mirror of
https://github.com/alrayyes/wiki.git
synced 2024-11-29 06:26:22 +00:00
139 lines
3.1 KiB
Markdown
139 lines
3.1 KiB
Markdown
---
|
|
id: cd472cb3-445c-4c81-a133-d6ef13eec545
|
|
title: JavaScript RegExp Named Capture Groups
|
|
---
|
|
|
|
# Introduction
|
|
|
|
This was introduced in [ES2018](20201030095105-es2018)
|
|
|
|
# Proposal
|
|
|
|
RegExp Named Capture Groups[^1]
|
|
|
|
# Syntax
|
|
|
|
``` javascript
|
|
const RE_DATE = /(?<year>[0-9]{4})-(?<month>[0-9]{2})-(?<day>[0-9]{2})/;
|
|
|
|
const matchObj = RE_DATE.exec("1999-12-31");
|
|
|
|
console.log(matchObj.groups.year); // 1999
|
|
console.log(matchObj.groups.month); // 12
|
|
console.log(matchObj.groups.day); // 31
|
|
```
|
|
|
|
## Indexed entries
|
|
|
|
Named capture groups also created indexed entries, like [numbered
|
|
capture
|
|
groups](20201104095851-javascript_regexp_numbered_capture_groups)
|
|
|
|
``` javascript
|
|
const RE_DATE = /(?<year>[0-9]{4})-(?<month>[0-9]{2})-(?<day>[0-9]{2})/;
|
|
|
|
const matchObj = RE_DATE.exec("1999-12-31");
|
|
|
|
console.log(matchObj[1]); // 1999
|
|
console.log(matchObj[2]); // 12
|
|
console.log(matchObj[3]); // 31
|
|
```
|
|
|
|
## Destructuring
|
|
|
|
[Destructuring](20201103111746-destructuring_objects) works well with
|
|
this:
|
|
|
|
``` javascript
|
|
const RE_DATE = /(?<year>[0-9]{4})-(?<month>[0-9]{2})-(?<day>[0-9]{2})/;
|
|
|
|
const {groups: {day, year}} = RE_DATE.exec('1999-12-31');
|
|
console.log(year); // 1999
|
|
console.log(day); // 31
|
|
```
|
|
|
|
## Backreferences
|
|
|
|
`\k<name>` matches the string that was previously matched by the named
|
|
capture group `name`:
|
|
|
|
``` javascript
|
|
const RE_TWICE = /^(?<word>[a-z]+)!\k<word>$/;
|
|
RE_TWICE.test("abc!abc"); // true
|
|
RE_TWICE.test("abc!ab"); // false
|
|
```
|
|
|
|
Backreference syntax for [numbered capture
|
|
groups](20201104095851-javascript_regexp_numbered_capture_groups) works
|
|
as well:
|
|
|
|
``` javascript
|
|
const RE_TWICE = /^(?<word>[a-z]+)!\1$/;
|
|
RE_TWICE.test('abc!abc'); // true
|
|
RE_TWICE.test('abc!ab'); // false
|
|
```
|
|
|
|
Both can be mixed:
|
|
|
|
``` javascript
|
|
const RE_TWICE = /^(?<word>[a-z]+)!\k<word>!\1$/;
|
|
RE_TWICE.test("abc!abc!abc"); // true
|
|
RE_TWICE.test("abc!abc!ab"); // false
|
|
```
|
|
|
|
## repalce() and named capture groups
|
|
|
|
[replace()](20201104102343-javascript_regexp_replace_method) supports
|
|
named capture groups in two ways:
|
|
|
|
### Mention names in replacement string
|
|
|
|
``` javascript
|
|
const RE_DATE = /(?<year>[0-9]{4})-(?<month>[0-9]{2})-(?<day>[0-9]{2})/;
|
|
console.log("1999-12-31".replace(RE_DATE, "$<month>/$<day>/$<year>"));
|
|
// 12/31/1999
|
|
```
|
|
|
|
### Each replacement function receives an additional parameter that holds an object with data captured via named groups
|
|
|
|
``` javascript
|
|
const RE_DATE = /(?<year>[0-9]{4})-(?<month>[0-9]{2})-(?<day>[0-9]{2})/;
|
|
console.log(
|
|
"1999-12-31".replace(
|
|
RE_DATE,
|
|
(
|
|
g0,
|
|
y,
|
|
m,
|
|
d,
|
|
offset,
|
|
input,
|
|
{ year, month, day } // (A)
|
|
) => month + "/" + day + "/" + year
|
|
)
|
|
);
|
|
// 12/31/1999
|
|
```
|
|
|
|
[Rest parameters](20200922162500-rest_parameters) can also be used to
|
|
access the last argument:
|
|
|
|
``` javascript
|
|
const RE_DATE = /(?<year>[0-9]{4})-(?<month>[0-9]{2})-(?<day>[0-9]{2})/;
|
|
console.log(
|
|
"1999-12-31".replace(RE_DATE, (...args) => {
|
|
const { year, month, day } = args[args.length - 1];
|
|
return month + "/" + day + "/" + year;
|
|
})
|
|
);
|
|
// 12/31/1999
|
|
```
|
|
|
|
# See also
|
|
|
|
- [Numbered Capture
|
|
Groups](20201104095851-javascript_regexp_numbered_capture_groups)
|
|
|
|
# Footnotes
|
|
|
|
[^1]: <https://github.com/tc39/proposal-regexp-named-groups>
|