Named Capture Groups — (?<name>...) for Self-Documenting Regex

Named Capture Groups

Named capture groups make regular expressions self-documenting and decouple your code from the order of groups in the pattern. They were standardized for JavaScript in ES2018.

Syntax

(?<name>pattern)

Example: Date Parsing

const m = "2024-06-15".match(/(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/);
m.groups.year   // "2024"
m.groups.month  // "06"
m.groups.day    // "15"

Use in Replacements

Reference named groups with $<name>:

"2024-06-15".replace(
  /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/,
  "$<month>/$<day>/$<year>"
)
// "06/15/2024"

Backreferences

\k<name> references a named group within the pattern itself:

/(?<word>\w+)\s+\k<word>/.test("hello hello")  // true

Useful for finding repeated words or matching paired delimiters.

Tested Examples

Pattern	Input	groups
(?<area>\d{3})-(?<num>\d{4})	"555-1234"	area=555, num=1234
(?<name>\w+)\s+\k<name>	"go go"	name=go
(?<proto>https?):\/\/(?<host>[^/]+)	"https://x.io/path"	proto=https, host=x.io
#(?<hex>[a-f0-9]{6})	"#ff8800"	hex=ff8800

Why Named Groups Improve Code

Without names:

const [, year, month, day] = m;

With names:

const { year, month, day } = m.groups;

When the pattern evolves (a new group is inserted), positional code breaks; named-group code keeps working.

Restrictions on Names

• Names must be valid JavaScript identifiers
• Each name must be unique within the pattern
• ASCII letters, digits, underscores, and dollar signs are allowed; names cannot start with a digit

Mixing Named and Numbered Groups

Both work together. (?<key>k)(v) exposes groups.key and m[2] for the unnamed group.

Practical Recommendation

Use named groups whenever a pattern has more than two captures, or when the captures are ambiguous ((\d+)/(\d+)/(\d+) could be MM/DD/YYYY or DD/MM/YYYY). Self-documentation pays for itself the first time you revisit the regex six months later.