Python's `(?P<name>...)` in JavaScript: Fixing "Invalid group"

You pasted a working Python regex into JavaScript and got:

SyntaxError: Invalid regular expression: /(?P<year>\d{4})/: Invalid group

The one-character diagnosis: JavaScript doesn't accept the P. Python writes named groups as (?P<name>...); JavaScript writes them (?<name>...). Same feature, one letter of syntax drift, and JS's parser hard-rejects the Python spelling.

Delete the P:

Python:      (?P<year>\d{4})-(?P<month>\d{2})-(?P<day>\d{2})
JavaScript:  (?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})

const re = /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/;
const m = "2026-07-08".match(re);
m.groups.year;  // "2026"
m.groups.month; // "07"

That's genuinely the whole fix for group definitions. A safe mechanical conversion for a whole pattern:

const jsPattern = pyPattern.replace(/\(\?P</g, "(?<");

(Safe because (?P< has no other meaning in Python regex, and the sequence can't legitimately appear elsewhere — outside of a character class, which is why a plain .replace is fine in practice for patterns you've eyeballed.)

Python got named groups first — 1997, and the P literally stands for Python (Guido's naming, inherited from an era when regex dialects flagged their extensions). When ECMAScript 2018 finally added named groups two decades later, TC39 followed .NET's cleaner (?<name>...) syntax instead. Most of the ecosystem has converged on the JS/.NET form — Go accepts both spellings (JS-style since Go 1.22) — but Python's stdlib re still accepts only (?P<name>...), even in the newest releases. So the conversion is asymmetric: going Python → JS you drop the P; going JS → Python you must add it back, or Python throws unknown extension ?<n at position 1 at you (the n being the first letter of your group's name).

Named groups are usually just the first error JS throws at a Python pattern. The complete drift list:

Access-side differences worth 10 seconds:

# Python
m = re.search(r"(?P<year>\d{4})", s)
m.group("year")
m.groupdict()          # {'year': '2026'}

// JavaScript
const m = s.match(/(?<year>\d{4})/);
m.groups.year;
m.groups;              // { year: '2026' } — plain object, undefined for non-participating groups

One behavioral gotcha: in Python, a named group that didn't participate returns None from .group(); in JS it's undefined on .groups, and m.groups itself is undefined when the pattern has no named groups at all — guard accordingly if you're writing generic code.

The named capture group pattern runs in JS, Python, and Go side by side in RegexPro's tester — paste your pattern in each flavor and watch which syntax each engine accepts.

Related guides: Python backreferences in JavaScript · Named backreferences in Go