Regular expressions, groups & matchAll

Parse text with named groups

Lesson 17 of 20 · 11 min · Module 5: Real-world data

Logs, CSV exports, and chat commands are text with a structure. Regular expressions with named groups turn such text into objects you can work with.

Start this lesson →

Free. No installation: you write and run the code in your browser, and it is checked automatically.

By the end, you’ll be able to…

Capture fields with named groups
Anchor a pattern to the whole line
Turn text into structured data

The idea

(?<level>[A-Z]+) captures part of a match under a name, available as match.groups.level. Anchor a pattern with ^ and $ so the whole line must match. text.matchAll(/…/g) returns every match in a text.

const m = "[email protected]".match(/^(?<user>[^@]+)@(?<domain>.+)$/);
console.log(m.groups.user, m.groups.domain);

01Names beat positions

match[3] breaks as soon as someone adds a group earlier in the pattern. match.groups.level keeps working and explains itself.

02Anchors matter

Without ^ and $, the pattern would match a valid-looking part of "junk 2026-10-18 …" and accept a broken line. Anchors require the entire line to fit.

console.log(/\d+/.test("abc123"), /^\d+$/.test("abc123"));

Output

true false

03Define the pattern once

A regular expression stored in a constant is created once and reused for every line, which keeps the parser fast and the pattern in one place.

Your turn

Write parseLogLine(line) for lines like 2026-10-18 08:07:03 ERROR Payment service timeout, returning { date, time, level, message }, or null when the line does not match (levels are uppercase words; there may be several spaces after the level). Write countLevels(text) that returns an object counting each level across all lines, ignoring lines that do not match.

What the checks look for

Parses a log line into named fields
Allows several spaces after the level
Returns null for lines that do not match
Counts levels and skips other lines

Try it in the editor →

Take this with you

Named groups turn lines of text into objects.

Go a little further

Return the messages of all ERROR lines between two given times.