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.
The simplest quantifier is a number in curly braces: pattern:{n}.
A quantifier is appended to a character (or a character class, or a [...] set etc) and specifies how many we need.
It has a few advanced forms, let's see examples:
The 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 range: {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
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 sequences of digits of length `3` or more:
```js run
alert( "I'm not 12, but 345678 years old".match(/\d{3,}/) ); // "345678"
```
Let's return to the string +7(903)-123-45-67.
A number is a sequence of one or more digits in a row. So the regexp is pattern:\d{1,}:
let str = "+7(903)-123-45-67";
let numbers = str.match(/\d{1,}/g);
alert(numbers); // 7,903,123,45,67There are shorthands for most used quantifiers:
+
: 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, `pattern:colou?r` finds both `match:color` and `match: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.
For example, `pattern:\d0*` 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
// 1 not matched, as 0+ requires at least one zero
```
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+
: 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, for HTML tags we could use a simpler regexp: `pattern:<\w+>`.
...But because `pattern:\w` means any English 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.
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.