A part of a pattern can be enclosed in parentheses (...). 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 (go)+ finds one or more 'go':

alert( 'Gogogo now!'.match(/(go)+/i) ); // "Gogogo"

Without parentheses, the pattern /go+/ means g, followed by o repeated one or more times. For instance, goooo or gooooooooo.

Parentheses group the word (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: [-.\w]+@([\w-]+\.)+[\w-]{2,20}.

  1. The first part [-.\w]+ (before @) may include any alphanumeric word characters, a dot and a dash, to match john.smith.
  2. Then @, and the domain. It may be a subdomain like host.site.com.uk, so we match it as "a word followed by a dot ([\w-]+\.) (repeated), and then the last part must be a word: com or 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:

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 repeating (...)+. 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 of each and allows to reference it in the pattern or in the replacement string.

For instance, we’d like to find HTML tags <.*?>, and process them.

Let’s wrap the inner content into parentheses, like this: <(.*?)>.

We’ll get them into an array:

let str = '<h1>Hello, world!</h1>';
let reg = /<(.*?)>/;

alert( str.match(reg) ); // Array: ["<h1>", "h1"]

The call to String#match returns groups only if the regexp has no /.../g flag.

If we need all matches with their groups then we can use .matchAll or regexp.exec as described in Methods of RegExp and String:

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 <(.*?)>, 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 <span class="my"> we may be interested in:

  1. The tag content as a whole: span class="my".
  2. The tag name: span.
  3. The tag attributes: class="my".

Let’s add parentheses for them:

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:

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 ( till the corresponding ) – 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 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 a, then the result is:

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 ack:

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 (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 ?<name> immediately after the opening paren, like this:

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.

Wee can also use them in replacements, as $<name> (like $1..9, but name instead of a digit).

For instance, let’s rearrange the date into day.month.year:

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, then named groups object is always the last argument:

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:

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 the contents in results.

A group may be excluded by adding ?: in the beginning.

For instance, if we want to find (go)+, but don’t want to remember the contents (go) in a separate array item, we can write: (?:go)+.

In the example below we only get the name “John” as a separate member of the results array:

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 can be:
    • capturing (...), ordered left-to-right, accessible by number.
    • named capturing (?<name>...), accessible by name.
    • non-capturing (?:...), used only to apply quantifier to the whole groups.

Tasks

Write a RegExp that matches colors in the format #abc or #abcdef. That is: # followed by 3 or 6 hexadecimal digits.

Usage example:

let reg = /your regexp/g;

let str = "color: #3f3; background-color: #AA00ef; and: #abcd";

alert( str.match(reg) ); // #3f3 #AA00ef

P.S. This should be exactly 3 or 6 hex digits: values like #abcd should not match.

A regexp to search 3-digit color #abc: /#[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: /#[a-f0-9]{3}([a-f0-9]{3})?/i

We can do it in a smarter way though: /#([a-f0-9]{3}){1,2}/i.

Here the regexp [a-f0-9]{3} is in parentheses to apply the quantifier {1,2} to it as a whole.

In action:

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 #abc in #abcd. To prevent that we can add \b to the end:

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

Create a regexp that looks for positive numbers, including those without a decimal point.

An example of use:

let reg = /your regexp/g;

let str = "1.5 0 -5 12. 123.4.";

alert( str.match(reg) ); // 1.5, 12, 123.4 (ignores 0 and -5)

An non-negative integer number is \d+. We should exclude 0 as the first digit, as we don’t need zero, but we can allow it in further digits.

So that gives us [1-9]\d*.

A decimal part is: \.\d+.

Because the decimal part is optional, let’s put it in parentheses with the quantifier '?'.

Finally we have the regexp: [1-9]\d*(\.\d+)?:

let reg = /[1-9]\d*(\.\d+)?/g;

let str = "1.5 0 -5 12. 123.4.";

alert( str.match(reg) );   // 1.5, 0, 12, 123.4

Write a regexp that looks for all decimal numbers including integer ones, with the floating point and negative ones.

An example of use:

let reg = /your regexp/g;

let str = "-1.5 0 2 -123.4.";

alert( str.match(re) ); // -1.5, 0, 2, -123.4

A positive number with an optional decimal part is (per previous task): \d+(\.\d+)?.

Let’s add an optional - in the beginning:

let reg = /-?\d+(\.\d+)?/g;

let str = "-1.5 0 2 -123.4.";

alert( str.match(reg) );   // -1.5, 0, 2, -123.4

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:

let [a, op, b] = parse("1.2 * 3.4");

alert(a); // 1.2
alert(op); // *
alert(b); // 3.4

A regexp for a number is: -?\d+(\.\d+)?. We created it in previous tasks.

An operator is [-+*/]. We put the dash - 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 /.../.

We need a number, an operator, and then another number. And optional spaces between them.

The full regular expression: -?\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: (-?\d+(\.\d+)?)\s*([-+*/])\s*(-?\d+(\.\d+)?).

In action:

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] == "1.2" (first group (-?\d+(\.\d+)?) – the first number, including the decimal part)
  • result[2] == ".2" (second group(\.\d+)? – the first decimal part)
  • result[3] == "+" (third group ([-+*\/]) – the operator)
  • 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.

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 ?: to the beginning: (?:\.\d+)?.

The final solution:

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
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