29 Februari 2020

Materi ini hanya tersedia dalam bahasa berikut: English, EspaÃ±ol, FranÃ§ais, Italiano, æ—¥æœ¬èªž, Ð ÑƒÑÑÐºÐ¸Ð¹, TÃ¼rkÃ§e, Ð£ÐºÑ€Ð°Ñ—Ð½ÑÑŒÐºÐ°, OÊ»zbek, ç®€ä½“ä¸æ–‡. Tolong, bantu kami menerjemahkan ke dalam Indonesia.

Quantifiers +, *, ? and {n}

Letâ€™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 can append a quantifier.

Quantity {n}

The simplest quantifier is a number in curly braces: {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, letâ€™s see examples:

The exact count: {5}

\d{5} denotes exactly 5 digits, the same as \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: \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: \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 \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"

Letâ€™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 \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 â€œone or moreâ€, the same as {1,}.

For instance, \d+ looks for numbers:

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

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

?

Means â€œzero or oneâ€, the same as {0,1}. In other words, it makes the symbol optional.

For instance, the pattern ou?r looks for o followed by zero or one u, and then r.

So, colou?r finds both color and colour:

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

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

*

Means â€œzero or moreâ€, the same as {0,}. That is, the character may repeat any times or be absent.

For example, \d0* looks for a digit followed by any number of zeroes (may be many or none):

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 â€œbuilding blockâ€ of complex regular expressions, so letâ€™s see more examples.

Regexp for decimal fractions (a number with a floating point): \d+\.\d+

In action:

alert( "0 1 12.345 7890".match(/\d+\.\d+/g) ); // 12.345

Regexp for an â€œopening HTML-tag without attributesâ€, such as <span> or <p>.

The simplest one: /<[a-z]+>/i
```
alert( "<body> ... </body>".match(/<[a-z]+>/gi) ); // <body>
```
The regexp looks for character '<' followed by one or more Latin letters, and then '>'.
Improved: /<[a-z][a-z0-9]*>/i

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 â€œopening or closing HTML-tag without attributesâ€: /<\/?[a-z][a-z0-9]*>/i

We added an optional slash /? near the beginning of the pattern. 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 â€“ the longer and more complex it is.

For instance, for HTML tags we could use a simpler regexp: <\w+>. But as HTML has stricter restrictions for a tag name, <[a-z][a-z0-9]*> is more reliable.

Can we use <\w+> or we need <[a-z][a-z0-9]*>?

In real life both variants are acceptable. Depends on how tolerant we can be to â€œextraâ€ matches and whether itâ€™s difficult or not to remove them from the result by other means.

Tugas

How to find an ellipsis "..." ?

Create a regexp to find ellipsis: 3 (or more?) dots in a row.

Check it:

          let regexp = /your regexp/g;
alert( "Hello!... How goes?.....".match(regexp) ); // ..., .....
        

Solution:

let regexp = /\.{3,}/g;
alert( "Hello!... How goes?.....".match(regexp) ); // ..., .....

Please note that the dot is a special character, so we have to escape it and insert as \..

Regexp for HTML colors

Create a regexp to search HTML-colors written as #ABCDEF: first # and then 6 hexadecimal characters.

An example of use:

          let regexp = /...your regexp.../

let str = "color:#121212; background-color:#AA00ef bad-colors:f#fddee #fd2 #12345678";

alert( str.match(regexp) )  // #121212,#AA00ef

P.S. In this task we do not need other color formats like #123 or rgb(1,2,3) etc.

We need to look for # followed by 6 hexadecimal characters.

A hexadecimal character can be described as [0-9a-fA-F]. Or if we use the i flag, then just [0-9a-f].

Then we can look for 6 of them using the quantifier {6}.

As a result, we have the regexp: /#[a-f0-9]{6}/gi.

let regexp = /#[a-f0-9]{6}/gi;

let str = "color:#121212; background-color:#AA00ef bad-colors:f#fddee #fd2"

alert( str.match(regexp) );  // #121212,#AA00ef

The problem is that it finds the color in longer sequences:

alert( "#12345678".match( /#[a-f0-9]{6}/gi ) ) // #123456

To fix that, we can add \b to the end:

// color
alert( "#123456".match( /#[a-f0-9]{6}\b/gi ) ); // #123456

// not a color
alert( "#12345678".match( /#[a-f0-9]{6}\b/gi ) ); // null

Peta tutorial

komentar

baca ini sebelum berkomentarâ€¦