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en.javascript.info/9-regular-expressions/07-regexp-quantifiers/article.md
Ilya Kantor 1191dbb395 fixes
2019-06-26 09:30:35 +03:00

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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 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.

Quantity {n}

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:

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} 
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:

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); // 7,903,123,45,67

Shorthands

There are shorthands for most used quantifiers:

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

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

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:

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:

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

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

alert( "100 10 1".match(/\d0+/g) ); // 100, 10
// 1 not matched, as 0+ requires at least one zero

More examples

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

We look 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
Better regexp: according to the standard, HTML tag name may have a digit at any position except the first one, like <h1>. 
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. 
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 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.

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.