How to get retrieve all results of a regex with capturing groups in JS

I had this job to do

Basically I had a string that contained multiple URLs and I wanted to process them all using a regular expression.

This regular expression used capturing groups, which are great.

So, let’s start from getting one single result:

const text = 'hello1 bla bla hello2'

const regex = /hello\d/

text.match(regex)

/*
[ 'hello1', index: 0, input: 'hello1 bla bla hello2', groups: undefined ]
*/

Getting multiple results from a regex can be done using the g flag and it’s automatic, but now the result of match() is different, just the matched result:

const text = 'hello1 bla bla hello2'

const regex = /hello\d/g

console.log(text.match(regex))
//[ 'hello1', 'hello2' ]

Using matchAll() (an ES2020 feature) you can get a more detailed result set.

That returns an iterator object, so you need to use a loop to go through the results:

for (let match of text.matchAll(regex)) {
  console.log(match)
}

/*
[ 'hello1', index: 0, input: 'hello1 bla bla hello2', groups: undefined ]
[ 'hello2', index: 15, input: 'hello1 bla bla hello2', groups: undefined ]
*/

Let’s talk about capturing groups.

Say you have a text containing dates like this:

const text = '2015-01-02 2022-02-04 2040-12-02'

and you have a regex to match that date pattern, because maybe you want to get the years references:

const regex = /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/g

This uses named capturing groups.

Using text.match(regex) now will not give you any information about the groups:

const text = '2015-01-02 2022-02-04 2040-12-02'

const regex = /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/g

text.match(regex)

/*
[ '2015-01-02', '2022-02-04', '2040-12-02' ]
*/

but you can get this information using text.matchAll(regex):

const text = '2015-01-02 2022-02-04 2040-12-02'

const regex = /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/g
for (let match of text.matchAll(regex)) {
  console.log(match)
}

/*
[ '2015-01-02', '2015', '01', '02', index: 0, input: '2015-01-02 2022-02-04 2040-12-02', groups: [Object: null prototype] { year: '2015', month: '01', day: '02' } ]
[ '2022-02-04', '2022', '02', '04', index: 11, input: '2015-01-02 2022-02-04 2040-12-02', groups: [Object: null prototype] { year: '2022', month: '02', day: '04' } ]
[ '2040-12-02', '2040', '12', '02', index: 22, input: '2015-01-02 2022-02-04 2040-12-02', groups: [Object: null prototype] { year: '2040', month: '12', day: '02' } ]
*/

So you could extract the year information like this:

const text = '2015-01-02 2022-02-04 2040-12-02'

const regex = /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/g

for (let match of text.matchAll(regex)) {
  console.log(match.groups.year)
}

/*
'2015'
'2022'
'2040'
*/