up

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

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+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 #AA0ef #abc
+```
+
+There's 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 #AA0ef
+```

10-regular-expressions/09-regexp-groups/1-find-webcolor-3-or-6/task.md

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+# Find color in the format #abc or #abcdef
+
+Write a regexp that matches colors in the format `#abc` or `#abcdef`. That is: `#` followed by 3 or 6 hexadimal digits.
+
+Usage example:
+```js
+let reg = /your regexp/g;
+
+let str = "color: #3f3; background-color: #AA00ef; and: #abcd";
+
+alert( str.match(reg) ); // #3f3 #AA0ef
+```
+
+P.S. Should be exactly 3 or 6 hex digits: values like `#abcd` should not match.

10-regular-expressions/09-regexp-groups/3-find-decimal-positive-numbers/solution.md

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+
+An integer number is `pattern:\d+`.
+
+A decimal part is: `pattern:\.\d+`.
+
+Because the decimal part is optional, let's put it in parentheses with quantifier `pattern:'?'`.
+
+Finally we have the regexp: `pattern:\d+(\.\d+)?`:
+
+```js run
+let reg = /\d+(\.\d+)?/g;
+
+let str = "1.5 0 12. 123.4.";
+
+alert( str.match(re) );   // 1.5, 0, 12, 123.4
+```

10-regular-expressions/09-regexp-groups/3-find-decimal-positive-numbers/task.md

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+# Find positive numbers
+
+Create a regexp that looks for positive numbers, including those without a decimal point.
+
+An example of use:
+```js
+let reg = /your regexp/g;
+
+let str = "1.5 0 12. 123.4.";
+
+alert( str.match(reg) );   // 1.5, 0, 12, 123.4
+```

10-regular-expressions/09-regexp-groups/4-find-decimal-numbers/solution.md

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

10-regular-expressions/09-regexp-groups/4-find-decimal-numbers/task.md

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+# Find all numbers
+
+Write a regexp that looks for all decimal numbers including integer ones, with the floating point and negative ones.
+
+An example of use:
+
+```js
+let reg = /your regexp/g;
+
+let str = "-1.5 0 2 -123.4.";
+
+alert( str.match(re) ); // -1.5, 0, 2, -123.4
+```

10-regular-expressions/09-regexp-groups/5-parse-expression/solution.md

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+A regexp for a number is: `pattern:-?\d+(\.\d+)?`. We created it in previous tasks.
+
+An operator is `pattern:[-+*/]`. We put a dash `pattern:-` the first, because in the middle it would mean a character range, we don't need that.
+
+Note that a slash should be escaped inside a JavaScript regexp `pattern:/.../`.
+
+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+)?)`.
+
+In action:
+
+```js run
+let reg = /(-?\d+(\.\d+)?)\s*([-+*\/])\s*(-?\d+(\.\d+)?)/;
+
+alert( "1.2 + 12".match(reg) );
+```
+
+The result includes:
+
+- `result[0] == "1.2 + 12"` (full match)
+- `result[1] == "1"` (first parentheses)
+- `result[2] == "2"` (second parentheses -- the decimal part `(\.\d+)?`)
+- `result[3] == "+"` (...)
+- `result[4] == "12"` (...)
+- `result[5] == undefined` (the last decimal part is absent, so it's undefined)
+
+We need only numbers and the operator. We don't need decimal parts.
+
+So let's remove extra groups from capturing by added `pattern:?:`, for instance: `pattern:(?:\.\d+)?`.
+
+The final solution:
+
+```js run
+function parse(expr) {
+  let reg = /(-?\d+(?:\.\d+)?)\s*([-+*\/])\s*(-?\d+(?:\.\d+)?)/;
+
+  let result = expr.match(reg);
+
+  if (!result) return;
+  result.shift();
+
+  return result;
+}
+
+alert( parse("-1.23 * 3.45") );  // -1.23, *, 3.45
+```

10-regular-expressions/09-regexp-groups/5-parse-expression/task.md

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+# Parse an expression
+
+An arithmetical expression consists of 2 numbers and an operator between them, for instance:
+
+- `1 + 2`
+- `1.2 * 3.4`
+- `-3 / -6`
+- `-2 - 2`
+
+The operator is one of: `"+"`, `"-"`, `"*"` or `"/"`.
+
+There may be extra spaces at the beginning, at the end or between the parts.
+
+Create a function `parse(expr)` that takes an expression and returns an array of 3 items:
+
+1. The first number.
+2. The operator.
+3. The second number.
+
+For example:
+
+```js
+let [a, op, b] = parse("1.2 * 3.4");
+
+alert(a); // 1.2
+alert(op); // *
+alert(b); // 3.4
+```

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

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+# Capturing groups
+
+A part of the pattern can be enclosed in parentheses `pattern:(...)`. That's called a "capturing group".
+
+That has two effects:
+
+1. It allows to place a part of the match into a separate array item when using  [String#match](mdn:js/String/match) or [RegExp#exec](mdn:/RegExp/exec) methods.
+2. If we put a quantifier after the parentheses, it applies to the parentheses as a whole, not the last character.
+
+[cut]
+
+## 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}`.
+
+- The first part before `@` may include wordly characters, a dot and a dash `pattern:[-.\w]+`, like `match:john.smith`.
+- Then `pattern:@`
+- And then the domain. May be a second-level domain `site.com` or with subdomains like `host.site.com.uk`. We can match it as "a word followed by a dot" repeated one or more times for subdomains: `match:mail.` or `match:site.com.`, and then "a word" for the last part: `match:.com` or `match:.uk`.
+
+    The word followed by a dot is `pattern:(\w+\.)+` (repeated). The last word should not have a dot at the end, so it's just `\w{2,20}`. The quantifier `pattern:{2,20}` limits the length, because domain zones are like `.uk` or `.com` or `.museum`, but can't be longer than 20 characters.
+
+    So the domain pattern is `pattern:(\w+\.)+\w{2,20}`. Now we replace `\w` with `[\w-]`, because dashes are also allowed in domains, and we get the final result.
+
+That regexp is not perfect, but usually works. It's short and good enough to fix errors or occasional mistypes.
+
+For instance, here 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
+```
+
+
+## Contents of parentheses  
+
+Parentheses are numbered from left to right. The search engine remembers the content of each and allows to reference it in the pattern or in the replacement string.
+
+For instance, we can find an HTML-tag using a (simplified) pattern `pattern:<.*?>`. Usually we'd want to do something with the result after it.
+
+If we enclose the inner contents of `<...>` into parentheses, then we can access it like this:
+
+```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 has no `pattern:/.../g` flag.
+
+If we need all matches with their groups then we can use [RegExp#exec](mdn:js/RegExp/exec) method 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 match;
+
+while (match = reg.exec(str)) {
+  // first shows the match: <h1>,h1
+  // then shows the match: </h1>,/h1
+  alert(match);
+}
+```
+
+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.png)
+
+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]`.
+
+**If a group is optional and doesn't exist in the match, the corresponding `result` index 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"]`.
+
+## Non-capturing groups with ?:
+
+Sometimes we need parentheses to correctly apply a quantifier, but we don't want their contents in the array.
+
+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 put 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
+```