wiki/content/20201104100431-javascript_regexp_named_capture_groups.md

date	id	title
2020-11-04	cd472cb3-445c-4c81-a133-d6ef13eec545	JavaScript RegExp Named Capture Groups

Introduction

This was introduced in ES2018

Proposal

RegExp Named Capture Groups¹

Syntax

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

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 works well with this:

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:

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 works as well:

const RE_TWICE = /^(?<word>[a-z]+)!\1$/;
RE_TWICE.test('abc!abc'); // true
RE_TWICE.test('abc!ab'); // false

Both can be mixed:

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() supports named capture groups in two ways:

Mention names in replacement string

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

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 can also be used to access the last argument:

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

Footnotes

---

1. https://github.com/tc39/proposal-regexp-named-groups ↩︎