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Grouping and Back References

During replacements, within a match, you can group the characters with (), and back-reference them with $1-$9.

Additionally,$& references the whole match.$` references the left context.$' references the right context.


RESETRUNFULL
<!DOCTYPE html><html><body><script>


   var s = "abc";
    console.log(s.replace(/a(.)(.)/,"$2"));  // c
   console.log(s.replace(/a/,"$&$&"));  // aabc
   console.log(s.replace(/b/,"$`"));  // aac
   console.log(s.replace(/a/,"$'"));  // bcbc

</script></body><html>

You can name each capture group by starting the () group with ?<name>.


RESETRUNFULL
<!DOCTYPE html><html><body><script>

const r =
      /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/u;const d = r.exec("2020-07-29");/*d.groups.year === "2020"d.groups.month === "07"d.groups.day === "29"d[0] === "2020-07-29"d[1] === "2020"d[2] === "07"d[3] === "29"*/let {groups:{year,month,day}} = d;  // destructuringconsole.log(year,month,day);  // 2020 07 29

</script></body><html>