Python's `(?P=name)` in JavaScript: Use `\k<name>` Instead

Python's (?P=name) backreference syntax throws "Invalid group" in JS — swap in \k<name>, plus the replacement-string gotchas.

You ported a Python regex and JavaScript rejected it:

textOutput

SyntaxError: Invalid regular expression: /(?P<q>['"]).*?(?P=q)/: Invalid group

Two Python-isms are failing there, and they travel as a pair: (?P<name>...) defines a named group (covered here) and (?P=name) references it later in the same pattern. JavaScript spells that reference \k<name>.

The fix

textText

Python:      (?P<q>['"]).*?(?P=q)
JavaScript:  (?<q>['"]).*?\k<q>

jsJavaScript

const re = /(?<q>['"]).*?\k<q>/;
re.test(`'hello'`);   // true  — same quote closes
re.test(`'hello"`);   // false — mismatched quotes don't pair

Mechanical conversion for both forms at once:

jsJavaScript

const jsPattern = pyPattern
  .replace(/\(\?P</g, "(?<")      // group definitions
  .replace(/\(\?P=(\w+)\)/g, "\\k<$1>"); // in-pattern references

The three reference contexts — don't mix them up

Python uses one syntax family in patterns and another in replacements; JS does too, and they don't line up. This table prevents the second bug you were about to write:

Context	Python	JavaScript
Reference inside the pattern	`(?P=name)`	`\k<name>`
Reference in replacement string	`\g<name>` (in `re.sub`)	`$<name>` (in `.replace`)
Numbered reference in pattern	`\1`	`\1` (same!)
Numbered reference in replacement	`\1`	`$1`

Worked replacement example, both languages:

pythonPython

# Python — swap "Last, First" to "First Last"
re.sub(r"(?P<last>\w+), (?P<first>\w+)", r"\g<first> \g<last>", "Ebert, Andrew")

jsJavaScript

// JavaScript
"Ebert, Andrew".replace(/(?<last>\w+), (?<first>\w+)/, "$<first> $<last>");

Note the numbered form \1 inside a pattern is identical in both languages — if your Python pattern used \1 instead of (?P=name), it runs in JS unchanged. Only the named spelling drifts.

Gotchas that survive the conversion

\k without a named group is treated as a literal k in non-Unicode JS regexes (Annex B legacy behavior) — so a typo like \k<qoute> may silently match the letter "k" instead of erroring. Add the u flag (/pattern/u) and JS will throw on invalid \k, turning silent wrong-matches into loud errors. Recommended always.
Case-insensitive backreferences behave the same in both engines ((?i) semantics apply to the reference too), but remember JS wants the /i flag, not inline (?i).
Go doesn't accept either spelling — no backreferences at all in RE2. If Go is in your deployment mix, see named backreferences in Go for the capture-and-verify workaround before you standardize on a pattern.

Test it live

Paste your pattern into RegexPro's tester and run it against JS, Python, and Go simultaneously — the fastest way to find out which of the three spellings you actually need. Start from the duplicate word or named capture group patterns to see the syntax families side by side.

All guides Open the RegexPro tester →