Permalink
Switch branches/tags
Nothing to show
Find file Copy path
Fetching contributors…
Cannot retrieve contributors at this time
134 lines (85 sloc) 4.47 KB

Quantifiers +, *, ? and {n}

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 in not digits, but full numbers: 7, 903, 123, 45, 67.

A number is a sequence of 1 or more digits \d. The instrument to say how many we need is called quantifiers.

Quantity {n}

The most obvious quantifier is a number in figure quotes: pattern:{n}. A quantifier is put after a character (or a character class and so on) and specifies exactly how many we need.

It also has advanced forms, here we go with examples:

Exact count: {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:

```js run
alert( "I'm 12345 years old".match(/\d{5}/) ); //  "12345"
```

We can add `\b` to exclude longer numbers: `pattern:\b\d{5}\b`.

The count from-to: {3,5} : To find numbers from 3 to 5 digits we can put the limits into figure brackets: pattern:\d{3,5}

```js run
alert( "I'm not 12, but 1234 years old".match(/\d{3,5}/) ); // "1234"
```

We can omit the upper limit. Then a regexp `pattern:\d{3,}` looks for numbers of `3` and more digits:

```js run
alert( "I'm not 12, but 345678 years old".match(/\d{3,}/) ); // "345678"
```

In case with the string +7(903)-123-45-67 we need numbers: one or more digits in a row. That is pattern:\d{1,}:

let str = "+7(903)-123-45-67";

let numbers = str.match(/\d{1,}/g);

alert(numbers); // 7,903,123,45,67

Shorthands

Most often needed quantifiers have shorthands:

+ : Means "one or more", the same as {1,}.

For instance, `pattern:\d+` looks for numbers:

```js run
let str = "+7(903)-123-45-67";

alert( str.match(/\d+/g) ); // 7,903,123,45,67
```

? : Means "zero or one", the same as {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`.

So it can find `match:or` in the word `subject:color` and `match:our` in `subject:colour`:

```js run
let str = "Should I write color or colour?";

alert( str.match(/colou?r/g) ); // color, colour
```

* : Means "zero or more", the same as {0,}. That is, the character may repeat any times or be absent.

The example below looks for a digit followed by any number of zeroes:

```js run
alert( "100 10 1".match(/\d0*/g) ); // 100, 10, 1
```

Compare it with `'+'` (one or more):

```js run
alert( "100 10 1".match(/\d0+/g) ); // 100, 10
```

More examples

Quantifiers are used very often. They are one of the main "building blocks" for complex regular expressions, so let's see more examples.

Regexp "decimal fraction" (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:

```js run
alert( "<body> ... </body>".match(/<[a-z]+>/gi) ); // <body>
```

We look for character `pattern:'<'` followed by one or more English letters, and then  `pattern:'>'`.

Regexp "open HTML-tag without attributes" (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>.

```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 : 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
alert( "<h1>Hi!</h1>".match(/<\/?[a-z][a-z0-9]*>/gi) ); // <h1>, </h1>
```
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, HTML tags could use a simpler regexp: `pattern:<\w+>`.

Because `pattern:\w` means any English letter or a digit or `'_'`, the regexp also matches non-tags, for instance `match:<_>`. But it's much simpler than `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 filter them out by other means.