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