match(/<[^>]+>/) ); // <a test="<>
-```
-
-To correctly handle such situations we need a more complex regular expression. It will have the form  `pattern:<tag (key=value)*>`.
-
-1. For the `tag` name: `pattern:\w+`,
-2. For the `key` name: `pattern:\w+`,
-3. And the `value`: a quoted string `pattern:"[^"]*"`.
-
-If we substitute these into the pattern above and throw in some optional spaces `pattern:\s`, the full regexp becomes: `pattern:<\w+(\s*\w+="[^"]*"\s*)*>`.
-
-That regexp is not perfect! It doesn't support all the details of HTML syntax, such as unquoted values, and there are other ways to improve, but let's not add complexity. It will demonstrate the problem for us.
-
-The regexp seems to work:
-
-```js run
-let reg = /<\w+(\s*\w+="[^"]*"\s*)*>/g;
-
-let str='...<a test="<>" href="#">... <b>...';
-
-alert( str.match(reg) ); // <a test="<>" href="#">, <b>
-```
-
-Great! It found both the long tag `match:<a test="<>" href="#">` and the short one `match:<b>`.
-
-Now, that we've got a seemingly working solution, let's get to the infinite backtracking itself.
-
-## Infinite backtracking
-
-If you run our regexp on the input below, it may hang the browser (or another JavaScript host):
-
-```js run
-let reg = /<\w+(\s*\w+="[^"]*"\s*)*>/g;
-
-let str = `<tag a="b"  a="b"  a="b"  a="b"  a="b"  a="b"  a="b"  a="b"
-  a="b"  a="b"  a="b"  a="b"  a="b"  a="b"  a="b"  a="b"  a="b" a="b"  a="b"  a="b"  a="b"`;
-
-*!*
-// The search will take a long, long time
-alert( str.match(reg) );
-*/!*
-```
-
-Some regexp engines can handle that search, but most of them can't.
-
-What's the matter? Why a simple regular expression "hangs" on such a small string?
-
-Let's simplify the regexp by stripping the tag name and the quotes. So that we look only for `key=value` attributes: `pattern:<(\s*\w+=\w+\s*)*>`.
-
-Unfortunately, the regexp still hangs:
-
-```js run
-// only search for space-delimited attributes
-let reg = /<(\s*\w+=\w+\s*)*>/g;
-
-let str = `<a=b  a=b  a=b  a=b  a=b  a=b  a=b  a=b
-  a=b  a=b  a=b  a=b  a=b  a=b  a=b  a=b  a=b  a=b  a=b  a=b  a=b  a=b`;
-
-*!*
-// the search will take a long, long time
-alert( str.match(reg) );
-*/!*
-```
-
-Here we end the demo of the problem and start looking into what's going on, why it hangs and how to fix it.
-
-## Detailed example
-
-To make an example even simpler, let's consider `pattern:(\d+)*$`.
-
-This regular expression also has the same problem. In most regexp engines that search takes a very long time (careful -- can hang):
-
-```js run
-alert( '12345678901234567890123456789123456789z'.match(/(\d+)*$/) );
-```
-
-So what's wrong with the regexp?
-
-First, one may notice that the regexp is a little bit strange. The quantifier `pattern:*` looks extraneous. If we want a number, we can use `pattern:\d+$`.
-
-Indeed, the regexp is artificial. But the reason why it is slow is the same as those we saw above. So let's understand it, and then the previous example will become obvious.
-
-What happens during the search of `pattern:(\d+)*$` in the line `subject:123456789z`?
-
-1. First, the regexp engine tries to find a number `pattern:\d+`. The plus `pattern:+` is greedy by default, so it consumes all digits:
-
-    ```
-    \d+.......
-    (123456789)z
-    ```
-2. Then it tries to apply the star quantifier, but there are no more digits, so it the star doesn't give anything.
-
-3. Then the pattern expects to see the string end `pattern:$`, and in the text we have `subject:z`, so there's no match:
-
-    ```
-               X
-    \d+........$
-    (123456789)z
-    ```
-
-4. As there's no match, the greedy quantifier `pattern:+` decreases the count of repetitions (backtracks).
-
-    Now `\d+` doesn't take all digits, but all except the last one:
-    ```
-    \d+.......
-    (12345678)9z
-    ```
-5. Now the engine tries to continue the search from the new position (`9`).
-
-    The star `pattern:(\d+)*` can be applied -- it gives the number `match:9`:
-
-    ```
-
-    \d+.......\d+
-    (12345678)(9)z
-    ```
-
-    The engine tries to match `$` again, but fails, because meets `subject:z`:
-
-    ```
-                 X
-    \d+.......\d+
-    (12345678)(9)z
-    ```
-
-
-5. There's no match, so the engine will continue backtracking, decreasing the number of repetitions for `pattern:\d+` down to 7 digits. So the rest of the string `subject:89` becomes the second `pattern:\d+`:
-
-    ```
-                 X
-    \d+......\d+
-    (1234567)(89)z
-    ```
-
-    ...Still no match for `pattern:$`.
-
-    The search engine backtracks again. Backtracking generally works like this: the last greedy quantifier decreases the number of repetitions until it can. Then the previous greedy quantifier decreases, and so on. In our case the last greedy quantifier is the second `pattern:\d+`, from `subject:89` to `subject:8`, and then the star takes `subject:9`:
-
-    ```
-                   X
-    \d+......\d+\d+
-    (1234567)(8)(9)z
-    ```
-6. ...Fail again. The second and third `pattern:\d+` backtracked to the end, so the first quantifier shortens the match to `subject:123456`, and the star takes the rest:
-
-    ```
-                 X
-    \d+.......\d+
-    (123456)(789)z
-    ```
-
-    Again no match. The process repeats: the last greedy quantifier releases one character (`9`):
-
-    ```
-                   X
-    \d+.....\d+ \d+
-    (123456)(78)(9)z
-    ```
-7. ...And so on.
-
-The regular expression engine goes through all combinations of `123456789` and their subsequences. There are a lot of them, that's why it takes so long.
-
-What to do?
-
-Should we turn on the lazy mode?
-
-Unfortunately, it doesn't: if we replace `pattern:\d+` with `pattern:\d+?`, that still hangs:
-
-```js run
-// sloooooowwwwww
-alert( '12345678901234567890123456789123456789z'.match(/(\d+?)*$/) );
-```
-
-Lazy quantifiers actually do the same, but in the reverse order.
-
-Just think about how the search engine would work in this case.
-
-Some regular expression engines have tricky built-in checks to detect infinite backtracking or other means to work around them, but there's no universal solution.
-
-## Back to tags
-
-In the example above, when we search `pattern:<(\s*\w+=\w+\s*)*>` in the string `subject:<a=b  a=b  a=b  a=b` -- the similar thing happens.
-
-The string has no `>` at the end, so the match is impossible, but the regexp engine doesn't know about it. The search backtracks trying different combinations of `pattern:(\s*\w+=\w+\s*)`:
-
-```
-(a=b a=b a=b) (a=b)
-(a=b a=b) (a=b a=b)
-(a=b) (a=b a=b a=b)
-...
-```
-
-As there are many combinations, it takes a lot of time.
-
-## How to fix?
-
-The backtracking checks many variants that are an obvious fail for a human.
-
-For instance, in the pattern `pattern:(\d+)*$` a human can easily see that `pattern:(\d+)*` does not need to backtrack `pattern:+`. There's no difference between one or two `\d+`:
-
-```
-\d+........
-(123456789)z
-
-\d+...\d+....
-(1234)(56789)z
-```
-
-Let's get back to more real-life example: `pattern:<(\s*\w+=\w+\s*)*>`. We want it to find pairs `name=value` (as many as it can).
-
-What we would like to do is to forbid backtracking.
-
-There's totally no need to decrease the number of repetitions.
-
-In other words, if it found three `name=value` pairs and then can't find `>` after them, then there's no need to decrease the count of repetitions. There are definitely no `>` after those two (we backtracked one `name=value` pair, it's there):
-
-```
-(name=value) name=value
-```
-
-Modern regexp engines support so-called "possessive" quantifiers for that. They are like greedy, but don't backtrack at all. Pretty simple, they capture whatever they can, and the search continues. There's also another tool called "atomic groups" that forbid backtracking inside parentheses.
-
-Unfortunately, but both these features are not supported by JavaScript.
-
-### Lookahead to the rescue
-
-We can forbid backtracking using lookahead.
-
-The pattern to take as much repetitions as possible without backtracking is: `pattern:(?=(a+))\1`.
-
-In other words:
-- The lookahead `pattern:?=` looks for the maximal count `pattern:a+` from the current position.
-- And then they are "consumed into the result" by the backreference `pattern:\1` (`pattern:\1` corresponds to the content of the second parentheses, that is `pattern:a+`).
-
-There will be no backtracking, because lookahead does not backtrack. If, for
-example, it found 5 instances of `pattern:a+` and the further match failed,
-it won't go back to the 4th instance.
-
-```smart
-There's more about the relation between possessive quantifiers and lookahead in articles [Regex: Emulate Atomic Grouping (and Possessive Quantifiers) with LookAhead](http://instanceof.me/post/52245507631/regex-emulate-atomic-grouping-with-lookahead) and [Mimicking Atomic Groups](http://blog.stevenlevithan.com/archives/mimic-atomic-groups).
-```
-
-So this trick makes the problem disappear.
-
-Let's fix the regexp for a tag with attributes from the beginning of the chapter`pattern:<\w+(\s*\w+=(\w+|"[^"]*")\s*)*>`. We'll use lookahead to prevent backtracking of `name=value` pairs:
-
-```js run
-// regexp to search name=value
-let attrReg = /(\s*\w+=(\w+|"[^"]*")\s*)/
-
-// use new RegExp to nicely insert its source into (?=(a+))\1
-let fixedReg = new RegExp(`<\\w+(?=(${attrReg.source}*))\\1>`, 'g');
-
-let goodInput = '...<a test="<>" href="#">... <b>...';
-
-let badInput = `<tag a=b  a=b  a=b  a=b  a=b  a=b  a=b  a=b
-  a=b  a=b  a=b  a=b  a=b  a=b  a=b  a=b  a=b  a=b  a=b  a=b  a=b`;
-
-alert( goodInput.match(fixedReg) ); // <a test="<>" href="#">, <b>
-alert( badInput.match(fixedReg) ); // null (no results, fast!)
-```
-
-Great, it works! We found both a long tag  `match:<a test="<>" href="#">` and a small one `match:<b>`, and (!) didn't hang the engine on the bad input.
-
-Please note the `attrReg.source` property. `RegExp` objects provide access to their source string in it. That's convenient when we want to insert one regexp into another.

9-regular-expressions/16-regexp-sticky/article.md

@@ -0,0 +1,127 @@
+
+# Sticky flag "y", searching at position
+
+The flag `pattern:y` allows to perform the search at the given position in the source string.
+
+To grasp the use case of `pattern:y` flag, and see how great it is, let's explore a practical use case.
+
+One of common tasks for regexps is "lexical analysis": we get a text, e.g. in a programming language, and analyze it for structural elements.
+
+For instance, HTML has tags and attributes, JavaScript code has functions, variables, and so on.
+
+Writing lexical analyzers is a special area, with its own tools and algorithms, so we don't go deep in there, but there's a common task: to read something at the given position.
+
+E.g. we have a code string `subject:let varName = "value"`, and we need to read the variable name from it, that starts at position `4`.
+
+We'll look for variable name using regexp `pattern:\w+`. Actually, JavaScript variable names need a bit more complex regexp for accurate matching, but here it doesn't matter.
+
+A call to `str.match(/\w+/)` will find only the first word in the line. Or all words with the flag `pattern:g`. But we need only one word at position `4`.
+
+To search from the given position, we can use method `regexp.exec(str)`.
+
+If the `regexp` doesn't have flags `pattern:g` or `pattern:y`, then this method looks for the first match in the string `str`, exactly like `str.match(regexp)`. Such simple no-flags case doesn't interest us here.
+
+If there's flag `pattern:g`, then it performs the search in the string `str`, starting from position stored in its `regexp.lastIndex` property. And, if it finds a match, then sets `regexp.lastIndex` to the index immediately after the match.
+
+When a regexp is created, its `lastIndex` is `0`.
+
+So, successive calls to `regexp.exec(str)` return matches one after another.
+
+An example (with flag `pattern:g`):
+
+```js run
+let str = 'let varName';
+
+let regexp = /\w+/g;
+alert(regexp.lastIndex); // 0 (initially lastIndex=0)
+
+let word1 = regexp.exec(str);
+alert(word1[0]); // let (1st word)
+alert(regexp.lastIndex); // 3 (position after the match)
+
+let word2 = regexp.exec(str);
+alert(word2[0]); // varName (2nd word)
+alert(regexp.lastIndex); // 11 (position after the match)
+
+let word3 = regexp.exec(str);
+alert(word3); // null (no more matches)
+alert(regexp.lastIndex); // 0 (resets at search end)
+```
+
+Every match is returned as an array with groups and additional properties.
+
+We can get all matches in the loop:
+
+```js run
+let str = 'let varName';
+let regexp = /\w+/g;
+
+let result;
+
+while (result = regexp.exec(str)) {
+  alert( `Found ${result[0]} at position ${result.index}` );
+  // Found let at position 0, then
+  // Found varName at position 4
+}
+```
+
+Such use of `regexp.exec` is an alternative to method `str.matchAll`.
+
+Unlike other methods, we can set our own `lastIndex`, to start the search from the given position.
+
+For instance, let's find a word, starting from position `4`:
+
+```js run
+let str = 'let varName = "value"';
+
+let regexp = /\w+/g; // without flag "g", property lastIndex is ignored
+
+*!*
+regexp.lastIndex = 4;
+*/!*
+
+let word = regexp.exec(str);
+alert(word); // varName
+```
+
+We performed a search of `pattern:\w+`, starting from position `regexp.lastIndex = 4`.
+
+Please note: the search starts at position `lastIndex` and then goes further. If there's no word at position `lastIndex`, but it's somewhere after it, then it will be found:
+
+```js run
+let str = 'let varName = "value"';
+
+let regexp = /\w+/g;
+
+*!*
+regexp.lastIndex = 3;
+*/!*
+
+let word = regexp.exec(str);
+alert(word[0]); // varName
+alert(word.index); // 4
+```
+
+...So, with flag `pattern:g` property `lastIndex` sets the starting position for the search.
+
+**Flag `pattern:y` makes `regexp.exec` to look exactly at position `lastIndex`, not before, not after it.**
+
+Here's the same search with flag `pattern:y`:
+
+```js run
+let str = 'let varName = "value"';
+
+let regexp = /\w+/y;
+
+regexp.lastIndex = 3;
+alert( regexp.exec(str) ); // null (there's a space at position 3, not a word)
+
+regexp.lastIndex = 4;
+alert( regexp.exec(str) ); // varName (word at position 4)
+```
+
+As we can see, regexp `pattern:/\w+/y` doesn't match at position `3` (unlike the flag  `pattern:g`), but matches at position `4`.
+
+Imagine, we have a long text, and there are no matches in it, at all. Then searching with flag `pattern:g` will go till the end of the text, and this will take significantly more time than the search with flag `pattern:y`.
+
+In such tasks like lexical analysis, there are usually many searches at an exact position. Using flag `pattern:y` is the key for a good performance.

9-regular-expressions/17-regexp-methods/article.md

@@ -0,0 +1,348 @@
+# Methods of RegExp and String
+
+In this article we'll cover various methods that work with regexps in-depth.
+
+## str.match(regexp)
+
+The method `str.match(regexp)` finds matches for `regexp` in the string `str`.
+
+It has 3 modes:
+
+1. If the `regexp` doesn't have flag `pattern:g`, then it returns the first match as an array with capturing groups and properties `index` (position of the match), `input` (input string, equals `str`):
+
+    ```js run
+    let str = "I love JavaScript";
+
+    let result = str.match(/Java(Script)/);
+
+    alert( result[0] );     // JavaScript (full match)
+    alert( result[1] );     // Script (first capturing group)
+    alert( result.length ); // 2
+
+    // Additional information:
+    alert( result.index );  // 0 (match position)
+    alert( result.input );  // I love JavaScript (source string)
+    ```
+
+2. If the `regexp` has flag `pattern:g`, then it returns an array of all matches as strings, without capturing groups and other details.
+    ```js run
+    let str = "I love JavaScript";
+
+    let result = str.match(/Java(Script)/g);
+
+    alert( result[0] ); // JavaScript
+    alert( result.length ); // 1
+    ```
+
+3. If there are no matches, no matter if there's flag `pattern:g` or not, `null` is returned.
+
+    That's an important nuance. If there are no matches, we don't get an empty array, but `null`. It's easy to make a mistake forgetting about it, e.g.:
+
+    ```js run
+    let str = "I love JavaScript";
+
+    let result = str.match(/HTML/);
+
+    alert(result); // null
+    alert(result.length); // Error: Cannot read property 'length' of null
+    ```
+
+    If we want the result to be an array, we can write like this:
+
+    ```js
+    let result = str.match(regexp) || [];
+    ```
+
+## str.matchAll(regexp)
+
+[recent browser="new"]
+
+The method `str.matchAll(regexp)` is a "newer, improved" variant of `str.match`.
+
+It's used mainly to search for all matches with all groups.
+
+There are 3 differences from `match`:
+
+1. It returns an iterable object with matches instead of an array. We can make a regular array from it using `Array.from`.
+2. Every match is returned as an array with capturing groups (the same format as `str.match` without flag `pattern:g`).
+3. If there are no results, it returns not `null`, but an empty iterable object.
+
+Usage example:
+
+```js run
+let str = '<h1>Hello, world!</h1>';
+let regexp = /<(.*?)>/g;
+
+let matchAll = str.matchAll(regexp);
+
+alert(matchAll); // [object RegExp String Iterator], not array, but an iterable
+
+matchAll = Array.from(matchAll); // array now
+
+let firstMatch = matchAll[0];
+alert( firstMatch[0] );  // <h1>
+alert( firstMatch[1] );  // h1
+alert( firstMatch.index );  // 0
+alert( firstMatch.input );  // <h1>Hello, world!</h1>
+```
+
+If we use `for..of` to loop over `matchAll` matches, then we don't need `Array.from`, разумеется, не нужен.
+
+## str.split(regexp|substr, limit)
+
+Splits the string using the regexp (or a substring) as a delimiter.
+
+We can use `split` with strings, like this:
+
+```js run
+alert('12-34-56'.split('-')) // array of [12, 34, 56]
+```
+
+But we can split by a regular expression, the same way:
+
+```js run
+alert('12, 34, 56'.split(/,\s*/)) // array of [12, 34, 56]
+```
+
+## str.search(regexp)
+
+The method `str.search(regexp)` returns the position of the first match or `-1` if none found:
+
+```js run
+let str = "A drop of ink may make a million think";
+
+alert( str.search( /ink/i ) ); // 10 (first match position)
+```
+
+**The important limitation: `search` only finds the first match.**
+
+If we need positions of further matches, we should use other means, such as finding them all with `str.matchAll(regexp)`.
+
+## str.replace(str|regexp, str|func)
+
+This is a generic method for searching and replacing, one of most useful ones. The swiss army knife for searching and replacing.  
+
+We can use it without regexps, to search and replace a substring:
+
+```js run
+// replace a dash by a colon
+alert('12-34-56'.replace("-", ":")) // 12:34-56
+```
+
+There's a pitfall though.
+
+**When the first argument of `replace` is a string, it only replaces the first match.**
+
+You can see that in the example above: only the first `"-"` is replaced by `":"`.
+
+To find all hyphens, we need to use not the string `"-"`, but a regexp `pattern:/-/g`, with the obligatory `pattern:g` flag:
+
+```js run
+// replace all dashes by a colon
+alert( '12-34-56'.replace( *!*/-/g*/!*, ":" ) )  // 12:34:56
+```
+
+The second argument is a replacement string. We can use special character in it:
+
+| Symbols | Action in the replacement string |
+| Symbols | Action in the replacement string |
+|--------|--------|
+|`$&`|inserts the whole match|
+|<code>$&#096;</code>|inserts a part of the string before the match|
+|`$'`|inserts a part of the string after the match|
+|`$n`|if `n` is a 1-2 digit number, inserts the contents of n-th capturing group, for details see [](info:regexp-groups)|
+|`$<name>`|inserts the contents of the parentheses with the given `name`, for details see [](info:regexp-groups)|
+|`$$`|inserts character `$` |
+
+For instance:
+
+```js run
+let str = "John Smith";
+
+// swap first and last name
+alert(str.replace(/(john) (smith)/i, '$2, $1')) // Smith, John
+```
+
+**For situations that require "smart" replacements, the second argument can be a function.**
+
+It will be called for each match, and the returned value will be inserted as a replacement.
+
+The function is called with arguments `func(match, p1, p2, ..., pn, offset, input, groups)`:
+
+1. `match` -- the match,
+2. `p1, p2, ..., pn` -- contents of capturing groups (if there are any),
+3. `offset` -- position of the match,
+4. `input` -- the source string,
+5. `groups` -- an object with named groups.
+
+If there are no parentheses in the regexp, then there are only 3 arguments: `func(str, offset, input)`.
+
+For example, let's uppercase all matches:
+
+```js run
+let str = "html and css";
+
+let result = str.replace(/html|css/gi, str => str.toUpperCase());
+
+alert(result); // HTML and CSS
+```
+
+Replace each match by its position in the string:
+
+```js run
+alert("Ho-Ho-ho".replace(/ho/gi, (match, offset) => offset)); // 0-3-6
+```
+
+In the example below there are two parentheses, so the replacement function is called with 5 arguments: the first is the full match, then 2 parentheses, and after it (not used in the example) the match position and the source string:
+
+```js run
+let str = "John Smith";
+
+let result = str.replace(/(\w+) (\w+)/, (match, name, surname) => `${surname}, ${name}`);
+
+alert(result); // Smith, John
+```
+
+If there are many groups, it's convenient to use rest parameters to access them:
+
+
+Если в регулярном выражении много скобочных групп, то бывает удобно использовать остаточные аргументы для обращения к ним:
+
+```js run
+let str = "John Smith";
+
+let result = str.replace(/(\w+) (\w+)/, (...match) => `${match[2]}, ${match[1]}`);
+
+alert(result); // Smith, John
+```
+
+Or, if we're using named groups, then `groups` object with them is always the last, so we can obtain it like this:
+
+```js run
+let str = "John Smith";
+
+let result = str.replace(/(?<name>\w+) (?<surname>\w+)/, (...match) => {
+  let groups = match.pop();
+
+  return `${groups.surname}, ${groups.name}`;
+});
+
+alert(result); // Smith, John
+```
+
+Using a function gives us the ultimate replacement power, because it gets all the information about the match, has access to outer variables and can do everything.
+
+## regexp.exec(str)
+
+The method `regexp.exec(str)` method returns a match for `regexp` in the string `str`.  Unlike previous methods, it's called on a regexp, not on a string.
+
+It behaves differently depending on whether the regexp has flag `pattern:g`.
+
+If there's no `pattern:g`, then `regexp.exec(str)` returns the first match exactly as  `str.match(regexp)`. This behavior doesn't bring anything new.
+
+But if there's flag `pattern:g`, then:
+- A call to `regexp.exec(str)` returns the first match and saves the position immediately after it in the property `regexp.lastIndex`.
+- The next such call starts the search from position `regexp.lastIndex`, returns the next match and saves the position after it in `regexp.lastIndex`.
+- ...And so on.
+- If there are no matches, `regexp.exec` returns `null` and resets `regexp.lastIndex` to `0`.
+
+So, repeated calls return all matches one after another, using property `regexp.lastIndex` to keep track of the current search position.
+
+In the past, before the method `str.matchAll` was added to JavaScript, calls of `regexp.exec` were used in the loop to get all matches with groups:
+
+```js run
+let str = 'More about JavaScript at https://javascript.info';
+let regexp = /javascript/ig;
+
+let result;
+
+while (result = regexp.exec(str)) {
+  alert( `Found ${result[0]} at position ${result.index}` );
+  // Found JavaScript at position 11, then
+  // Found javascript at position 33
+}
+```
+
+This works now as well, although for newer browsers `str.matchAll` is usually more convenient.
+
+**We can use `regexp.exec` to search from a given position by manually setting `lastIndex`.**
+
+For instance:
+
+```js run
+let str = 'Hello, world!';
+
+let regexp = /\w+/g; // without flag "g", lastIndex property is ignored
+regexp.lastIndex = 5; // search from 5th position (from the comma)
+
+alert( regexp.exec(str) ); // world
+```
+
+If the regexp has flag `pattern:y`, then the search will be performed exactly at the  position `regexp.lastIndex`, not any further.
+
+Let's replace flag `pattern:g` with `pattern:y` in the example above. There will be no matches, as there's no word at position `5`:
+
+```js run
+let str = 'Hello, world!';
+
+let regexp = /\w+/y;
+regexp.lastIndex = 5; // search exactly at position 5
+
+alert( regexp.exec(str) ); // null
+```
+
+That's convenient for situations when we need to "read" something from the string by a regexp at the exact position, not somewhere further.
+
+## regexp.test(str)
+
+The method `regexp.test(str)` looks for a match and returns `true/false` whether it exists.
+
+For instance:
+
+```js run
+let str = "I love JavaScript";
+
+// these two tests do the same
+alert( *!*/love/i*/!*.test(str) ); // true
+alert( str.search(*!*/love/i*/!*) != -1 ); // true
+```
+
+An example with the negative answer:
+
+```js run
+let str = "Bla-bla-bla";
+
+alert( *!*/love/i*/!*.test(str) ); // false
+alert( str.search(*!*/love/i*/!*) != -1 ); // false
+```
+
+If the regexp has flag `pattern:g`, then `regexp.test` looks from `regexp.lastIndex` property and updates this property, just like `regexp.exec`.
+
+So we can use it to search from a given position:
+
+```js run
+let regexp = /love/gi;
+
+let str = "I love JavaScript";
+
+// start the search from position 10:
+regexp.lastIndex = 10;
+alert( regexp.test(str) ); // false (no match)
+```
+
+````warn header="Same global regexp tested repeatedly on different sources may fail"
+If we apply the same global regexp to different inputs, it may lead to wrong result, because `regexp.test` call advances `regexp.lastIndex` property, so the search in another string may start from non-zero position.
+
+For instance, here we call `regexp.test` twice on the same text, and the second time fails:
+
+```js run
+let regexp = /javascript/g;  // (regexp just created: regexp.lastIndex=0)
+
+alert( regexp.test("javascript") ); // true (regexp.lastIndex=10 now)
+alert( regexp.test("javascript") ); // false
+```
+
+That's exactly because `regexp.lastIndex` is non-zero in the second test.
+
+To work around that, we can set `regexp.lastIndex = 0` before each search. Or instead of calling methods on regexp, use string methods `str.match/search/...`, they don't use `lastIndex`.
+````

9-regular-expressions/20-regexp-unicode/article.md

@@ -1,89 +0,0 @@
-
-# Unicode: flag "u"
-
-The unicode flag `/.../u` enables the correct support of surrogate pairs.
-
-Surrogate pairs are explained in the chapter <info:string>.
-
-Let's briefly review them here. In short, normally characters are encoded with 2 bytes. That gives us 65536 characters maximum. But there are more characters in the world.
-
-So certain rare characters are encoded with 4 bytes, like `𝒳` (mathematical X) or `😄` (a smile).
-
-Here are the unicode values to compare:
-
-| Character  | Unicode | Bytes  |
-|------------|---------|--------|
-| `a` | 0x0061 |  2 |
-| `≈` | 0x2248 |  2 |
-|`𝒳`| 0x1d4b3 | 4 |
-|`𝒴`| 0x1d4b4 | 4 |
-|`😄`| 0x1f604 | 4 |
-
-So characters like `a` and `≈` occupy 2 bytes, and those rare ones take 4.
-
-The unicode is made in such a way that the 4-byte characters only have a meaning as a whole.
-
-In the past JavaScript did not know about that, and many string methods still have problems. For instance, `length` thinks that here are two characters:
-
-```js run
-alert('😄'.length); // 2
-alert('𝒳'.length); // 2
-```
-
-...But we can see that there's only one, right? The point is that `length` treats 4 bytes as two 2-byte characters. That's incorrect, because they must be considered only together (so-called "surrogate pair").
-
-Normally, regular expressions also treat "long characters" as two 2-byte ones.
-
-That leads to odd results, for instance let's try to find `pattern:[𝒳𝒴]` in the string `subject:𝒳`:
-
-```js run
-alert( '𝒳'.match(/[𝒳𝒴]/) ); // odd result (wrong match actually, "half-character")
-```
-
-The result is wrong, because by default the regexp engine does not understand surrogate pairs.
-
-So, it thinks that `[𝒳𝒴]` are not two, but four characters:
-1. the left half of `𝒳` `(1)`,
-2. the right half of `𝒳` `(2)`,
-3. the left half of `𝒴` `(3)`,
-4. the right half of `𝒴` `(4)`.
-
-We can list them like this:
-
-```js run
-for(let i=0; i<'𝒳𝒴'.length; i++) {
-  alert('𝒳𝒴'.charCodeAt(i)); // 55349, 56499, 55349, 56500
-};
-```
-
-So it finds only the "left half" of `𝒳`.
-
-In other words, the search works like `'12'.match(/[1234]/)`: only `1` is returned.
-
-## The "u" flag
-
-The `/.../u` flag fixes that.
-
-It enables surrogate pairs in the regexp engine, so the result is correct:
-
-```js run
-alert( '𝒳'.match(/[𝒳𝒴]/u) ); // 𝒳
-```
-
-Let's see one more example.
-
-If we forget the `u` flag and accidentally use surrogate pairs, then we can get an error:
-
-```js run
-'𝒳'.match(/[𝒳-𝒴]/); // SyntaxError: invalid range in character class
-```
-
-Normally, regexps understand `[a-z]` as a "range of characters with codes between codes of `a` and `z`.
-
-But without `u` flag, surrogate pairs are assumed to be a "pair of independent characters", so `[𝒳-𝒴]` is like `[<55349><56499>-<55349><56500>]` (replaced each surrogate pair with code points). Now we can clearly see that the range `56499-55349` is unacceptable, as the left range border must be less than the right one.
-
-Using the `u` flag makes it work right:
-
-```js run
-alert( '𝒴'.match(/[𝒳-𝒵]/u) ); // 𝒴
-```

9-regular-expressions/21-regexp-unicode-properties/article.md

@@ -1,86 +0,0 @@
-
-# Unicode character properties \p
-
-[Unicode](https://en.wikipedia.org/wiki/Unicode), the encoding format used by JavaScript strings, has a lot of properties for different characters (or, technically, code points). They describe which "categories" character belongs to, and a variety of technical details.
-
-In regular expressions these can be set by `\p{…}`. And there must be flag `'u'`.
-
-For instance, `\p{Letter}` denotes a letter in any of language. We can also use `\p{L}`, as `L` is an alias of `Letter`, there are shorter aliases for almost every property.
-
-Here's the main tree of properties:
-
-- Letter `L`:
-  - lowercase `Ll`, modifier `Lm`, titlecase `Lt`, uppercase `Lu`, other `Lo`
-- Number `N`:
-  - decimal digit `Nd`, letter number `Nl`, other `No`
-- Punctuation `P`:
-  - connector `Pc`, dash `Pd`, initial quote `Pi`, final quote `Pf`, open `Ps`, close `Pe`, other `Po`
-- Mark `M` (accents etc):
-  - spacing combining `Mc`, enclosing `Me`, non-spacing `Mn`
-- Symbol `S`:
-  - currency `Sc`, modifier `Sk`, math `Sm`, other `So`
-- Separator `Z`:
-  - line `Zl`, paragraph `Zp`, space `Zs`
-- Other `C`:
-  - control `Cc`, format `Cf`, not assigned `Cn`, private use `Co`, surrogate `Cs`
-
-```smart header="More information"
-Interested to see which characters belong to a property? There's a tool at <http://cldr.unicode.org/unicode-utilities/list-unicodeset> for that.
-
-You could also explore properties at [Character Property Index](http://unicode.org/cldr/utility/properties.jsp).
-
-For the full Unicode Character Database in text format (along with all properties), see <https://www.unicode.org/Public/UCD/latest/ucd/>.
-```
-
-There are also other derived categories, like:
-- `Alphabetic` (`Alpha`), includes Letters `L`, plus letter numbers `Nl` (e.g. roman numbers Ⅻ), plus some other symbols `Other_Alphabetic` (`OAltpa`).
-- `Hex_Digit` includes hexadecimal digits: `0-9`, `a-f`.
-- ...Unicode is a big beast, it includes a lot of properties.
-
-For instance, let's look for a 6-digit hex number:
-
-```js run
-let reg = /\p{Hex_Digit}{6}/u; // flag 'u' is required
-
-alert("color: #123ABC".match(reg)); // 123ABC
-```
-
-There are also properties with a value. For instance, Unicode "Script" (a writing system) can be Cyrillic, Greek, Arabic, Han (Chinese) etc, the [list is long]("https://en.wikipedia.org/wiki/Script_(Unicode)").
-
-To search for characters in certain scripts ("alphabets"), we should supply `Script=<value>`, e.g. to search for cyrillic letters: `\p{sc=Cyrillic}`, for Chinese glyphs: `\p{sc=Han}`, etc:
-
-```js run
-let regexp = /\p{sc=Han}+/gu; // get chinese words
-
-let str = `Hello Привет 你好 123_456`;
-
-alert( str.match(regexp) ); // 你好
-```
-
-## Building multi-language \w
-
-The pattern `pattern:\w` means "wordly characters", but doesn't work for languages that use non-Latin alphabets, such as Cyrillic and others. It's just a shorthand for `[a-zA-Z0-9_]`, so `pattern:\w+` won't find any Chinese words etc.
-
-Let's make a "universal" regexp, that looks for wordly characters in any language. That's easy to do using Unicode properties:
-
-```js
-/[\p{Alphabetic}\p{Mark}\p{Decimal_Number}\p{Connector_Punctuation}\p{Join_Control}]/u
-```
-
-Let's decipher. Just as `pattern:\w` is the same as `pattern:[a-zA-Z0-9_]`, we're making a set of our own, that includes:
-
-- `Alphabetic` for letters,
-- `Mark` for accents, as in Unicode accents may be represented by separate code points,
-- `Decimal_Number` for numbers,
-- `Connector_Punctuation` for the `'_'` character and alike,
-- `Join_Control` -– two special code points with hex codes `200c` and `200d`, used in ligatures e.g. in arabic.
-
-Or, if we replace long names with aliases (a list of aliases [here](https://www.unicode.org/Public/UCD/latest/ucd/PropertyValueAliases.txt)):
-
-```js run
-let regexp = /([\p{Alpha}\p{M}\p{Nd}\p{Pc}\p{Join_C}]+)/gu;
-
-let str = `Hello Привет 你好 123_456`;
-
-alert( str.match(regexp) ); // Hello,Привет,你好,123_456
-```

9-regular-expressions/22-regexp-sticky/article.md

@@ -1,73 +0,0 @@
-
-# Sticky flag "y", searching at position
-
-To grasp the use case of `y` flag, and see how great it is, let's explore a practical use case.
-
-One of common tasks for regexps is "parsing": when we get a text and analyze it for logical components, build a structure.
-
-For instance, there are HTML parsers for browser pages, that turn text into a structured document. There are parsers for programming languages, like JavaScript, etc.
-
-Writing parsers is a special area, with its own tools and algorithms, so we don't go deep in there, but there's a very common question in them, and, generally, for text analysis: "What kind of entity is at the given position?".
-
-For instance, for a programming language variants can be like:
-- Is it a "name" `pattern:\w+`?
-- Or is it a number `pattern:\d+`?
-- Or an operator `pattern:[+-/*]`?
-- (a syntax error if it's not anything in the expected list)
-
-So, we should try to match a couple of regular expressions, and make a decision what's at the given position.
-
-In JavaScript, how can we perform a search starting from a given position? Regular calls start searching from the text start.
-
-We'd like to avoid creating substrings, as this slows down the execution considerably.
-
-One option is to use `regexp.exec` with `regexp.lastIndex` property, but that's not what we need, as this would search the text starting from `lastIndex`, while we only need to text the match *exactly* at the given position.
-
-Here's a (failing) attempt to use `lastIndex`:
-
-```js run
-let str = "(text before) function ...";
-
-// attempting to find function at position 5:
-let regexp = /function/g; // must use "g" flag, otherwise lastIndex is ignored
-regexp.lastIndex = 5
-
-alert (regexp.exec(str)); // function
-```
-
-The match is found, because `regexp.exec` starts to search from the given position and goes on by the text, successfully matching "function" later.
-
-We could work around that by checking if "`regexp.exec(str).index` property is `5`, and if not, ignore the match. But the main problem here is performance. The regexp engine does a lot of unnecessary work by scanning at further positions. The delays are clearly noticeable if the text is long, because there are many such searches in a parser.
-
-## The "y" flag
-
-So we've came to the problem: how to search for a match exactly at the given position.
-
-That's what `y` flag does. It makes the regexp search only at the `lastIndex` position.
-
-Here's an example
-
-```js run
-let str = "(text before) function ...";
-
-*!*
-let regexp = /function/y;
-regexp.lastIndex = 5;
-*/!*
-
-alert (regexp.exec(str)); // null (no match, unlike "g" flag!)
-
-*!*
-regexp.lastIndex = 14;
-*/!*
-
-alert (regexp.exec(str)); // function (match!)
-```
-
-As we can see, now the regexp is only matched at the given position.
-
-So what `y` does is truly unique, and very important for writing parsers.
-
-The `y` flag allows to test a regular expression exactly at the given position and when we understand what's there, we can move on -- step by step examining the text.
-
-Without the flag the regexp engine always searches till the end of the text, that takes time, especially if the text is large. So our parser would be very slow. The `y` flag is exactly the right thing here.

9-regular-expressions/index.md

@@ -1,7 +1,3 @@
 # Regular expressions
 
 Regular expressions is a powerful way of doing search and replace in strings.
-
-In JavaScript regular expressions are implemented using objects of a built-in `RegExp` class and integrated with strings.
-
-Please note that regular expressions vary between programming languages. In this tutorial we concentrate on JavaScript. Of course there's a lot in common, but they are a somewhat different in Perl, Ruby, PHP etc.